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Color quantization is an important operation with many applications in graphics and image processing. Most quantization methods are essentially based on data clustering algorithms. However, despite its popularity as a general purpose clustering algorithm, k-means has not received much respect in the color quantization literature because of its high computational requirements and sensitivity to initialization. In this paper, we investigate the performance of k-means as a color quantizer. We implement fast and exact variants of k-means with several initialization schemes and then compare the resulting quantizers to some of the most popular quantizers in the literature. Experiments on a diverse set of images demonstrate that an efficient implementation of k-means with an appropriate initialization strategy can in fact serve as a very effective color quantizer.
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arxiv:1101.0395
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This is a continuation of a project on large deviations for the empirical measures of zeros of random holomorphic sections of random line bundles over a Riemann surface X. In a previous article with O. Zeitouni (arXiv:0904.4271), we proved an LDP for random polynomials in the genus zero case. In higher genus, there is a Picard variety of line bundles and so the line bundle L is a random variable as well as the section s. The space of pairs (L, s) is known as the "vortex moduli space". The zeros of (L, s) fill out the configuration space $X^{(N)}$ of $N$ points of $X$. The LDP shows that the configurations concentrate at one equilibrium measure exponentially fast. The new features of the proof involve Abel-Jacobi theory, the prime form and bosonization.
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arxiv:1101.0417
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We introduce real second-order freeness in second-order noncommutative probability spaces. We demonstrate that under this definition, three real models of random matrices, namely real Ginibre matrices, Gaussian orthogonal matrices, and real Wishart matrices, are asymptotically second-order free. These ensembles do not satisfy the complex definition of second-order freeness satisfied by their complex analogues. We use a combinatorial approach to the matrix calculations similar to the genus expansion for complex random matrices, but in which nonorientable surfaces appear, demonstrating the commonality between the real models and the distinction from their complex analogues, motivating this distinct definition. In the real case we find, in addition to the terms appearing in the complex case corresponding to annular spoke diagrams, an extra set of terms corresponding to annular spoke diagrams in which the two circles of the annulus are oppositely oriented, and in which the matrix transpose appears.
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arxiv:1101.0422
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In this paper, we consider the optimal dividends problem for a company whose cash reserves follow a general Levy process with certain positive jumps and arbitrary negative jumps. The objective is to find a policy which maximizes the expected discounted dividends until the time of ruin. Under appropriate conditions, we appeal to very recent results in the theory of potential analysis of subordinators to obtain the convexity properties of probability of ruin. We present conditions under which the optimal dividend strategy, among all admissible ones, takes the form of a barrier strategy.
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arxiv:1101.0449
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In this article, we study the radiative decays among the charmonium states with the heavy quark effective theory, and make predictions for the ratios among the radiative decay widths of an special multiplet to another multiplet. The predictions can be confronted with the experimental data in the future and put additional constraints in identifying the $X$, $Y$, $Z$ charmonium-like mesons.
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arxiv:1101.0474
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Novel material properties can be realized by designing waves' dispersion relations in artificial crystals. The crystal's structural length scales may range from nano- (light) up to centimeters (sound waves). Because of their emergent properties these materials are called metamaterials. Different to photonics, where the dielectric constant dominantly determines the index of refraction, in a ferromagnet the spin-wave index of refraction can be dramatically changed already by the magnetization direction. This allows a different flexibility in realizing dynamic wave guides or spin-wave switches. The present review will give an introduction into the novel functionalities of spin-wave devices, concepts for spin-wave based computing and magnonic crystals. The parameters of the magnetic metamaterials are adjusted to the spin-wave k-vector such that the magnonic band structure is designed. However, already the elementary building block of an antidot lattice, the singular hole, owns a strongly varying internal potential determined by its magnetic dipole field and a localization of spin-wave modes. Photo-magnonics reveal a way to investigate the control over the interplay between localization and delocalization of the spin-wave modes using femtosecond lasers, which is a major focus of this review. We will discuss the crucial parameters to realize free Bloch states and how, by contrast, a controlled localization might allow to gradually turn on and manipulate spin-wave interactions in spin-wave based devices in the future.
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arxiv:1101.0479
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Chaotic instanton approach allows to describe analytically the influence of the polychromatic perturbation on quantum properties of nonlinear systems. Double well system with single, multiple and polychromatic kicked perturbation is considered in the paper to compare quantitative analytical predictions with the results of numerical calculations. Chaotic instantons are responsible for the enhancement of tunneling far away from the exact (avoided) level crossings in framework of the approach used. Time-independent averaged system is used for regular approximation of the chaotic instanton solution in order to take into account its contribution to the ground quasienergy doublet splitting. Formula for the ground quasienergy levels' splitting was derived averaging trajectory action in the stochastic layer in the framework of chaotic instanton approach. Results of quasienergy spectrum numerical calculations and simulations for tunneling dynamics are in good agreement with the obtained analytical predictions.
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arxiv:1101.0505
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We introduce the notion of "functional extension" of a set X, by means of two natural algebraic properties of the operator * on unary functions. We study the connections with ultrapowers of structures with universe X, and we give a simple characterization of those functional extensions that correspond to limit ultrapower extensions. In particular we obtain a purely algebraic proof of Keisler's characterization of nonstandard (= complete elementary) extensions.
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arxiv:1101.0548
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The article concerns the subalgebra U_v^+(w) of the quantized universal enveloping algebra of the complex Lie algebra sl_{n+1} associated with a particular Weyl group element of length 2n. We verify that U_v^+(w) can be endowed with the structure of a quantum cluster algebra of type A_n. The quantum cluster algebra is a deformation of the ordinary cluster algebra Geiss-Leclerc-Schroeer attached to w using the representation theory of the preprojective algebra. Furthermore, we prove that the quantum cluster variables are, up to a power of v, elements in the dual of Lusztig's canonical basis under Kashiwara's bilinear form.
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arxiv:1101.0580
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We show that geometrical scaling exhibited by the $p_{\rm T}$ spectra measured by the CMS collaboration at the LHC is substantially improved if the exponent $\lambda$ of the saturation scale depends on $p_{\rm T}$. This dependence is shown to be the same as the dependence of small $x$ exponent of $F_2$ structure function in deep inelastic scattering taken at the scale $Q=p_{\rm T}/2$
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arxiv:1101.0585
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We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years.
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arxiv:1101.0620
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In the scenario where only superpartners were produced at the Large Hadron Collider, how one could determine whether the supersymmetric model pointed out is 4-dimensional or higher-dimensional ? We propose and develop a series of tests for discriminating between a pure supersymmetry (SUSY) and a SUSY realized within the well-motivated warped geometry a la Randall-Sundrum (RS). Two of these tests make use of some different patterns arising in the squark/slepton mass spectrum. The other distinctive RS SUSY feature is the possibly larger (even dominant) Higgs boson decay branching ratios into sleptons, compared to pure SUSY. Techniques for pinning up the presence of soft SUSY breaking terms on the TeV-brane are also suggested, based on the analysis of stop pair production at the International Linear Collider. For all these phenomenological studies, we had first to derive the 4-dimensional (4D) effective couplings and mass matrices of the sfermions and Higgs bosons in RS SUSY. The localization of Higgs bosons, characteristic of RS, leads to singularities in their couplings which are regularized by the exchange contribution of infinite towers of Kaluza-Klein (KK) scalar modes with Dirichlet-Dirichlet boundary conditions. A general method is provided for this regularization, based on the completeness relation. The sfermion masses are obtained either from integrating out those specific KK towers or by treating their mixing effects. Finally, we show at the one-loop level how all quadratic divergences in the Higgs mass cancel out for any cut-off, due to 5D SUSY and to 5D anomaly cancellation; the analytical way followed here also allows a justification of the infinite KK summation required for the so-called KK regularization in 5D SUSY, which has motivated a rich literature.
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arxiv:1101.0634
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Assume $M$ is a closed 3-manifold whose universal covering is not $S^3$. We show that the obstruction to extend the Ricci flow is the boundedness $L^{3/2}$-norm of the scalar curvature $R(t)$, i.e, the Ricci flow can be extended over time $T$ if and only if the $||R(t)||_{L^{3/2}}$ is uniformly bounded for $0 \leq t < T$ . On the other hand, if the fundamental group of $M$ is finite and the $||R(t)||_{L^{\3/2}}$ is bounded for all time under the Ricci flow, then $M$ is diffeomorphic to a 3-dimensional spherical space-form.
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arxiv:1101.0637
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Inspired by recent work of S. K. Donaldson on constant scalar curvature metrics on toric complex surfaces, we study obstructions to the extension of the Calabi flow on a polarized toric variety. Under some technical assumptions, we prove that the Calabi flow can be extended for all time.
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arxiv:1101.0638
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(Abridged) The motion of a Schwarzschild black hole with velocity $v_0 = \beta_0 c$ through a constant magnetic field $B_0$ in vacuum induces a component of the electric field along the magnetic field, generating a non-zero second Poincare electromagnetic invariant $ ^* F \cdot F \neq 0$. This will produce (e.g., via radiative effects and vacuum breakdown) an electric charge density of the order of $\rho_{\rm ind}= B_0 \beta_0 /(2 \pi e R_G)$, where $R_G = 2 G M/c^2$ is the Schwarzschild radius and $M$ is the mass of the black hole; the charge density $\rho_{\rm ind}$ is similar to the Goldreich-Julian density. The magnetospheres of moving black holes resemble in many respects the magnetospheres of rotationally-powered pulsars, with pair formation fronts and outer gaps, where the sign of the induced charge changes. As a result, the black hole will generate bipolar electromagnetic jets each consisting of two counter-aligned current flows (four current flows total), each carrying an electric current of the order $I \approx e B_0 R_G \beta_0$. The electromagnetic power of the jets is $L \approx (G M)^2 B_0^2 \beta_0^2/c^3$; for a particular case of merging black holes the resulting Poynting power is $ L \approx {(G M)^3 B_0^2 /(c^5 R)}$, where $R$ is the radius of the orbit. In addition, in limited regions near the horizon the first electromagnetic invariant changes sign, so that the induced electric field becomes larger than the magnetic field, $E>B$. The total energy loss from a system of merging BHs is a sum of two components with similar powers, one due to the rotation of space-time within the orbit, driven by the non-zero angular momentum in the system, and the other due to the linear motion of the BHs through the magnetic field.
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arxiv:1101.0639
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In the isostructural oxides Ca3CoMO6 (M = Co, Rh, Ir), the CoMO6 chains made up of face-sharing CoO6 trigonal prisms and MO6 octahedra are separated by Ca atoms. We analyzed the magnetic and electronic properties of these oxides on the basis of density functional theory calculations including on-site repulsion and spin-orbit coupling, and examined the essential one-electron pictures hidden behind results of these calculations. Our analysis reveals an intimate interplay between Jahn-Teller instability, uniaxial magnetism, spin arrangement, metal-metal interaction, and spin-orbit coupling in governing the magnetic and electronic properties of these oxides. These oxides undergo a Jahn-Teller distortion but their distortions are weak, so that their trigonal-prism Con+ (n = 2, 3) ions still give rise to strong easy-axis anisotropy along the chain direction. As for the d-state split pattern of these ions, the electronic and magnetic properties of Ca3CoMO6 (M = Co, Rh, Ir) are consistent with d0 < (d2, d-2) < (d1, d-1), but not with (d2, d-2) < d0 < (d1, d-1). The trigonal-prism Co3+ ion in Ca3Co2O6 has the L = 2 configuration (d0)1(d2, d-2)3(d1, d-1)2 because of the metal-metal interaction between adjacent Co3+ ions in each Co2O6 chain, which is mediated by their z2 orbitals, and the spin-orbit coupling of the trigonal-prism Co3+ ion. The spins in each CoMO6 chain of Ca3CoMO6 prefer the ferromagnetic arrangement for M = Co and Rh, but the antiferromagnetic arrangement for M = Ir. The octahedral M4+ ion of Ca3CoMO6 has the (1a)1(1e)4 configuration for M = Rh but the (1a)2(1e)3 configuration for M = Ir, which arises from the difference in the spin-orbit coupling of the M4+ ions and the Co...M metal-metal interactions.
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arxiv:1101.0644
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The anomalous Hall effect in metal-insulator-semiconductor structures having thin (Ga,Mn)As layers as a channel has been studied in a wide range of Mn and hole densities changed by the gate electric field. Strong and unanticipated temperature dependence, including a change of sign, of the anomalous Hall conductance $\sigma_{xy}$ has been found in samples with the highest Curie temperatures. For more disordered channels, the scaling relation between $\sigma_{xy}$ and $\sigma_{xx}$, similar to the one observed previously for thicker samples, is recovered.
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arxiv:1101.0652
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We propose the use of intra-particle entanglement to enhance the security of a practical implementation of the Bennett-Brassard-1984 (BB84) quantum key distribution scheme. Intra-particle entanglement is an attractive resource since it can be easily generated using only linear optics. Security is studied under a simple model of incoherent attack for protocols involving two or all five mutually unbiased bases. In terms of efficiency of secret key generation and tolerable error rate, the latter is found to be superior to the former. We find that states that allow secrecy distillation are necessarily entangled, though they may be local. Since more powerful attacks by Eve obviously exist, our result implies that security is a strictly stronger condition than entanglement for these protocols.
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arxiv:1101.0660
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In this paper we present the method and experimental results of the investigation of a longitudinal component of relativistic electron electromagnetic field in the shadow area of a transversal component. We show experimentally, that in a region, comparable with the formation length area no shadowing effect of the longitudinal component of relativistic electron electromagnetic field appears. This is important for understanding of possibility of the shadowing effect in Smith-Purcell radiation and some other radiation types.
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arxiv:1101.0662
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In the framework of noncompact Kaluza-Klein theory, we investigate a $(4+1)$-dimensional universe consisting of a $(4+1)$ dimensional Robertson-Walker type metric coupled to a $(4+1)$ dimensional energy-momentum tensor. The matter part consists of an energy density together with a pressure subject to 4D part of the $(4+1)$ dimensional energy-momentum tensor. The dark part consists of just a dark pressure $\bar{p}$, corresponding to the extra-dimension endowed by a scalar field, with no element of dark energy. It is shown that for a flat universe, coupled with the non-vacuum states of the scalar field, the reduced field equations subject to suitable equations of state for matter and dark part may reveal inflationary behavior at early universe, deceleration for radiation dominant era and then acceleration in matter dominant era.
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arxiv:1101.0666
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In this comment we explain the discrepancies mentioned by the authors between their results and ours about the influence of the gravitational quadrupole moment in the perturbative calculation of corrections to the precession of the periastron of quasielliptical Keplerian equatorial orbits around a point mass. The discrepancy appears to be consequence of two different calculations of the angular momentum of the orbits.
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arxiv:1101.0700
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Guo and the second author have shown that the closure $[I]$ in the Drury-Arveson space of a homogeneous principal ideal $I$ in $\mathbb{C}[z_1,...,z_n]$ is essentially normal. In this note, the authors extend this result to the closure of any principal polynomial ideal in the Bergman space. In particular, the commutators and cross-commutators of the restrictions of the multiplication operators are shown to be in the Schatten $p $-class for $p>n$. The same is true for modules generated by polynomials with vector-valued coefficients. Further, the maximal ideal space $X_I$ of the resulting $C^\ast$-algebra for the quotient module is shown to be contained in $Z(I)\cap \partial\mathbb{B}_n$, where $Z(I)$ is the zero variety for $I$, and to contain all points in $\partial\mathbb{B}_n$ that are limit points of $Z(I)\cap \mathbb{B}_n$. Finally, the techniques introduced enable one to study a certain class of weight Bergman spaces on the ball.
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arxiv:1101.0774
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Competition is one of the most fundamental phenomena in physics, biology and economics. Recent studies of the competition between innovations have highlighted the influence of switching costs and interaction networks, but the problem is still puzzling. We introduce a model that reveals a novel multi-percolation process, which governs the struggle of innovations trying to penetrate a market. We find that innovations thrive as long as they percolate in a population, and one becomes dominant when it is the only one that percolates. Besides offering a theoretical framework to understand the diffusion of competing innovations in social networks, our results are also relevant to model other problems such as opinion formation, political polarization, survival of languages and the spread of health behavior.
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arxiv:1101.0775
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In the magnetar model, the quiescent non-thermal soft X-ray emission from Anomalous X-ray Pulsars and Soft-Gamma Repeaters is thought to arise from resonant comptonization of thermal photons by charges moving in a twisted magnetosphere. Robust inference of physical quantities from observations is difficult, because the process depends strongly on geometry and current understanding of the magnetosphere is not very deep. The polarization of soft X-ray photons is an independent source of information, and its magnetospheric imprint remains only partially explored. In this paper we calculate how resonant cyclotron scattering would modify the observed polarization signal relative to the surface emission, using a multidimensional Monte Carlo radiative transfer code that accounts for the gradual coupling of polarization eigenmodes as photons leave the magnetosphere. We employ a globally-twisted, self-similar, force-free magnetosphere with a power-law momentum distribution, assume a blackbody spectrum for the seed photons, account for general relativistic light deflection close to the star, and assume that vacuum polarization dominates the dielectric properties of the magnetosphere. The latter is a good approximation if the pair multiplicity is not much larger than unity. Phase-averaged polarimetry is able to provide a clear signature of the magnetospheric reprocessing of thermal photons and to constrain mechanisms generating the thermal emission. Phase-resolved polarimetry, in addition, can characterize the spatial extent and magnitude of the magnetospheric twist angle at ~100 stellar radii, and discern between uni- or bidirectional particle energy distributions, almost independently of every other parameter in the system. We discuss prospects for detectability with GEMS.
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arxiv:1101.0834
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We prove a formula expressing the motivic integral (\cite{ls}) of a K3 surface over $\bC((t))$ with semi-stable reduction in terms of the associated limit Hodge structure. Secondly, for every smooth variety over a non-archimedean field we define an analogue of the monodromy pairing, constructed by Grothendieck in the case of Abelian varieties, and prove that our monodromy pairing is a birational invariant of the variety. Finally, we propose a conjectural formula for the motivic integral of maximally degenerated K3 surfaces over an arbitrary non-archimedean field and prove this conjecture for Kummer K3 surfaces.
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arxiv:1101.0874
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The superallowed beta-decay rates that provide stringent constraints on physics beyond the Standard Model of particle physics are affected by nuclear structure effects through isospin-breaking corrections. The self-consistent isospin- and angular-momentum-projected nuclear density functional theory is used for the first time to compute those corrections for a number of Fermi transitions in nuclei from A=10 to A=74. The resulting leading element of the CKM matrix, |V_{ud}|= 0.97447(23), agrees well with the recent result by Towner and Hardy [Phys. Rev. C {\bf 77}, 025501 (2008)].
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arxiv:1101.0939
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We study the bandwidth and the pathwidth of multi-dimensional grids. It can be shown for grids, that these two parameters are equal to a more basic graph parameter, the vertex boundary width. Using this fact, we determine the bandwidth and the pathwidth of three-dimensional grids, which were known only for the cubic case. As a by-product, we also determine the two parameters of multi-dimensional grids with relatively large maximum factors.
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arxiv:1101.0964
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Polar molecules in selected quantum states can be guided, decelerated and trapped using electric fields created by microstructured electrodes on a chip. Here we explore how non-adiabatic transitions between levels in which the molecules are trapped and levels in which the molecules are not trapped can be suppressed. We use 12-CO and 13-CO (a 3-Pi(1), v=0) molecules, prepared in the upper Lambda-doublet component of the J=1 rotational level, and study the trap loss as a function of an offset magnetic field. The experimentally observed suppression (enhancement) of the non-adiabatic transitions for 12-CO (13-CO) with increasing magnetic field is quantitatively explained.
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arxiv:1101.1027
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We discuss perturbative O(g^2a) matching with static heavy quarks and domain-wall light quarks for lattice operators relevant to B-meson decays and $B^0$-$\bar{B}^0$ mixing. The chiral symmetry of the light domain-wall quarks does not prohibit operator mixing at O(a) for these operators. The O(a) corrections to physical quantities are non-negligible and must be included to obtain high-precision simulation results for CKM physics. We provide results using plaquette, Symanzik, Iwasaki and DBW2 gluon actions and applying APE, HYP1 and HYP2 link-smearing for the static quark action.
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arxiv:1101.1072
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We report about the search for short-term variability in the high-energy gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2 s^-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (< 2-3 hours) -- the smallest measured to date at MeV energies --, which can constrain the size of the gamma-ray emitting region. The results obtained in the present work favor the hypothesis that gamma rays are generated inside the broad-line region.
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arxiv:1101.1085
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We give the Lagrangian formulation of a generic non-minimally extended Einstein-Maxwell theory with an action that is linear in the curvature and quadratic in the electromagnetic field. We derive the coupled field equations by a first order variational principle using the method of Lagrange multipliers. We look for solutions describing plane-fronted Einstein-Maxwell waves with parallel rays. We give a family of exact solutions associated with a partially massless spin-2 photon and a partially massive spin-2 graviton.
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arxiv:1101.1177
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We give an upper bound on the set of spherical classes in $H_*QX$ when $X = P,S^1$. This is related to the Curtis conjecture on spherical classes in $H_*Q_0S^0$. The results also provide some control over the bordism classes on of immersions when $X$ is a Thom complex.
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arxiv:1101.1215
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Binary systems, in particular eclipsing binaries, are essential sources of our knowledge of the fundamental properties of stars. The ages of binaries, members of open clusters, are constrained by their own fundamental properties and by those of the hosting cluster. The ages of eleven open clusters are here found by constructing models for the components of twelve eclipsing binaries. The difference between the ages we found and the ages of the clusters derived from isochrone fitting is up to 40%. For the binary system V497 Cep in NGC 7160, the difference is about 100%. Binary systems whose primary component is aboutto complete main-sequence life time, such as V453 Cyg and V906 Sco, are the most suitable systems for age determination. Using model results for these stars, we derive an expression for sensitive and uncomplicated relative age determination of binary systems (age divided by the main-sequence life time of the primary star). The expression is given as logarithm of radii ratio divided by a logarithm of mass ratio. Two advantages of this expression are that (i) it is nearly independent of assumed chemical composition of the models because of the appearance of the ratio of radii, and (ii) the ratios of radii and masses are observationally much more precise than their absolute values. We also derive another expression using luminosities rather than radii and compare results.
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arxiv:1101.1217
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The integrable systems known as "AKS systems" admit a natural formulation in terms of a Hamiltonian picture. The Lagrangian side of these systems are far less known; a version in these terms can be found in a work of Feher et al. The purpose of these notes in to provide a novel description of AKS systems in terms of a variational problem different from the usual in mechanics. Additionally, and using techniques borrowed from an article of M. Gotay, it was possible to build the Hamiltonian side of this variational problem, allowing us to establish the equivalence with the usual approach to these integrable systems.
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arxiv:1101.1292
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We report the observation of prominent Shubnikov-de Haas oscillations in a Topological Insulator, Bi$_2$Te$_2$Se, with large bulk resistivity (6 $\Omega$cm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter $k_F\ell$ = 41, with a surface mobility ($\mu_s\sim$ 2,800 cm$^2$/Vs) much larger than the bulk mobility ($\mu_b\sim$ 50 cm$^2$/Vs). The plot of the index fields $B_{\nu}$ vs. filling factor $\nu$ shows a $\frac12$-shift, consistent with massless, Dirac states. Evidence for fractional-filling states is seen in an 11-T field.
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arxiv:1101.1315
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We show for the first time that collectively jammed disordered packings of three-dimensional monodisperse frictionless hard spheres can be produced and tuned using a novel numerical protocol with packing density $\phi$ as low as 0.6. This is well below the value of 0.64 associated with the maximally random jammed state and entirely unrelated to the ill-defined ``random loose packing'' state density. Specifically, collectively jammed packings are generated with a very narrow distribution centered at any density $\phi$ over a wide density range $\phi \in [0.6,~0.74048\ldots]$ with variable disorder. Our results support the view that there is no universal jamming point that is distinguishable based on the packing density and frequency of occurence. Our jammed packings are mapped onto a density-order-metric plane, which provides a broader characterization of packings than density alone. Other packing characteristics, such as the pair correlation function, average contact number and fraction of rattlers are quantified and discussed.
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arxiv:1101.1327
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The superb precision of an atomic clock is derived from its stability. Atomic clocks based on optical (rather than microwave) frequencies are attractive because of their potential for high stability, which scales with operational frequency. Nevertheless, optical clocks have not yet realized this vast potential, due in large part to limitations of the laser used to excite the atomic resonance. To address this problem, we demonstrate a cavity-stabilized laser system with a reduced thermal noise floor, exhibiting a fractional frequency instability of $2 \times 10^{-16}$. We use this laser as a stable optical source in a Yb optical lattice clock to resolve an ultranarrow 1 Hz transition linewidth. With the stable laser source and the signal to noise ratio (S/N) afforded by the Yb optical clock, we dramatically reduce key stability limitations of the clock, and make measurements consistent with a clock instability of $5 \times 10^{-16} / \sqrt{\tau}$.
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arxiv:1101.1351
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We prove the existence of good log minimal models for dlt pairs of numerical log Kodaira dimension 0.
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arxiv:1101.1394
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We investigate the orbital diamagnetism of a weak-doped bilayer graphene (BLG) in spatially smoothly varying magnetic field and obtain the general analytic expression of the orbital susceptibility of BLG, with finite wave number and Fermi energy, at zero temperature. We find that the magnetic field screening factor of BLG is dependent with the wave number, which results in a more complicated screening behavior compared with that of monolayer graphene (MLG). We also study the induced magnetization, electric current in BLG, under nonuniform magnetic field, and find that they are qualitatively different from that in MLG and two-dimensional electron gas (2DEG). However, similar to the MLG, the magnetic object placed above BLG is repelled by a diamagnetic force from BLG, approximately equivalent to a force produced by its mirror image on the other side of BLG with a reduced amplitude dependent with the typical length of the systems. BLG shows crossover behaviors in the responses to the external magnetic field as the intermediate between MLG and 2DEG.
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arxiv:1101.1457
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Sunward-flowing voids above post-coronal mass ejection (CME) flare arcades were first discovered using the soft X-ray telescope (SXT) aboard Yohkoh and have since been observed with TRACE (extreme ultra-violet (EUV)), SOHO/LASCO (white light), SOHO/SUMER (EUV spectra), and Hinode/XRT (soft X-rays (SXR)). Supra-arcade downflow (SAD) observations suggest that they are the cross-sections of thin flux tubes retracting from a reconnection site high in the corona. Supra-arcade downflowing loops (SADLs) have also been observed under similar circumstances and are theorized to be SADs viewed from a perpendicular angle. Previous studies have presented detailed SAD observations for a small number of flares. In this paper we present a substantial SADs and SADLs flare catalog. We have applied semi-automatic detection software to several of these events to detect and track individual downflows thereby providing statistically significant samples of parameters such as velocity, acceleration, area, magnetic flux, shrinkage energy, and reconnection rate. We discuss these measurements, how they were obtained, and potential impact on reconnection models.
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arxiv:1101.1540
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We measured the magnetic field dependence of the critical current of high quality Nb-based planar Josephson tunnel junctions in the presence of a controllable non-uniform field distribution. We found skewed and slowly changing magnetic diffraction patterns quite dissimilar from the Fraunhofer-like ones typical of a homogeneous field. Our findings can be well interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl. Phys. vol.108, 033906 (2010)] for a uniform magnetic field gradient leading to Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances can be suppressed by an asymmetric magnetic field profile.
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arxiv:1101.1610
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The Hirzebruch genus of complex-oriented manifolds associated to the Gamma-function lifts to a ring-homomorphism defined by a family of deformations of the Dirac operator, parametrized by the homogeneous space Sp/U.
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arxiv:1101.1647
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We investigate two quantities of interest in a delay-tolerant mobile ad hoc network: the network capacity region and the minimum energy function. The network capacity region is defined as the set of all input rates that the network can stably support considering all possible scheduling and routing algorithms. Given any input rate vector in this region, the minimum energy function establishes the minimum time average power required to support it. In this work, we consider a cell-partitioned model of a delay-tolerant mobile ad hoc network with general Markovian mobility. This simple model incorporates the essential features of locality of wireless transmissions as well as node mobility and enables us to exactly compute the corresponding network capacity and minimum energy function. Further, we propose simple schemes that offer performance guarantees that are arbitrarily close to these bounds at the cost of an increased delay.
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arxiv:1101.1712
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The helium trimer is studied using two- and three-body soft-core potentials. Realistic helium-helium potentials present an extremely strong short-range repulsion and support a single, very shallow, bound state. The description of systems with more than two helium atoms is difficult due to the very large cancellation between kinetic and potential energy. We analyze the possibility of describing the three helium system in the ultracold regime using a gaussian representation of a widely used realistic potential, the LM2M2 interaction. However, in order to describe correctly the trimer ground state a three-body force has to be added to the gaussian interaction. With this potential model the two bound states of the trimer and the low energy scattering helium-dimer phase shifts obtained with the LM2M2 potential are well reproduced.
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arxiv:1101.1719
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The Galactic microquasar GRS 1915+105 exhibits at least sixteen types of variability classes. Transitions from one class to another could take place in a matter of hours. In some of the classes, the spectral state transitions (burst-off to burst-on and vice versa) were found to take place in a matter of few to few tens of seconds. In the literature, there is no attempt to understand in which order these classes were exhibited. Since the observation was not continuous, the appearances of these classes seem to be in random order. Our goal is to find a natural sequence of these classes and compare with the existing observations. We also wish to present a physical interpretation of the sequence so obtained using two component advective flow model of black hole accretion. In the present paper, we compute the ratios of the power-law photons and the black body photons in the spectrum of each class and call these ratios as the `Comptonizing efficiency'(CE). We sequence the classes from the low to the high value of CE. We clearly find that each variability class could be characterized by a unique average Comptonizing efficiency. The sequence of the classes based on this parameter seem to be corroborated by a handful of the observed transitions caught by Rossi X-ray timing explorer and the Indian payload Indian X-ray Astronomy Experiment and we believe that future observation of the object would show that the transitions can only take place between consecutive classes in this sequence. Since the power-law photons are produced by inverse Comptonization of the intercepted soft-photons from the Keplerian disk, a change in CE actually corresponds to a change in geometry of the compton cloud. Thus we claim that the size of the Compton cloud gradually rises from very soft class to the very hard class.
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arxiv:1101.1739
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We study the entropy-corrected version of the new agegraphic dark energy (NADE) model and dark matter in a spatially non-flat Universe and in the framework of Ho\v{r}ava-Lifshitz cosmology. For the two cases containing noninteracting and interacting entropy-corrected NADE (ECNADE) models, we derive the exact differential equation that determines the evolution of the ECNADE density parameter. Also the deceleration parameter is obtained. Furthermore, using a parametrization of the equation of state parameter of the ECNADE model as $\omega_{\Lambda}(z)=\omega_0+\omega_1 z$, we obtain both $\omega_0$ and $\omega_1$. We find that in the presence of interaction, the equation of state parameter $\omega_0$ of this model can cross the phantom divide line which is compatible with the observation.
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arxiv:1101.1774
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One of the leading issues in high-$\rm T_c$ copper oxide superconductors is the origin of the pseudogap phase in the underdoped regime of their phase diagram. Using polarized neutron diffraction, a novel magnetic order has been identified as an hidden order parameter of the pseudogap as the transition temperature corresponds to what is expected for the pseudogap. The observed magnetic order preserves translational symmetry as predicted for orbital moments in the circulating current theory. Being now reported in three different cuprates familles, it appears as a universal phenomenon whatever the crystal structure (with single CuO$_2$ layer or bilayer per unit cell). To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase of high-$\rm T_c$ cuprates.
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arxiv:1101.1786
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We show that the mapping class group of a handlebody of genus at least 2 (with any number of marked points or spots) is exponentially distorted in the mapping class group of its boundary surface. The same holds true for solid tori with at least two marked points or spots.
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arxiv:1101.1838
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People need a model to study tachyons whose prediction can be tested easily. The dispersion relation w^2=k^2C^2-a^2C^2 of a low-frequency electromagnetic field in good conductors is equivalent to the energy-momentum equation E^2=p^2C^2-m^2C^4 of a tachyon where the proportionality coefficient is h^2. An experiment in 1980s to measure the phase velocity Vp [1] can be regarded as an indirect evidence of the superluminal velocity V>>c of those photons just equals the rate of energy flow S/w of the field.Instability of the tachyonic field corresponds to the Joule heat. To detect the speed of energy is difficult and we plan to modulate signals to observe the information velocity (speed of points of non-analyticity)[2].
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arxiv:1101.1840
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The Ap\'ery polynomials are given by $$A_n(x)=\sum_{k=0}^n\binom nk^2\binom{n+k}k^2x^k\ \ (n=0,1,2,\ldots).$$ (Those $A_n=A_n(1)$ are Ap\'ery numbers.) Let $p$ be an odd prime. We show that $$\sum_{k=0}^{p-1}(-1)^kA_k(x)\equiv\sum_{k=0}^{p-1}\frac{\binom{2k}k^3}{16^k}x^k\pmod{p^2},$$ and that $$\sum_{k=0}^{p-1}A_k(x)\equiv\left(\frac xp\right)\sum_{k=0}^{p-1}\frac{\binom{4k}{k,k,k,k}}{(256x)^k}\pmod{p}$$ for any $p$-adic integer $x\not\equiv 0\pmod p$. This enables us to determine explicitly $\sum_{k=0}^{p-1}(\pm1)^kA_k$ mod $p$, and $\sum_{k=0}^{p-1}(-1)^kA_k$ mod $p^2$ in the case $p\equiv 2\pmod3$. Another consequence states that $$\sum_{k=0}^{p-1}(-1)^kA_k(-2)\equiv\begin{cases}4x^2-2p\pmod{p^2}&\mbox{if}\ p=x^2+4y^2\ (x,y\in\mathbb Z),\\0\pmod{p^2}&\mbox{if}\ p\equiv3\pmod4.\end{cases}$$ We also prove that for any prime $p>3$ we have $$\sum_{k=0}^{p-1}(2k+1)A_k\equiv p+\frac 76p^4B_{p-3}\pmod{p^5}$$ where $B_0,B_1,B_2,\ldots$ are Bernoulli numbers.
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arxiv:1101.1946
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We determine the mean velocity dispersion of six Galactic outer halo globular clusters, AM 1, Eridanus, Pal 3, Pal 4, Pal 15, and Arp 2 in the weak acceleration regime to test classical vs. modified Newtonian dynamics (MOND). Owing to the non-linearity of MOND's Poisson equation, beyond tidal effects, the internal dynamics of clusters is affected by the external field in which they are immersed. For the studied clusters, particle accelerations are much lower than the critical acceleration a_0 of MOND, but the motion of stars is neither dominated by internal accelerations (a_i >> a_e) nor external accelerations (a_e >> a_i). We use the N-body code N-MODY in our analysis, which is a particle-mesh-based code with a numerical MOND potential solver developed by Ciotti, Londrillo, and Nipoti (2006) to derive the line-of-sight velocity dispersion by adding the external field effect. We show that Newtonian dynamics predicts a low-velocity dispersion for each cluster, while in modified Newtonian dynamics the velocity dispersion is much higher. We calculate the minimum number of measured stars necessary to distinguish between Newtonian gravity and MOND with the Kolmogorov-Smirnov test. We also show that for most clusters it is necessary to measure the velocities of between 30 to 80 stars to distinguish between both cases. Therefore the observational measurement of the line-of-sight velocity dispersion of these clusters will provide a test for MOND.
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arxiv:1101.1952
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In close eclipsing binaries, measurements of the variations in binary's eclipse timing may be used to infer information about the existence of circumbinary objects. To determine the possibility of the detection of such variations with CoRoT and Kepler space telescopes, we have carried out an extensive study of the dynamics of a binary star system with a circumbinary planet, and calculated its eclipse timing variations (ETV) for different values of the mass-ratio and orbital elements of the binary and the perturbing body. Here, we present the results of our study and assess the detectability of the planet by comparing the resulting values of ETVs with the temporal sensitivity of CoRoT and Kepler. Results point to extended regions in the parameter-space where the perturbation of a planet may become large enough to create measurable variations in the eclipse timing of the secondary star. Many of these variations point to potentially detectable ETVs and the possible existence of Jovian-type planets.
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arxiv:1101.1994
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We study the deformed conformal-Poincare symmetries consistent with the Snyder--de Sitter space. A relativistic particle model invariant under these deformed symmetries is given. This model is used to provide a gauge independent derivation of the Snyder--de Sitter algebra. Our results are valid in the leading order in the parameters appearing in the model.
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arxiv:1101.2021
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We present the XMM-Newton follow-up for confirmation of Planck cluster candidates. Twenty-five candidates have been observed to date using snapshot (~10 ksec) exposures, ten as part of a pilot programme to sample a low range of signal-to-noise ratios (4<S/N<6), and a further 15 in a programme to observe a sample of S/N>5 candidates. The sensitivity and spatial resolution of XMM-Newton allows unambiguous discrimination between clusters and false candidates. The 4 false candidates have S/N <= 4.1. A total of 21 candidates are confirmed as extended X-ray sources. Seventeen are single clusters, the majority of which are found to have highly irregular and disturbed morphologies (about ~70%). The remaining four sources are multiple systems, including the unexpected discovery of a supercluster at z=0.45. For 20 sources we are able to derive a redshift estimate from the X-ray Fe K line (albeit of variable quality). The new clusters span the redshift range 0.09 <= z <= 0.54, with a median redshift of z~0.37. A first determination is made of their X-ray properties including the characteristic size, which is used to improve the estimate of the SZ Compton parameter, Y_SZ. The follow-up validation programme has helped to optimise the Planck candidate selection process. It has also provided a preview of the X-ray properties of these newly-discovered clusters, allowing comparison with their SZ properties, and to the X-ray and SZ properties of known clusters observed in the Planck survey. Our results suggest that Planck may have started to reveal a non-negligible population of massive dynamically perturbed objects that is under-represented in X-ray surveys. However, despite their particular properties, these new clusters appear to follow the Y_SZ-Y_X relation established for X-ray selected objects, where Y_X is the product of the gas mass and temperature.
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arxiv:1101.2025
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This paper presents the first results of comparison of Planck along with IRAS data with Green Bank Telescope 21-cm observations in 14 fields covering more than 800 square degrees at high Galactic latitude. Galactic dust emission for fields with average HI column density lower than 2 x 10^20 cm^-2 is well correlated with 21-cm emission. The residual emission in these fields, once the HI-correlated emission is removed, is consistent with the expected statistical properties of the cosmic infrared background fluctuations. Fields with larger column densities show significant excess dust emission compared to the HI column density. Regions of excess lie in organized structures that suggest the presence of hydrogen in molecular form, though they are not always correlated with CO emission. Dust emission from intermediate-velocity clouds is detected with high significance. Its spectral properties are consistent with, compared to the local ISM values, significantly hotter dust (T~20 K), lower sub-millimeter dust opacity, and a relative abundance of very small grains to large grains about four times higher. These results are compatible with expectations for clouds that are part of the Galactic fountain in which there is dust shattering and fragmentation. Correlated dust emission in HVCs is not detected; the average of the 99.9% confidence upper limits to the emissivity is 0.15 times the local ISM value at 857 and 3000 GHz, in accordance with gas phase evidence for lower metallicity and depletion in these clouds. Unexpected anti-correlated variations of the dust temperature and emission cross-section per H atom are identified in the local ISM and IVCs, a trend that continues into molecular environments. This suggests that dust growth through aggregation, seen in molecular clouds, is active much earlier in the cloud condensation and star formation processes.
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arxiv:1101.2036
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We extend our previous theoretical study of Mn-based orthorhombic metamagnets to those that possess large nearest neighbour Mn-Mn separations (d1>3.22A). Based on our calculations, we design and synthesize a series of alloys, CoMnGe_{1-x}P_{x}, to experimentally demonstrate the validity of the model. Unusually, we predict and prepare several metamagnets from two ferromagnetic end-members, thus demonstrating a new example of how to vary crystal structure, within the Pnma symmetry group, to provide highly tunable metamagnetism.
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arxiv:1101.2099
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We extend the quantum dimer model (QDM) introduced by Rokhsar and Kivelson so as to construct a concrete example of the model which exhibits the first-order phase transition between different valence-bond solids suggested recently by Batista and Trugman and look for the possibility of other exotic dimer states. We show that our model contains three exotic valence-bond phases (herringbone, checkerboard and dimer smectic) in the ground-state phase diagram and that it realizes the phase transition from the staggered valence-bond solid to the herringbone one. The checkerboard phase has four-fold rotational symmetry, while the dimer smectic, in the absence of quantum fluctuations, has massive degeneracy originating from partial ordering only in one of the two spatial directions. A resonance process involving three dimers resolves this massive degeneracy and dimer smectic gets ordered (order from disorder).
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arxiv:1101.2111
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The AGILE gamma-ray satellite provides large sky exposure levels ($\geq 10^9$ cm$^2$ s per year on the Galactic Plane) with sensitivity peaking at $E\sim$100 MeV where the bulk of pulsar energy output is typically released. Its $\sim$1 $\mu$s absolute time tagging capability makes it perfectly suited for the study of gamma-ray pulsars. AGILE collected a large number of gamma-ray photons from EGRET pulsars ($\geq$40,000 pulsed counts for Vela) in two years of observations unveiling new interesting features at sub-millisecond level in the pulsars' high-energy light-curves, gamma-ray emission from pulsar glitches and Pulsar Wind Nebulae. AGILE detected about 20 nearby and energetic pulsars with good confidence through timing and/or spatial analysis. Among the newcomers we find pulsars with very high rotational energy losses, such as the remarkable PSR B1509--58 with a magnetic field in excess of 10$^{13}$ Gauss, and PSR J2229+6114 providing a reliable identification for the previously unidentified EGRET source 3EG 2227+6122. Moreover, the powerful millisecond pulsar B1821--24, in the globular cluster M28, is detected during a fraction of the observations.
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arxiv:1101.2125
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The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess.They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential. However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers ($N = Z$), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing, in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in $N = Z = 46$ nucleus $^{92}$Pd. Gamma rays emitted following the $^{58}$Ni($^{36}$Ar,2$n$)$^{92}$Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution {\gamma}-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron- proton correlations in these $N = Z$ nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.
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arxiv:1101.2187
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We use deep wide-field photometry from the Large Binocular Camera to study the stellar and structural properties of the recently discovered Andromeda X and Andromeda XVII (And X and And XVII) dwarf galaxies. Using the mean apparent magnitude of the horizontal branch (HB), we derive distances of 621 +- 20 kpc to And X and 734+- 23 kpc to And XVII, closer by >60 kpc than the previous estimates which were based on red giant branch (RGB) observations. Thus our results warrant against the use of the RGB tip method for determining distances to systems with sparsely populated RGBs, and show how crucial HB observations are in obtaining accurate distances in systems such as these. We find that And X is a relatively faint (MV = -7.36), highly elongated (e = 0.48) system at a distance of 174 +- 62 kpc from Andromeda. And XVII is brighter (MV = -8.61) with an M31-centric distance of 73 kpc which makes it one of the closest satellites to Andromeda. Both galaxies are metal-poor: we derive <[Fe/H]>=-2.2 for And X, while And XVII shows <[Fe/H]> = -2.0, consistent with the relation of higher luminosity dwarfs being more metal- rich. Additionally, both galaxies show considerable intrinsic spreads in metallicity (0.2 and 0.3 dex for And X and And XVII respectively), consistent with multiple stellar populations.
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arxiv:1101.2201
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This paper is a study of non-linear effects of RF Amplifiers on Communication Systems Performance. High speed data communication is made possible by Multilevel Modulation schemes. This paper presents a study of these non linear effects on multilevel Modulation schemes like MPSK and MQAM. We make use of Bit Error Ratio (BER) as performance measure. BER vs SNR (Signal-to-Noise Ratio) curves provide comparison between the non linear effects caused by Gain Compression in particular.
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arxiv:1101.2242
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We prove that the canonical Thurston obstruction for a sub-hyperbolic semi-rational branched covering exists if the branched covering is not CLH-equivalent to a rational map.
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arxiv:1101.2285
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In this paper we study the (2,k)-generation of the finite classical groups SL(4,q), Sp(4,q), SU(4,q^2) and their projective images. Here k is the order of an arbitrary element of SL(2,q), subject to the necessary condition k>= 3. When q is even we allow also k=4.
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arxiv:1101.2358
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Motivated by a phenomenon of phase transition in a model of alignment of self-propelled particles, we obtain a kinetic mean-field equation which is nothing else than the Doi equation (also called Smoluchowski equation) with dipolar potential. In a self-contained article, using only basic tools, we analyze the dynamics of this equation in any dimension. We first prove global well-posedness of this equation, starting with an initial condition in any Sobolev space. We then compute all possible steady-states. There is a threshold for the noise parameter: over this threshold, the only equilibrium is the uniform distribution, and under this threshold, there is also a family of non-isotropic equilibria. We give a rigorous prove of convergence of the solution to a steady-state as time goes to infinity. In particular we show that in the supercritical case, the only initial conditions leading to the uniform distribution in large time are those with vanishing momentum. For any positive value of the noise parameter, and any initial condition, we give rates of convergence towards equilibrium, exponentially for both supercritical and subcritical cases and algebraically for the critical case.
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arxiv:1101.2380
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It has been known since 1981 that if one fixes an orientable surface $S$ of genus $g$, then there is a real number $\lambda_{min,g} > 1$ that is the dilatation of a pA diffeomorphism of $S$, and every other pA diffeomorphism of $S$ has dilatation $\geq \lambda_{min,g}$. We will show how a little-known theorem about digraphs gives some insight into $\lambda_{min,g}$.
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arxiv:1101.2383
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We develop Jacobson's refinement of Engel's Theorem for Leibniz algebras. We then note some consequences of the result.
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arxiv:1101.2438
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We consider various configurations of T-branes which are non-abelian bound states of branes and were recently introduced by Cecotti, Cordova, Heckman and Vafa. They are a refinement of the concept of monodromic branes featured in phenomenological F-theory models. We are particularly interested in the T-branes corresponding to Z3 and Z4 monodromies, which are used to break E7 or E8 gauge groups to SU(5) GUT. Our results imply that the up-type and down-type Yukawa couplings for the breaking of E7 are zero, whereas up-type and down-type Yukawa couplings, together with right handed neutrino Yukawas are non-zero for the case of the breaking of E8. The dimension four proton decay mediating term is avoided in models with either E7 or E8 breaking.
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arxiv:1101.2455
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We examine thermal transport in graphene supported on SiO2 using molecular dynamics simulations. Coupling to the substrate reduces the thermal conductivity (TC) of supported graphene by an order of magnitude, due to damping of the flexural acoustic (ZA) phonons. However, increasing the strength of the graphene-substrate interaction enhances the TC of supported graphene, contrary to expectations. The enhancement is due to the coupling of graphene ZA modes to the substrate Rayleigh waves, which linearizes the dispersion and increases the group velocity of the hybridized modes. These findings suggest that the TC of two-dimensional supported graphene is tunable through surface interactions, providing a novel possibility for controlled energy flow in nanomaterials.
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arxiv:1101.2463
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Properties of four infinite families of special functions of two real variables, based on the compact simple Lie group G2, are compared and described. Two of the four families (called here C- and S-functions) are well known, whereas the other two (S^L- and S^S-functions) are not found elsewhere in the literature. It is shown explicitly that all four families have similar properties. In particular, they are orthogonal when integrated over a finite region F of the Euclidean space, and they are discretely orthogonal when their values, sampled at the lattice points F_M \subset F, are added up with a weight function appropriate for each family. Products of ten types among the four families of functions, namely CC, CS, SS, SS^L, CS^S, SS^L, SS^S, S^SS^S, S^LS^S and S^LS^L, are completely decomposable into the finite sum of the functions. Uncommon arithmetic properties of the functions are pointed out and questions about numerous other properties are brought forward.
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arxiv:1101.2502
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All-optical active plasmonic devices are of fundamental importance for designing efficient nanophotonic circuits. We theoretically propose and numerically investigate an active plasmonic device made up of a nonlinear epsilon-near-zero metamaterial slab of thickness smaller than 100 nanometers lying on a linear epsilon-near-zero metamaterial substrate. We predict that, in free-space coupling configuration, the device, operating at low-intensity, would display plasmon mediated hysteresis behavior since the phase difference between the reflected and the incident optical waves turns out to be multi-valued and dependent on the history of the excitation process. Such an hysteresis behavior would allow to regard the proposed device as a compact memory unit whose state is accessible by measuring either the mentioned phase difference or the power, which is multi-valued as well, carried by the nonlinear plasmon wave. Since multiple plasmon powers comprise both positive and negative values, the device would also operate as a switch of the plasmon power direction at each jump along an hysteresis loop.
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arxiv:1101.2541
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The linear and renormalized nonlinear kinetic theory of drift instability of plasma shear flow across the magnetic field, which has the Kelvin's method of shearing modes or so-called non-modal approach as its foundation, is developed. The developed theory proves that the time-dependent effect of the finite ion Larmor radius is the key effect, which is responsible for the suppression of drift turbulence in an inhomogeneous electric field. This effect leads to the non-modal decrease of the frequency and growth rate of the unstable drift perturbations with time. We find that turbulent scattering of the ion gyrophase is the dominant effect, which determines extremely rapid suppression of drift turbulence in shear flow.
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arxiv:1101.2567
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Monochromatic photons could be produced in the annihilation or decay of dark matter particles. At high energies, the search for such line features in the cosmic gamma-ray spectrum is essentially background free because plausible astrophysical processes are not expected to produce such a signal. The observation of a gamma-ray line would hence be a `smoking-gun' signature for dark matter, making the search for such signals particularly attractive. Among the different dark matter models predicting gamma-ray lines, the local supersymmetric extension of the standard model with small R-parity violation and gravitino LSP is of particular interest because it provides a framework where primordial nucleosynthesis, gravitino dark matter and thermal leptogenesis are naturally consistent. Using the two-years Fermi LAT data, we present a dedicated search for gamma-ray lines coming from dark matter annihilation or decay in the Galactic halo. Taking into account the full detector response, and using a binned profile likelihood method, we search for significant line features in the energy spectrum of the diffuse flux observed in different regions of the sky. No evidence for a line signal at the 5-sigma level is found for photon energies between 1 and 300 GeV, and conservative bounds on dark matter decay rates and annihilation cross sections are presented. Implications for gravitino dark matter in presence of small R-parity violation are discussed, as well as the impact of our results on the prospect for seeing long-lived neutralinos or staus at the LHC.
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arxiv:1101.2610
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HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt and is the massive component of a 69-day spectroscopic binary. The binary parameters and properties of the Be star measured using high-dispersion spectra obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young (<50-80 Myr), then the companion is a K V star, but, following another, older evolutionary scenario, the companion is a horizontal-branch star or possibly a white dwarf star. In the latter scenario, a past episode of mass transfer from an evolved star onto a less massive dwarf star would be responsible for the peculiar nature of the present-day, fast-rotating Be star.
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arxiv:1101.2622
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Polarised radio synchrotron emission from interstellar, intracluster and intergalactic magnetic fields is affected by frequency-dependent Faraday depolarisation. The maximum polarised intensity depends on the physical properties of the depolarising medium. New-generation radio telescopes like LOFAR, SKA and its precursors need a wide range of frequencies to cover the full range of objects. The optimum frequency of maximum polarised intensity (PI) is computed for the cases of depolarisation in magneto-ionic media by regular magnetic fields (differential Faraday rotation) or by turbulent magnetic fields (internal or external Faraday dispersion), assuming that the Faraday spectrum of the medium is dominated by one component or that the medium is turbulent. Polarised emission from bright galaxy disks, spiral arms and cores of galaxy clusters are best observed at wavelengths below a few centimeters (at frequencies beyond about 10 GHz), halos of galaxies and clusters around decimeter wavelengths (at frequencies below about 2 GHz). Intergalactic filaments need observations at meter wavelengths (frequencies below 300 MHz). Sources with extremely large intrinsic $|RM|$ or RM dispersion can be searched with mm-wave telescopes. Measurement of the PI spectrum allows us to derive the average Faraday rotation measure $|RM|$ or the Faraday dispersion within the source, as demonstrated for the case of the spiral galaxy NGC 6946. Periodic fluctuations in PI at low frequencies are a signature of differential Faraday rotation. Internal and external Faraday dispersion can be distinguished by the different slopes of the PI spectrum at low frequencies. A wide band around the optimum frequency is important to distinguish between varieties of depolarisation effects.
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arxiv:1101.2631
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We classify (up to affine equivalence) all 7-dimensional flat manifolds with a cyclic holonomy group.
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arxiv:1101.2633
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We propose a threshold detector with an operation principle, based on a parametric period-doubling bifurcation in an externally pumped nonlinear resonance circuit. The ac-driven resonance circuit includes a dc-current-biased Josephson junction ensuring parametric frequency conversion (period-doubling bifurcation) due to its quadratic nonlinearity. A sharp onset of oscillations at the half-frequency of the drive allows for detection of small variations of an effective inductance and, therefore, the read-out of the quantum state of a coupled Josephson qubit. The bifurcation characteristics of this circuit are compared with those of the conventional Josephson bifurcation amplifier, and its possible advantages are discussed.
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arxiv:1101.2712
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A Hausdorff topological group $(G,\tau)$ is called an $s$-group and $\tau$ is called an $s$-topology if there is a set $S$ of sequences in $G$ such that $\tau$ is the finest Hausdorff group topology on $G$ in which every sequence of $S$ converges to the unit. The class $\mathbf{S}$ of all $s$-groups contains all sequential Hausdorff groups and it is finitely multiplicative. A quotient group of an $s$-group is an $s$-group. For a non-discrete topological group $(G,\tau)$ the following three assertions are equivalent: 1) $(G,\tau)$ is an $s$-group, 2) $(G,\tau)$ is a quotient group of a Graev free topological group over a metrizable space, 3) $(G,\tau)$ is a quotient group of a Graev free topological group over a sequential Tychonoff space. The Abelian version of this characterization of $s$-groups holds as well.
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arxiv:1101.2754
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The anomalous low energy behaviour observed in metals with strong electron correlation, such as in the heavy fermion materials, is believed to arise from the scattering of the itinerant electrons with low energy spin fluctuations. In systems with magnetic impurities this scattering leads to the Kondo effect and a low energy renormalized energy scale, the Kondo temperature $T_{\rm K}$. It has been generally assumed that these low energy scales can only be accessed by a non-perturbative approach due to the strength of the local inter-electron interactions. Here we show that it is possible to circumvent this difficulty by first suppressing the spin fluctuations with a large magnetic field. As a first step field-dependent renormalized parameters are calculated using standard perturbation theory. A renormalized perturbation theory is then used to calculate the renormalized parameters for a reduced magnetic field strength. The process can be repeated and the flow of the renormalized parameters continued to zero magnetic field. We illustrate the viability of this approach for the single impurity Anderson model. The results for the renormalized parameters, which flow as a function of magnetic field, can be checked with those from numerical renormalization group and Bethe ansatz calculations.
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arxiv:1101.2840
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We study the focusing of light through random photonic materials using wavefront shaping. We explore a novel approach namely binary amplitude modulation. To this end, the light incident to a random photonic medium is spatially divided into a number of segments. We identify the segments that give rise to fields that are out of phase with the total field at the intended focus and assign these a zero amplitude, whereas the remaining segments maintain their original amplitude. Using 812 independently controlled segments of light, we find the intensity at the target to be 75 +/- 6 times enhanced over the average intensity behind the sample. We experimentally demonstrate focusing of light through random photonic media using both an amplitude only mode liquid crystal spatial light modulator and a MEMS-based spatial light modulator. Our use of Micro Electro-Mechanical System (MEMS)-based digital micromirror devices for the control of the incident light field opens an avenue to high speed implementations of wavefront shaping.
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arxiv:1101.2860
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We study the effect of rho^0-gamma mixing in e^+e^- to pi^+pi^- and its relevance for the comparison of the square modulus of the pion from-factor |F^(e)_pi|^2, as measured in e^+e^- annihilation experiments, and |F^(tau)_pi|^2 the corresponding quantity obtained after accounting for known isospin breaking effects by an isospin rotation from the tau-decay spectra. After correcting the tau data for the missing rho-gamma mixing contribution, besides the other known isospin symmetry violating corrections, the pi pi I=1 part of the hadronic vacuum polarization contribution to the muon g-2 are fully compatible between tau based and e^+e^- based evaluations. tau data thus confirm result obtained with e^+e^- data. Our evaluation based on all e^+e^- data including more recent BaBar and KLOE data yields a_mu^(had)=690.75(4.72) x 10^{-10} (e^+e^- based), while including tau data we find a_mu^(had)=690.96(4.65) X 10^{-10} (e^+e^-+tau based). This backs the ~3 sigma deviation between theory and experiment. For the tau di-pion branching fraction we find B^{CVC}_{pi pi^0}=25.20 \pm0.0.17\pm0.28 from e^+e^-+CVC, while B_{pi pi^0}=25.34\pm0.0.06\pm0.08 is evaluated directly from the tau spectra.
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arxiv:1101.2872
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Recent LHC results concerning full jet-quenching in PbPb collisions have been presented in terms of a jet asymmetry parameter, measuring the imbalance between the transverse momenta of leading and subleading jets. We examine the potential sensitivity of this distribution to fluctuations from the heavy-ion background. Our results suggest that a detailed estimate of the true fluctuations would be of benefit in extracting quantitative information about jet quenching. We also find that the apparent impact of fluctuations on the jet asymmetry distribution can depend significantly on the choice of low-pt threshold used for the simulation of the hard pp events.
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arxiv:1101.2878
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We present the discovery of a wide (67 AU) substellar companion to the nearby (21 pc) young solar-metallicity M1 dwarf CD-35 2722, a member of the ~100 Myr AB Doradus association. Two epochs of astrometry from the NICI Planet-Finding Campaign confirm that CD-35 2722 B is physically associated with the primary star. Near-IR spectra indicate a spectral type of L4\pm1 with a moderately low surface gravity, making it one of the coolest young companions found to date. The absorption lines and near-IR continuum shape of CD-35 2722 B agree especially well the dusty field L4.5 dwarf 2MASS J22244381-0158521, while the near-IR colors and absolute magnitudes match those of the 5 Myr old L4 planetary-mass companion, 1RXS J160929.1-210524 b. Overall, CD-35 2722 B appears to be an intermediate-age benchmark for L-dwarfs, with a less peaked H-band continuum than the youngest objects and near-IR absorption lines comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR spectra and find T=1700-1900 K and log(g) =4.5\pm0.5. The spectra also show that the radial velocities of components A and B agree to within \pm10 km/s, further confirming their physical association. Using the age and bolometric luminosity of CD-35 2722 B, we derive a mass of 31\pm8 Mjup from the Lyon/Dusty evolutionary models. Altogether, young late-M to mid-L type companions appear to be over-luminous for their near-IR spectral type compared to field objects, in contrast to the under-luminosity of young late-L and early-T dwarfs.
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arxiv:1101.2893
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We produce examples in the cohomology of algebraic groups which answer two questions of Parshall and Scott. Specifically, if $G=SL_2$, then we show: (a) $\dim \Ext_G^2(L,L)$ can be arbitrarily large for a simple module $L$; and (b) the sequence $\max_{L-\text{irred}}\dim H^k(G,L)$ grows exponentially fast with $k$.
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arxiv:1101.3004
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This is an introduction to finite simple groups, in particular sporadic groups, intended for physicists. After a short review of group theory, we enumerate the $1+1+16=18$ families of finite simple groups, as an introduction to the sporadic groups. These are described next, in three levels of increasing complexity, plus the six isolated "pariah" groups. The (old) five Mathieu groups make up the first, smallest order level. The seven groups related to the Leech lattice, including the three Conway groups, constitute the second level. The third and highest level contains the Monster group $\mathbb M$, plus seven other related groups. Next a brief mention is made of the remaining six pariah groups, thus completing the $5+7+8+6=26$ sporadic groups. The review ends up with a brief discussion of a few of physical applications of finite groups in physics, including a couple of recent examples which use sporadic groups.
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arxiv:1101.3055
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I review recent lattice calculations performed with Nf=2 and Nf=2+1 dynamical fermions which provide a precise computation of the BK bag parameter. I also report on Nf=2 dynamical quark simulations aiming at the computation of the full basis of the Delta_S=2 four-fermion operator matrix elements that are relevant to models beyond the Standard Model.
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arxiv:1101.3069
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In this paper we analyze American style of floating strike Asian call options belonging to the class of financial derivatives whose payoff diagram depends not only on the underlying asset price but also on the path average of underlying asset prices over some predetermined time interval. The mathematical model for the option price leads to a free boundary problem for a parabolic partial differential equation. Applying fixed domain transformation and transformation of variables we develop an efficient numerical algorithm based on a solution to a non-local parabolic partial differential equation for the transformed variable representing the synthesized portfolio. For various types of averaging methods we investigate the dependence of the early exercise boundary on model parameters.
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arxiv:1101.3071
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The paper is devoted to the investigation of finite dimensional commutative nilpotent (associative) algebras N over an arbitrary base field of characteristic zero. Due to the lack of a general structure theory for algebras of this type (as opposed to the semi-simple case) we associate various objects to every N which encode the algebra structure. Our main results are in the subclass of algebras having 1-dimensional annihilator, that is, are maximal ideals of Gorenstein algebras of finite vector dimension > 1. Associated structural objects are then, for instance, a class of mutually affinely equivalent algebraic hypersurfaces S in N, and a class of so-called nil-polynomials p, whose degree is the nil-index of N. Then N can be reconstructed from S and even from the quadratic plus cubic part of p. If the algebra N is graded the hypersurface S is affinely homogeneous. The paper closes with an example of an N of dimension 23 and nil-index 5, for which S is not affinely homogeneous.
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arxiv:1101.3088
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In this paper, we propose a novel S/D engineering for dual-gated Bilayer Graphene (BLG) Field Effect Transistor (FET) using doped semiconductors (with a bandgap) as source and drain to obtain unipolar complementary transistors. To simulate the device, a self-consistent Non-Equilibrium Green's Function (NEGF) solver has been developed and validated against published experimental data. Using the simulator, we predict an on-off ratio in excess of $10^4$ and a subthreshold slope of ~110mV/decade with excellent scalability and current saturation, for a 20nm gate length unipolar BLG FET. However, the performance of the proposed device is found to be strongly dependent on the S/D series resistance effect. The obtained results show significant improvements over existing reports, marking an important step towards bilayer graphene logic devices.
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arxiv:1101.3123
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We analyze the desynchronization bifurcation in the coupled R\"ossler oscillators. After the bifurcation the coupled oscillators move away from each other with a square root dependence on the parameter. We define system transverse Lyapunov exponents and in the desynchronized state one is positive while the other is negative implying that one oscillator is trying to fly away while the other is holding it. We give a simple model of coupled integrable systems that shows a similar phenomena and can be treated as the normal form for the desynchronization bifurcation. We conclude that the desynchronization is a pitchfork bifurcation of the transverse manifold.
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arxiv:1101.3130
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Parton luminosities are convenient for estimating how the physics potential of Large Hadron Collider experiments depends on the energy of the proton beams. I quantify the advantage of increasing the beam energy from 3.5 TeV to 4 TeV. I present parton luminosities, ratios of parton luminosities, and contours of fixed parton luminosity for $gg$, $u\bar{d}$, $qq$, and $gq$ interactions over the energy range relevant to the Large Hadron Collider, along with example analyses for specific processes. This note extends the analysis presented in arXiv:0908.3660. Full-size figures are available as pdf files at http://lutece.fnal.gov/PartonLum11/
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arxiv:1101.3201
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A closed quantum system is defined as completely controllable if an arbitrary unitary transformation can be executed using the available controls. In practice, control fields are a source of unavoidable noise. Can one design control fields such that the effect of noise is negligible on the time-scale of the transformation? Complete controllability in practice requires that the effect of noise can be suppressed for an arbitrary transformation. The present study considers a paradigm of control, where the Lie-algebraic structure of the control Hamiltonian is fixed, while the size of the system increases, determined by the dimension of the Hilbert space representation of the algebra. We show that for large quantum systems, generic noise in the controls dominates for a typical class of target transformations i.e., complete controllability is destroyed by the noise.
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arxiv:1101.3248
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A review of charm semileptonic decays results obtained at B factories is presented. It focuses on form factors measurements in $D^0\to K^- \ell^+ \nu$, $D^0\to \pi^- \ell^+ \nu$, $D_s^+ \to K^+K^- e^+\nu_e$, and $D^+ \to K^-\pi^+ e^+\nu_e$. For the last two decay channels, the contribution and characteristics of other components in the final state, in addition to the main vector contribution, is also studied.
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arxiv:1101.3258
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Charged asymptotically AdS black branes in five dimensions are sometimes unstable to the condensation of charged scalar fields. For fields of infinite charge and squared mass -4 Herzog was able to analytically determine the phase transition temperature and compute the endpoint of this instability in the neighborhood of the phase transition. We generalize Herzog's construction by perturbing away from infinite charge in an expansion in inverse charge and use the solutions so obtained as input for the fluid gravity map. Our tube wise construction of patched up locally hairy black brane solutions yields a one to one map from the space of solutions of superfluid dynamics to the long wavelength solutions of the Einstein Maxwell system. We obtain explicit expressions for the metric, gauge field and scalar field dual to an arbitrary superfluid flow at first order in the derivative expansion. Our construction allows us to read off the the leading dissipative corrections to the perfect superfluid stress tensor, current and Josephson equations. A general framework for dissipative superfluid dynamics was worked out by Landau and Lifshitz for zero superfluid velocity and generalized to nonzero fluid velocity by Clark and Putterman. Our gravitational results do not fit into the 13 parameter Clark-Putterman framework. Purely within fluid dynamics we present a consistent new generalization of Clark and Putterman's equations to a set of superfluid equations parameterized by 14 dissipative parameters. The results of our gravitational calculation fit perfectly into this enlarged framework. In particular we compute all the dissipative constants for the gravitational superfluid.
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arxiv:1101.3332
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We examine observational signatures of dark matter annihilation in the Milky Way arising from electroweak bremsstrahlung contributions to the annihilation cross section. It has been known for some time that photon bremsstrahlung may significantly boost DM annihilation yields. Recently, we have shown that electroweak bremsstrahlung of W and Z gauge bosons can be the dominant annihilation channel in some popular models with helicity-suppressed 2 --> 2 annihilation. W/Z-bremsstrahlung is particularly interesting because the gauge bosons produced via annihilation subsequently decay to produce large correlated fluxes of electrons, positrons, neutrinos, hadrons (including antiprotons) and gamma rays, which are all of importance in indirect dark matter searches. Here we calculate the spectra of stable annihilation products produced via gamma/W/Z-bremsstrahlung. After modifying the fluxes to account for the propagation through the Galaxy, we set upper bounds on the annihilation cross section via a comparison with observational data. We show that stringent cosmic ray antiproton limits preclude a sizable dark matter contribution to observed cosmic ray positron fluxes in the class of models for which the bremsstrahlung processes dominate.
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arxiv:1101.3357
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We give a summary of recent results on the explicit local form of the second-order symmetric Lorentzian manifolds in arbitrary dimension, and its global version. These spacetimes turn out to be essentially a specific subclass of plane waves.
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arxiv:1101.3438
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Experimental data on inclusive cross sections of the hadrons produced in high energy proton-(anti)proton collisions are analyzed in the z-scaling approach. Saturation of the scaling function PSI(z) for z<0.1 (low transverse momenta) was found. The first results on charged hadron spectra in pp collisions obtained by the CMS Collaboration at the LHC have confirmed the saturation down to the value of z=0.05. The CMS data on neutral K-meson production at s^{1/2}=7 TeV extend the saturation region even to a lower value of z=0.002 in the new energy domain. A microscopic scenario of hadron production at a constituent level based on the z-scaling is discussed in the saturation regime.
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arxiv:1101.3439
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The terahertz spectra of the dynamic conductivity and radiation absorption coefficient in germanium-silicon heterostructures with arrays of Ge hut clusters (quantum dots) have been measured for the first time in the frequency range of 0.3-1.2 THz at room temperature. It has been found that the effective dynamic conductivity and effective radiation absorption coefficient in the heterostructure due to the presence of germanium quantum dots in it are much larger than the respective quantities of both the bulk Ge single crystal and Ge/Si(001) without arrays of quantum dots. The possible microscopic mechanisms of the detected increase in the absorption in arrays of quantum dots have been discussed.
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arxiv:1101.3527
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Telescopes like Herschel and the Atacama Large Millimeter/submillimeter Array (ALMA) are creating new opportunities to study sources in the far infrared (FIR), a wavelength region dominated by cold dust emission. Probing cold dust in active galaxies allows for study of the star formation history of active galactic nuclei (AGN) hosts. The FIR is also an important spectral region for observing AGN which are heavily enshrouded by dust, such as Compton thick AGN. By using information from deep X-ray surveys and cosmic X-ray background synthesis models, we compute Cloudy photoionization simulations which are used to predict the spectral energy distribution (SED) of AGN in the FIR. Expected differential number counts of AGN and their host galaxies are calculated in the Herschel bands. The expected contribution of AGN and their hosts to the cosmic infrared background (CIRB) and the infrared luminosity density are also computed. Multiple star formation scenarios are investigated using a modified blackbody star formation SED. It is found that FIR observations at ~500 um are an excellent tool in determining the star formation history of AGN hosts. Additionally, the AGN contribution to the CIRB can be used to determine whether star formation in AGN hosts evolves differently than in normal galaxies. The contribution of Compton thick AGN to the bright end differential number counts and to the bright source infrared luminosity density is a good test of AGN evolution models where quasars are triggered by major mergers.
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arxiv:1101.3536
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We describe a new Godunov algorithm for relativistic magnetohydrodynamics (RMHD) that combines a simple, unsplit second order accurate integrator with the constrained transport (CT) method for enforcing the solenoidal constraint on the magnetic field. A variety of approximate Riemann solvers are implemented to compute the fluxes of the conserved variables. The methods are tested with a comprehensive suite of multidimensional problems. These tests have helped us develop a hierarchy of correction steps that are applied when the integration algorithm predicts unphysical states due to errors in the fluxes, or errors in the inversion between conserved and primitive variables. Although used exceedingly rarely, these corrections dramatically improve the stability of the algorithm. We present preliminary results from the application of these algorithms to two problems in RMHD: the propagation of supersonic magnetized jets, and the amplification of magnetic field by turbulence driven by the relativistic Kelvin-Helmholtz instability (KHI). Both of these applications reveal important differences between the results computed with Riemann solvers that adopt different approximations for the fluxes. For example, we show that use of Riemann solvers which include both contact and rotational discontinuities can increase the strength of the magnetic field within the cocoon by a factor of ten in simulations of RMHD jets, and can increase the spectral resolution of three-dimensional RMHD turbulence driven by the KHI by a factor of 2. This increase in accuracy far outweighs the associated increase in computational cost. Our RMHD scheme is publicly available as part of the Athena code.
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arxiv:1101.3573
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The recently observed high-spin rotational bands in odd-$A$ nuclei $^{247, 249}$Cm and $^{249}$Cf [Tandel \textit{et al.}, Phys. Rev. C 82 (2010) 041301R] are investigated by using the cranked shell model (CSM) with the pairing correlations treated by a particle-number conserving (PNC) method in which the blocking effects are taken into account exactly. The experimental moments of inertia and alignments and their variations with the rotational frequency $\omega$ are reproduced very well by the PNC-CSM calculations. By examining the $\omega$-dependence of the occupation probability of each cranked Nilsson orbital near the Fermi surface and the contributions of valence orbitals to the angular momentum alignment in each major shell, the level crossing and upbending mechanism in each nucleus is understood clearly.
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arxiv:1101.3607
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