Short-time fluctuations of displacements and work

A recent theorem giving the initial behavior of very short-time fluctuations of particle displacements in classical many-body systems is discussed. It has applications to equilibrium and non-equilibrium systems, one of which is a series expansion of the distribution of work fluctuations around a Gaussian function. To determine the time-scale at which this series expansion is valid, we present preliminary numerical results for a Lennard-Jones fluid. These results suggest that the series expansion converges up to time scales on the order of a picosecond, below which a simple Gaussian function for the distribution of the displacements can be used.Comment: 10 pages, 3 figure

Radiative Tau Lepton Pair Production as a Probe of Anomalous Electromagnetic Couplings of the Tau

We calculate the squared matrix element for the process e+ e- --> tau+ tau- gamma allowing for anomalous magnetic and electric dipole moments at the tau tau gamma vertex. No interferences are neglected and no approximations of light fermion masses are made. We show that anomalous moments affect not only the cross section, but also the shape of the photon energy and angular distributions. We also demonstrate that in the case of the anomalous magnetic dipole moment, the contribution from interference involving Standard Model and anomalous amplitudes is significant compared to the contribution from anomalous amplitudes alone. A program to perform the calculation is available and it may be employed as a Monte Carlo generator.Comment: 14 pages, 8 figures submitted to Nuclear Physics

Coherent two pion photoproduction on 12C

We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for $\pi^-\pi^+$ and $\pi^0\pi^0$ photoproduction on $^{12}C$. We find that due to the isospin symmetry the cross section for $\pi^0\pi^0$ production is very small and has a maximum when the pions propagate together. However, the kinematical region where the energies and polar angles of the two $\pi^0$ mesons are equal and their relative azimuthal angle $\phi=180^0$ is forbidden. Conversely in the $\pi^-\pi^+$ production the pions prefer to have a relative azimuthal angle 180$^0$ and the production of the pions propagating together is suppressed. The dominant one-body mechanism in both channels is related to the excitation of the $\Delta$ isobar. Hence the reaction can serve as a source of information about $\Delta$'s properties in nucleus. We have found that the reaction is sensitive to effects of the pion and $\Delta$ renormalization in the nuclear medium, similar to those found in the coherent $(\gamma,\pi^0)$ reaction, but magnified because of the presence of the two pions.Comment: 17 pages LATEX and 11 postscript figure

Young star clusters in M31

In our study of M31's globular cluster system with MMT/Hectospec, we have obtained high-quality spectra of 85 clusters with ages less than 1 Gyr. With the exception of Hubble V, the young cluster in NGC 205, we find that these young clusters have kinematics and spatial distribution consistent with membership in M31's young disk. Preliminary estimates of the cluster masses and structural parameters, using spectroscopically derived ages and HST imaging, confirms earlier suggestions that M31 has clusters similar to the LMC's young populous clusters.Comment: 4 pages, 1 figure, contributed talk at "Galaxies in the Local Volume" conference in Sydney, July 200

Improved results for N=(2,2) super Yang-Mills theory using supersymmetric discrete light-cone quantization

We consider the (1+1)-dimensional ${\cal N}=(2,2)$ super Yang--Mills theory which is obtained by dimensionally reducing ${\cal N}=1$ super Yang--Mills theory in four dimension to two dimensions. We do our calculations in the large-$N_c$ approximation using Supersymmetric Discrete Light Cone Quantization. The objective is to calculate quantities that might be investigated by researchers using other numerical methods. We present a precision study of the low-mass spectrum and the stress-energy correlator $$. We find that the mass gap of this theory closes as the numerical resolution goes to infinity and that the correlator in the intermediate $r$ region behaves like $r^{-4.75}$.Comment: 18 pages, 8 figure

Experimental indication on chiral symmetry restoration in meson spectrum

The spectroscopic predictions of the Ademollo-Veneziano-Weinberg dual model are critically tested in view of the modern experimental data. The predicted equidistance of masses squared for chiral partners is shown to be violated high in energies, instead one observes an approximate degeneracy of these quantities. This phenomenon can be interpreted as the restoration of Wigner-Weyl realization of chiral symmetry for highly excited states. The scale of complete restoration is expected to be 2.5 GeV. A multispin-parity cluster structure of meson spectrum is revealed.Comment: To be published in Phys. Lett. B, 10 pages, 1 figure, some comments and references are adde

Classification of protein interaction sentences via gaussian processes

The increase in the availability of protein interaction studies in textual format coupled with the demand for easier access to the key results has lead to a need for text mining solutions. In the text processing pipeline, classification is a key step for extraction of small sections of relevant text. Consequently, for the task of locating protein-protein interaction sentences, we examine the use of a classifier which has rarely been applied to text, the Gaussian processes (GPs). GPs are a non-parametric probabilistic analogue to the more popular support vector machines (SVMs). We find that GPs outperform the SVM and na\"ive Bayes classifiers on binary sentence data, whilst showing equivalent performance on abstract and multiclass sentence corpora. In addition, the lack of the margin parameter, which requires costly tuning, along with the principled multiclass extensions enabled by the probabilistic framework make GPs an appealing alternative worth of further adoption

Self-organization in a phonon laser

We make an adaptation of laser modelling equations to describe the behavior of a phonon laser (saser). Our saser consists of an AlGaAs/GaAs double barrier heterostructure designed to generate an intense beam of transversal acoustic (TA) phonons. To study our system, we begin with a Hamiltonian that describes the decay of primary longitudinal optical phonons (LO_1) into secondary (LO_2) and TA (LO_1 -> LO_2 + TA) and its inverse process (recombination). Using this Hamiltonian, a set of coupled equations of motion for the phonons is obtained. We also consider the interaction between the phonons and its reservoirs. These interactions are introduced in the equations of motion leading to a set of coupled Langevin equations. In order to obtain an expression to describe our saser we apply, in the Langevin equations, an adiabatic elimination of some variables of the subsystem. Following the method above we obtain the value of the injection threshold for the operation of our phonon laser. At this threshold occurs a phase transition from a disordered to a coherent state. It is shown that it is not necessary a big "optical" pumping to get a sasing region.Comment: 4 figure

Black hole collision with a scalar particle in four, five and seven dimensional anti-de Sitter spacetimes: ringing and radiation

In this work we compute the spectra, waveforms and total scalar energy radiated during the radial infall of a small test particle coupled to a scalar field into a $d$-dimensional Schwarzschild-anti-de Sitter black hole. We focus on $d=4, 5$ and 7, extending the analysis we have done for $d=3$. For small black holes, the spectra peaks strongly at a frequency $\omega \sim d-1$, which is the lowest pure anti-de Sitter (AdS) mode. The waveform vanishes exponentially as $t \to \infty$, and this exponential decay is governed entirely by the lowest quasinormal frequency. This collision process is interesting from the point of view of the dynamics itself in relation to the possibility of manufacturing black holes at LHC within the brane world scenario, and from the point of view of the AdS/CFT conjecture, since the scalar field can represent the string theory dilaton, and 4, 5, 7 are dimensions of interest for the AdS/CFT correspondence.Comment: 16 pages, 13 figures. Published versio

Iterative algorithms for total variation-like reconstructions in seismic tomography

A qualitative comparison of total variation like penalties (total variation, Huber variant of total variation, total generalized variation, ...) is made in the context of global seismic tomography. Both penalized and constrained formulations of seismic recovery problems are treated. A number of simple iterative recovery algorithms applicable to these problems are described. The convergence speed of these algorithms is compared numerically in this setting. For the constrained formulation a new algorithm is proposed and its convergence is proven.Comment: 28 pages, 8 figures. Corrected sign errors in formula (25