Unitarity of the tree approximation to the Glauber AA amplitude for large A

The nucleus-nucleus Glauber amplitude in the tree approximation is studied for heavy participant nuclei. It is shown that, contrary to previous published results, it is not unitary for realistic values of nucleon-nucleon cross-sections.Comment: 15 pages, 1 figure, 1 table. Submitted to Yad. Fi

Fluctuations of the number of participants and binary collisions in AA interactions at fixed centrality in the Glauber approach

In the framework of the classical Glauber approach, the analytical expressions for the variance of the number of wounded nucleons and binary collisions in AA interactions at a given centrality are presented. Along with the optical approximation term, they contain additional contact terms arising only in the case of nucleus-nucleus collisions. The magnitude of the additional contributions, e.g., for PbPb collisions at SPS energies, is larger than the contribution of the optical approximation at some values of the impact parameter. The sum of the additional contributions is in good agreement with the results of independent Monte Carlo simulations of this process. Due to these additional terms, the variance of the total number of participants for peripheral PbPb collisions and the variance of the number of collisions at all values of the impact parameter exceed several multiples of the Poisson variances. The correlator between the numbers of participants in colliding nuclei at fixed centrality is also analytically calculated.Comment: updated version; as published by Phys. Rev.

Effects of the Running of the QCD Coupling on the Energy Loss in the Quark-Gluon Plasma

Finite temperature modifies the running of the QCD coupling alpha_s(k,T) with resolution k. After calculating the thermal quark and gluon masses selfconsistently, we determine the quark-quark and quark-gluon cross sections in the plasma based on the running coupling. We find that the running coupling enhances these cross sections by factors of two to four depending on the temperature. We also compute the energy loss dE/dx of a high-energy quark in the plasma as a function of temperature. Our study suggests that, beside t-channel processes, inverse Compton scattering is a relevant process for a quantitative understanding of the energy loss of an incident quark in a hot plasma.Comment: 14 pages, 6 figure

Solitonic-exchange mechanism of surface~diffusion

We study surface diffusion in the framework of a generalized Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The model describes a lattice of atoms with a given concentration interacting by Morse-type forces, the lattice being subjected to a two-dimensional substrate potential which is periodic in one direction and nonconvex (Morse) in the transverse direction. The results are used to describe the complicated exchange-mediated diffusion mechanism recently observed in MD simulations [J.E. Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.

Enhanced response of the regular networks to local signals in presence of a fast impurity

We consider an array of inductively coupled Josephson junctions with a fast impurity (a junction with a smaller value of critical current), and study the consequences of imposing a small amplitude periodic signal at some point in the array. We find that when external signal is imposed at the impurity, the response of the array is boosted and a small amplitude signal can be detected throughout the array. When the signal is imposed elsewhere, minor effects is seen on the dynamics of the array. The same results have been also seen in presence of a single fast spiking neuron in a chain of diffusively coupled FitzHugh-Nagumo neurons.Comment: 6 pages, 5 figures, arXiv admin note: substantial text overlap with arXiv:1108.460

Mechanics of torque generation in the bacterial flagellar motor

The bacterial flagellar motor (BFM) is responsible for driving bacterial locomotion and chemotaxis, fundamental processes in pathogenesis and biofilm formation. In the BFM, torque is generated at the interface between transmembrane proteins (stators) and a rotor. It is well-established that the passage of ions down a transmembrane gradient through the stator complex provides the energy needed for torque generation. However, the physics involved in this energy conversion remain poorly understood. Here we propose a mechanically specific model for torque generation in the BFM. In particular, we identify two fundamental forces involved in torque generation: electrostatic and steric. We propose that electrostatic forces serve to position the stator, while steric forces comprise the actual 'power stroke'. Specifically, we predict that ion-induced conformational changes about a proline 'hinge' residue in an $\alpha$-helix of the stator are directly responsible for generating the power stroke. Our model predictions fit well with recent experiments on a single-stator motor. Furthermore, we propose several experiments to elucidate the torque-speed relationship in motors where the number of stators may not be constant. The proposed model provides a mechanical explanation for several fundamental features of the flagellar motor, including: torque-speed and speed-ion motive force relationships, backstepping, variation in step sizes, and the puzzle of swarming experiments

Percolation of strings and the first RHIC data on multiplicity and tranverse momentum distributions

The dependence of the multiplicity on the number of collisions and the transverse momentum distribution for central and peripheral Au-Au collisions are studied in the model of percolation of strings relative to the experimental conditions at RHIC. The comparison with the first RHIC data shows a good agreement.Comment: RevTeX, 11 pages, 4 eps figures included using epsfi

On Free Quotients of Complete Intersection Calabi-Yau Manifolds

In order to find novel examples of non-simply connected Calabi-Yau threefolds, free quotients of complete intersections in products of projective spaces are classified by means of a computer search. More precisely, all automorphisms of the product of projective spaces that descend to a free action on the Calabi-Yau manifold are identified.Comment: 39 pages, 3 tables, LaTe