1,501 research outputs found

### Non Perturbative Renormalization Group, momentum dependence of $n$-point functions and the transition temperature of the weakly interacting Bose gas

We propose a new approximation scheme to solve the Non Perturbative
Renormalization Group equations and obtain the full momentum dependence of
$n$-point functions. This scheme involves an iteration procedure built on an
extension of the Local Potential Approximation commonly used within the Non
Perturbative Renormalization Group. Perturbative and scaling regimes are
accurately reproduced. The method is applied to the calculation of the shift
$\Delta T_c$ in the transition temperature of the weakly repulsive Bose gas, a
quantity which is very sensitive to all momenta intermediate between these two
regions. The leading order result is in agreement with lattice calculations,
albeit with a theoretical uncertainty of about 25%. The next-to-leading order
differs by about 10% from the best accepted result

### Stars and statistical physics: a teaching experience

The physics of stars, their workings and their evolution, is a goldmine of
problems in statistical mechanics and thermodynamics. We discuss many examples
that illustrate the possibility of deepening student's knowledge of statistical
mechanics by an introductory study of stars. The matter constituting the
various stellar objects provides examples of equations of state for classical
or quantal and relativistic or non-relativistic gases. Maximum entropy can be
used to characterize thermodynamic and gravitational equilibrium which
determines the structure of stars and predicts their instability above a
certain mass. Contraction accompanying radiation induces either heating or
cooling, which explains the formation of stars above a minimum mass. The
characteristics of the emitted light are understood from black-body radiation
and more precisely from the Boltzmann-Lorentz kinetic equation for photons. The
luminosity is governed by the transport of heat by photons from the center to
the surface. Heat production by thermonuclear fusion is determined by
microscopic balance equations. The stability of the steady state of stars is
controlled by the interplay of thermodynamics and gravitation.Comment: latex gould_last.tex, 4 files, submitted to Am. J. Phy

### DCC: Attractive Idea Seeks Serious Confirmation

The theoretical ideas relevant for the physics of the disoriented chiral
condensate (DCC) are reviewed.Comment: 18 pages LaTex, based on invited lecture given by A.Krzywicki at the
workshop "Quark, plasma and beyond", Zif, Bielefeld, May 1996 ; a reference
is correcte

### Confinement, Turbulence and Diffraction Catastrophes

Many features of large N_c transition that occurs in the spectral density of
Wilson loops as a function of loop area (observed recently in numerical
simulations of Yang-Mills theory by Narayanan and Neuberger) can be captured by
a simple Burgers equation used to model turbulence. Spectral shock waves that
precede this asymptotic limit exhibit universal scaling with N_c, with indices
that can be related to Berry indices for diffraction catastrophes.Comment: Presented at PANIC 200

### Quark number susceptibilities from HTL-resummed thermodynamics

We compute analytically the diagonal quark number susceptibilities for a
quark-gluon plasma at finite temperature and zero chemical potential, and
compare with recent lattice results. The calculation uses the approximately
self-consistent resummation of hard thermal and dense loops that we have
developed previously. For temperatures between 1.5 to 5 $T_c$, our results
follow the same trend as the lattice data, but exceed them in magnitude by
about 5-10%. We also compute the lowest order contribution, of order
$\alpha_s^3\log(1/\alpha_s)$, to the off-diagonal susceptibility. This
contribution, which is not a part of our self-consistent calculation, is
numerically small, but not small enough to be compatible with a recent lattice
simulation.Comment: 13 pages, 5 figures, uses elsart.cls; v2: minor corrections; v3: sign
in eq.(1) correcte

### Comparing different hard-thermal-loop approaches to quark number susceptibilities

We compare our previously proposed hard-thermal-loop (HTL) resummed
calculation of quark number susceptibilities using a self-consistent two-loop
approximation to the quark density with a recent calculation of the same
quantity at the one-loop level in a variant of HTL-screened perturbation
theory. Besides pointing out conceptual problems with the latter approach, we
show that it severely over-includes the leading-order interaction effects while
including none of the plasmon term which after all is the reason to construct
improved resummation schemes.Comment: 6 pages, 6 figures. Revised version to appear in Eur. J. Phys.

### Proton-nucleus collisions in the color glass condensate framework

We discuss proton-nucleus collisions in the framework of the color glass
condensate. By assuming that the proton can be described as a low density color
source, we solve exactly the Yang-Mills equations corresponding to this type of
collision, and then use this solution in order to calculate inclusive gluon
production or quark-antiquark production. Our result shows that
k_T-factorization, while valid for gluon production, is violated for quark pair
production in proton-nucleus collisions.Comment: Talk given at SEWM2004, Helsinki, June 200

### Non-Perturbative Renormalization Group calculation of the scalar self-energy

We present the first numerical application of a method that we have recently
proposed to solve the Non Perturbative Renormalization Group equations and
obtain the n-point functions for arbitrary external momenta. This method leads
to flow equations for the n-point functions which are also differential
equations with respect to a constant background field. This makes them, a
priori, difficult to solve. However, we demonstrate in this paper that, within
a simple approximation which turns out to be quite accurate, the solution of
these flow equations is not more complicated than that of the flow equations
obtained in the derivative expansion. Thus, with a numerical effort comparable
to that involved in the derivative expansion, we can get the full momentum
dependence of the n-point functions. The method is applied, in its leading
order, to the calculation of the self-energy in a 3-dimensional scalar field
theory, at criticality. Accurate results are obtained over the entire range of
momenta.Comment: 29 page

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