2,969 research outputs found

### Non-Abelian Plane-Waves in the Quark-Gluon Plasma

We present new, non-abelian, solutions to the equations of motion which
describe the collective excitations of a quark-gluon plasma at high
temperature. These solutions correspond to longitudinal and transverse
plane-waves propagating through the plasma.Comment: 13 pages, LaTex, preprint Saclay-T94/01

### A generating functional for ultrasoft amplitudes in hot QCD

The effective amplitudes for gluon momentum p<<gT in hot QCD exhibit damping
as a result of collisions. The whole set of n-point amplitudes is shown to be
generated from one functional K(x,y;A), in addition to the induced current
j(x;A).Comment: 7 pages, no figure (some comments added

### The pressure of QED from the two-loop 2PI effective action

We compute the pressure of hot quantum electrodynamics from the two-loop
truncation of the 2PI effective action. Since the 2PI resummation guarantees
gauge-fixing independence only up to the order of the truncation, our result
for the pressure presents a gauge dependent contribution of O(e^4). We
numerically characterize the credibility of this gauge-dependent calculation
and find that the uncertainty due to gauge parameter dependence is under
control for xi<1. Our calculation also suggests that the choice of Landau gauge
may minimize gauge-dependent effects.Comment: 15 latex pages with 3 figure

### Correlation functions and dissipation in hot QCD

A recently proposed generating functional allows the construction of the full
set of n-point Green functions in QCD at high temperature and at distances
larger than 1/gT. One may then learn how the system maintains its thermal
equilibrium in the quantum field theory approach, i.e. which process
compensates for the important dissipation due to collisions. This system may be
characterized by quantities which have a classical limit. One finds that the
fluctuations of the coloured field are not gaussian ones. A comparison is made
with the semi-classical approach where a random noise is the source of
fluctuations.Comment: 21 pages, latex 2e, no figure Comments added in Introduction, in Sec

### Ultrasoft Amplitudes in Hot QCD

By using the Boltzmann equation describing the relaxation of colour
excitations in the QCD plasma, we obtain effective amplitudes for the ultrasoft
colour fields carrying momenta of order $g^2 T$. These amplitudes are of the
same order in $g$ as the hard thermal loops (HTL), which they generalize by
including the effects of the collisions among the hard particles. The ultrasoft
amplitudes share many of the remarkable properties of the HTL's: they are gauge
invariant, obey simple Ward identities, and, in the static limit, reduce to the
usual Debye mass for the electric fields. However, unlike the HTL's, which
correspond effectively to one-loop diagrams, the ultrasoft amplitudes resum an
infinite number of diagrams of the bare perturbation theory. By solving the
linearized Boltzmann equation, we obtain a formula for the colour conductivity
which accounts for the contributions of the hard and soft modes beyond the
leading logarithmic approximation.Comment: 38 pages, 10 figures, LaTeX. An extension of the previous results is
included (in Sec. 4.2); also, some minor modifications here and there. Final
version, to appear in Nucl. Phys.

### Perturbation theory and non-perturbative renormalization flow in scalar field theory at finite temperature

We use the non-perturbative renormalization group to clarify some features of
perturbation theory in thermal field theory. For the specific case of the
scalar field theory with O(N) symmetry, we solve the flow equations within the
local potential approximation. This approximation reproduces the perturbative
results for the screening mass and the pressure up to order g^3, and starts to
differ at order g^4. The method allows a smooth extrapolation to the regime
where the coupling is not small, very similar to that obtained from a simple
self-consistent approximation.Comment: 42 pages, 19 figures; v2: typos corrected and references added,
version accepted for publication in Nucl. Phys.

### Renormalization of \Phi-derivable approximations in scalar field theories

We discuss the renormalization of \Phi-derivable approximations for scalar
field theories. In such approximations, the self-energy is obtained as the
solution of a self-consistent equation which effectively resums infinite
subsets of diagrams of perturbation theory. We show that a consistent
renormalization can be carried out, and we provide an explicit construction of
the counterterms needed to eliminate the subdivergences. The counterterms are
calculated from the solution of an auxiliary gap equation which determines the
leading asymptotic part of the self-energy. This auxiliary gap equation may be
chosen as the gap equation of the massless theory at zero temperature. We
verify explicitly that the counterterms determined at zero temperature are
sufficient to eliminate the divergences which occur in finite temperature
calculations.Comment: 69 pages, 26 figure

### Meson correlation functions in a QCD plasma

The temporal pseudoscalar meson correlation function in a QCD plasma is
investigated in a range of temperatures exceeding $T_c$ and yet of experimental
interest. Only the flavour-singlet channel is considered and the imaginary time
formalism is employed for the finite temperature calculations. The behaviour of
the meson spectral function and of the temporal correlator is first studied in
the HTL approximation, where one replaces the free thermal quark propagators
with the HTL resummed ones. This procedure satisfactory describes the soft
fermionic modes, but its application to the propagation of hard quarks is not
reliable. An improved version of the so-called NLA scheme, which allows a
better treatment of the hard fermionic modes, is then proposed. The impact of
the improved NLA on the pseudoscalar temporal correlator is investigated.Comment: 35 pages, 15 figure

### Heavy ion collisions: puzzles and hopes

This talk is a brief summary of some theoretical issues in the field of hot
and dense QCD matter and ultra-relativistic heavy ion collisions.Comment: 6 pages, invited talk presented at the conference "Physics at the LHC
2010", DESY, June 7-12, 2010 Minor corrections in reference

- â€¦