38 research outputs found
The soft fermion dispersion relation at next-to-leading order in hot QED
We study next-to-leading order contributions to the soft static fermion
dispersion relation in hot QED. We derive an expression for the complete
next-to-leading order contribution to the retarded fermion self-energy. The
real and imaginary parts of this expression give the next-to-leading order
contributions to the mass and damping rate of the fermionic quasi-particle.
Many of the terms that are expected to contribute according to the traditional
power counting argument are actually subleading. We explain why the power
counting method over estimates the contribution from these terms. For the
electron damping rate in QED we obtain: . We check our method by calculating the next-to-leading order
contribution to the damping rate for the case of QCD with two flavours and
three coulours. Our result agrees with the result obtained previously in the
literature. The numerical evaluation of the nlo contribution to the mass is
left to a future publication.Comment: 15 pages, 5 figure
The Kohn-Luttinger Effect in Gauge Theories
Kohn and Luttinger showed that a many body system of fermions interacting via
short range forces becomes superfluid even if the interaction is repulsive in
all partial waves. In gauge theories such as QCD the interaction between
fermions is long range and the assumptions of Kohn and Luttinger are not
satisfied. We show that in a U(1) gauge theory the Kohn-Luttinger phenomenon
does not take place. In QCD attractive channels always exist, but there are
cases in which the primary pairing channel leaves some fermions ungapped. As an
example we consider the unpaired fermion in the 2SC phase of QCD with two
flavors. We show that it acquires a very small gap via a mechanism analogous to
the Kohn-Luttinger effect. The gap is too small to be phenomenologically
relevant.Comment: 5 pages, 2 figure, minor revisions, to appear in PR
Dissipative Hydrodynamics and Heavy Ion Collisions
Recent discussions of RHIC data emphasized the exciting possibility that the
matter produced in nucleus-nucleus collisions shows properties of a
near-perfect fluid. Here, we aim at delineating the applicability of fluid
dynamics, which is needed to quantify the size of corresponding dissipative
effects. We start from the equations for dissipative fluid dynamics, which we
derive from kinetic theory up to second order (Israel-Stewart theory) in a
systematic gradient expansion. In model studies, we then establish that for too
early initialization of the hydrodynamic evolution (\tau_0 \lsim 1 fm/c) or
for too high transverse momentum (p_T \gsim 1 GeV) in the final state, the
expected dissipative corrections are too large for a fluid description to be
reliable. Moreover, viscosity-induced modifications of hadronic transverse
momentum spectra can be accommodated to a significant degree in an ideal fluid
description by modifications of the decoupling stage. We argue that these
conclusions, drawn from model studies, can also be expected to arise in
significantly more complex, realistic fluid dynamics simulations of heavy ion
collisions.Comment: 18 pages, 5 figures, uses revtex4; v2: references added, typos
correcte
Mesonic Excitations of QGP: Study with an Effective Model
We study the correlations between quark-antiquark pairs in different quantum
number channels in a deconfined plasma by using an effective model of QCD.
Using the three flavour PNJL model, the finite temperature spectral functions
for different mesonic states are studied at zero and nonzero quark chemical
potentials. It is found that in the channel resonance structures survive
above the chiral transition temperature \tc, while the kaonic states seem to
get washed off just above \tc. The sensitivity of the structures to the
anomaly term are carefully investigated.Comment: 15page
The Phase Structure of the Polyakov--Quark-Meson Model
The relation between the deconfinement and chiral phase transition is
explored in the framework of an Polyakov-loop-extended two-flavor quark-meson
(PQM) model. In this model the Polyakov loop dynamics is represented by a
background temporal gauge field which also couples to the quarks. As a novelty
an explicit quark chemical potential and N_f-dependence in the Polyakov loop
potential is proposed by using renormalization group arguments. The behavior of
the Polyakov loop as well as the chiral condensate as function of temperature
and quark chemical potential is obtained by minimizing the grand canonical
thermodynamic potential of the system. The effect of the Polyakov loop dynamics
on the chiral phase diagram and on several thermodynamic bulk quantities is
presented.Comment: 13 pages, 12 figures, RevTex4; discussion of mu-dependence extended,
references added, version to be published in PR
Effective field theory and dispersion law of the phonons of a non-relativistic superfluid
We study the recently proposed effective field theory for the phonon of an
arbitrary non-relativistic superfluid. After computing the one-loop phonon
self-energy, we obtain the low temperature T contributions to the phonon
dispersion law at low momentum, and see that the real part of those can be
parametrized as a thermal correction to the phonon velocity. Because the
phonons are the quanta of the sound waves, at low momentum their velocity
should agree with the speed of sound. We find that our results match at order
T^4ln(T) with those predicted by Andreev and Khalatnikov for the speed of
sound, derived from the superfluid hydrodynamical equations and the phonon
kinetic theory. We get also higher order corrections of order T^4, which are
not reproduced pushing naively the kinetic theory computation. Finally, as an
application, we consider the cold Fermi gas in the unitarity limit, and find a
universal expression for the low T relative correction to the speed of sound
for these systems.Comment: 14 pages, 2 figures. References adde
Probing Color Response - Wakes in a Color Plasma
The wake induced in a hot QCD medium by a high momentum parton (jet
precursor) is calculated in the framework of linear response theory. Two
different scenarios are discussed: a weakly coupled quark gluon plasma (pQGP)
as described by hard-thermal loop (HTL) perturbation theory and a strongly
cupled QGP (sQGP) with the properties of a quantum liquid. In the latter case
the wake could exhibit a pronounced Mach cone structure. This physical
mechanism could be important for the understanding of preliminary data from the
PHENIX and STAR experiments at RHIC on the angular distribution of low-pt
secondaries stemming from the away-side jet which indicate maxima at
.Comment: Prepared for: Workshop on Correlations and Fluctuations in
Relativistic Nuclear Collisions, MIT, Cambridge, Massachusetts, USA, 21-23
April 200
Debye mass and heavy quark potential in a PNJL quark plasma
We calculate the Debye mass for the screening of the heavy quark potential in
a plasma of massless quarks coupled to the temporal gluon background governed
by the Polyakov loop potential within the PNJL model in RPA approximation. We
give a physical motivation for a recent phenomenological fit of lattice data by
applying the calculated Debye mass with its suppression in the confined phase
due to the Polyakov-loop to a description of the temperature dependence of the
singlet free energy for QCD with a heavy quark pair at infinite separation. We
compare the result to lattice data.Comment: 6 pages, 1 figure, contribution to Proceedings of the 6th
International Conference on "Critical Point and Onset of Deconfinement", to
appear in Phys. At. Nucl., vol. 7
Superconductivity from perturbative one-gluon exchange in high density quark matter
We study color superconductivity in QCD at asymptotically large chemical
potential. In this limit, pairing is dominated by perturbative one-gluon
exchange. We derive the Eliashberg equation for the pairing gap and solve this
equation numerically. Taking into account both magnetic and electric gluon
exchanges, we find with ,
verifying a recent result by Son. For chemical potentials that are of physical
interest, GeV, the calculation ceases to be reliable quantitatively,
but our results suggest that the gap can be as large as 100 MeV.Comment: 19 pages, 6 figures. I accidentally replaced the paper with an
outdated version. This version has typos corrected and will appear in PR
KMS conditions for 4-point Green functions at finite temperature
We study the 4-point function in the Keldysh formalism of the closed time
path formulation of real time finite temperature field theory.
We derive the KMS conditions for these functions and discuss the number of
4-point functions that are independent. We define a set of `physical' functions
which are linear combinations of the usual Keldysh functions. We show that
these functions satisfy simple KMS conditions. In addition, we consider a set
of integral equations which represent a resummation of ladder graphs. We show
that these integral equations decouple when one uses the physical functions
that we have defined. We discuss the generalization of these results to QED.Comment: 17 pages in Revtex with 2 figure