576 research outputs found
Renormalization in Self-Consistent Approximation schemes at Finite Temperature III: Global Symmetries
We investigate the symmetry properties for Baym's -derivable schemes.
We show that in general the solutions of the dynamical equations of motion,
derived from approximations of the -functional, do not fulfill the
Ward-Takahashi identities of the symmetry of the underlying classical action,
although the conservation laws for the expectation values of the corresponding
Noether currents are fulfilled exactly for the approximation. Further we prove
that one can define an effective action functional in terms of the
self-consistent propagators which is invariant under the operation of the same
symmetry group representation as the classical action. The requirements for
this theorem to hold true are the same as for perturbative approximations: The
symmetry has to be realized linearly on the fields and it must be free of
anomalies, i.e., there should exist a symmetry conserving regularization
scheme. In addition, if the theory is renormalizable in Dyson's narrow sense,
it can be renormalized with counter terms which do not violate the symmetry.Comment: 32 papges, 3 figures, uses ReVTeX 4, V2: Added one more reference,
V3: Corrected some typos, added two more sections about the large-N expansio
Chiral symmetry restoration in linear sigma models with different numbers of quark flavors
Chiral symmetry restoration at nonzero temperature is studied in the
framework of the O(4) linear sigma model and the U(N_f)_r x U(N_f)_l linear
sigma model with N_f=2,3, and 4 quark flavors. We investigate the temperature
dependence of the masses of the scalar and pseudoscalar mesons, and the
non-strange, strange, and charm condensates within the Hartree approximation as
derived from the Cornwall-Jackiw-Tomboulis formalism. We find that the masses
of the non-strange and strange mesons at nonzero temperature depend sensitively
on the particular symmetry of the model and the number of light quark flavors
N_f. On the other hand, due to the large charm quark mass, neither do charmed
mesons significantly affect the properties of the other mesons, nor do their
masses change appreciably in the temperature range around the chiral symmetry
restoration temperature. In the chiral limit, the transition temperatures for
chiral symmetry restoration are surprisingly close to those found in lattice
QCD.Comment: 28 pages, 8 figure
Heavy-Quark Diffusion, Flow and Recombination at RHIC
We discuss recent developments in assessing heavy-quark interaction in the
Quark-Gluon Plasma (QGP). While induced gluon radiation is expected to be the
main energy-loss mechanism for fast-moving quarks, we focus on elastic
scattering which prevails toward lower energies, evaluating both perturbative
(gluon-exchange) and nonperturbative (resonance formation) interactions in the
QGP. The latter are treated within an effective model for D- and B-meson
resonances above T_c as motivated by current QCD lattice calculations.
Pertinent diffusion and drag constants, following from a Fokker-Planck
equation, are implemented into an expanding fireball model for Au-Au collisions
at RHIC using relativistic Langevin simulations. Heavy quarks are hadronized in
a combined fragmentation and coalescence framework, and resulting
electron-decay spectra are compared to recent RHIC data. A reasonable
description of both nuclear suppression factors and elliptic flow up to momenta
of ~5 GeV supports the notion of a strongly interacting QGP created at RHIC.
Consequences and further tests of the proposed resonance interactions are
discussed.Comment: 8 pages, 14 figures, contribution to the proceedings for the
"International Conference on Strangeness in Quark Matter 2006
Choice Functions and Hard Choices
A hard choice is a situation in which an agent is unable to make a justifiable choice from a given menu of alternatives. Our objective is to present a systematic treatment of the axiomatic structure of such situations. To do so, we draw on and contribute to the study of choice functions that can be indecisive, i.e., that may fail to select a non-empty set for some menus. In this more general framework, we present new characterizations of two well-known choice rules, the maximally dominant choice rule and the top-cycle choice rule. Together with existing results, this yields an understanding of the circumstances in which hard choices arise
Heavy Flavor Production at STAR
e present measurements on meson production via direct reconstruction of
its hadronic decay channel in minimum bias +Au and Au+Au
collisions at =200 GeV with up to 3 GeV/.
Non-photonic electron spectra from the charm semi-leptonic decays are analyzed
from the same data set as well as in + collision at =200 GeV
using the STAR Time-of-Flight (TOF) and Barrel EMC (BEMC) detectors,
respectively. Results of the charm-decayed single muon (prompt muon) spectra
are also presented at low in Au+Au collisions measured by the TOF
detector. The charm production total cross-section per nucleon-nucleon
collision is measured to be 1.260.09(stat.)0.23(sys.) mb in minimum
bias Au+Au collisions, which is consistent with the scaling compared
to 1.40.4 mb in minimum bias +Au collisions, and supports the
idea that charm quarks should be produced mostly via parton fusion at early
stage in relativistic heavy-ion collisions. A Blast-Wave model fit to the low
( GeV/c) non-photonic electrons, prompt muons and spectra shows
that charm hadrons may kinetically freeze-out earlier than light hadrons with a
smaller collective velocity. The nuclear modification factors () of the
non-photonic electrons in central Au+Au collisions are significantly below
unity at 2 GeV/, which indicates a significant amount of energy
loss for heavy quarks in Au+Au collisions. The charm transverse momentum
distribution must have been modified by the hot and dense matter created in
central Au+Au collisions at RHIC.Comment: 7 pages, 4 figures, proceedings for the Strange Quark Matter 2006
conferenc
Large-space shell-model calculations for light nuclei
An effective two-body interaction is constructed from a new Reid-like
potential for a large no-core space consisting of six major shells and is used
to generate the shell-model properties for light nuclei from =2 to 6. (For
practical reasons, the model space is partially truncated for =6.) Binding
energies and other physical observables are calculated and compare favorably
with experiment.Comment: prepared using LaTex, 21 manuscript pages, no figure
Hard gluon damping in hot QCD
The gluon collisional width in hot QCD plasmas is discussed with emphasis on
temperatures near , where the coupling is large. Considering its effect on
the entropy, which is known from lattice calculations, it is argued that the
width, which in the perturbative limit is given by , should be sizeable at intermediate temperatures but has to be small close
to . Implications of these results for several phenomenologically relevant
quantities, such as the energy loss of hard jets, are pointed out.Comment: uses RevTex and graphic
The O(N) linear sigma model at finite temperature beyond the Hartree approximation
We study the O(N) linear sigma model with spontaneous symmetry breaking,
using a Hartree-like ansatz with a classical field and variational masses. We
go beyond the Hartree approximation by including the two-loop contribution, the
sunset diagram, using the 2PPI expansion. We have computed numerically the
effective potential at finite temperature. We find a phase transition of second
order, while it is first order in the Hartree approximation. We also discuss
some implications of the fact that in this order, the decay of the sigma into
two pions affects the thermal diagrams.Comment: 22 pages, 14 figures. v2: minor corrections, some more references.
v3: added new set of data, new appendix. Submitted to Phys.Rev.
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