401 research outputs found
Thermal fluctuations in the interacting pion gas
We derive the two-particle fluctuation correlator in a thermal gas of
pi-mesons to the lowest order in an interaction due to a resonance exchange. A
diagrammatic technique is used. We discuss how this result can be applied to
event-by-event fluctuations in heavy-ion collisions, in particular, to search
for the critical point of QCD. As a practical example, we determine the shape
of the rapidity correlator.Comment: 12 pages, 4 figures, RevTe
Pion dispersion relation at finite density and temperature
We study the behavior of the pion dispersion relation in a pion medium at
finite density and temperature. We introduce a pion chemical potential to
describe the finite pion number density and argue that such description is
valid during the hadronic phase of a relativistic heavy-ion collision between
chemical and thermal freeze-out. We make use of an effective Lagrangian that
explicitly respects chiral symmetry through the enforcement of the chiral Ward
identities. The pion dispersion relation is computed through the computation of
the pion self-energy in a non-perturbative fashion by giving an approximate
solution to the Schwinger-Dyson equation for this self-energy. The dispersion
relation is described in terms of a density and temperature dependent mass and
an index of refraction which is also temperature, density as well as momentum
dependent. The index of refraction is larger than unity for all values of the
momentum for finite and . We conclude by exploring some of the
possible consequences for the propagation of pions through the boundary between
the medium and vacuum.Comment: 7 pages, 5 figures, 3 new references, published versio
Lifetimes of electrons in the Shockley surface state band of Ag(111)
We present a theoretical many-body analysis of the electron-electron (e-e)
inelastic damping rate of electron-like excitations in the Shockley
surface state band of Ag(111). It takes into account ab-initio band structures
for both bulk and surface states. is found to increase more rapidly as
a function of surface state energy E than previously reported, thus leading to
an improved agreement with experimental data
Equation of State, Flow, Fluctuations and suppression
Radial flow observed at AGS/SPS energies is very strong, with collective
velocities of matter reaching about 0.5c for central collisions of the heaviest
ions. The lattice-based Equation of State (EOS) is however rather soft, due to
the QCD phase transition. We show that both statements are consistent only if
proper kinetic-based treatment of the freeze-out is made. In fact chemical and
thermal freeze-out happen at quite different conditions, especially at SPS.
Event-by-event fluctuations can shed new light on this problem. We also propose
new model of "anomalous" suppression found for PbPb collisions,
related it to prolonged lifetime of dense matter due the "softest point" of the
EOS.Comment: Plenary Talk at Quark Matter 97, Tsukuba, Dec.199
Chiral phase boundary of QCD at finite temperature
We analyze the approach to chiral symmetry breaking in QCD at finite
temperature, using the functional renormalization group. We compute the running
gauge coupling in QCD for all temperatures and scales within a simple truncated
renormalization flow. At finite temperature, the coupling is governed by a
fixed point of the 3-dimensional theory for scales smaller than the
corresponding temperature. Chiral symmetry breaking is approached if the
running coupling drives the quark sector to criticality. We quantitatively
determine the phase boundary in the plane of temperature and number of flavors
and find good agreement with lattice results. As a generic and testable
prediction, we observe that our underlying IR fixed-point scenario leaves its
imprint in the shape of the phase boundary near the critical flavor number:
here, the scaling of the critical temperature is determined by the
zero-temperature IR critical exponent of the running coupling.Comment: 39 pages, 8 figure
Twist-3 Distribute Amplitude of the Pion in QCD Sum Rules
We apply the background field method to calculate the moments of the pion
two-particles twist-3 distribution amplitude (DA) in QCD sum
rules. In this paper,we do not use the equation of motion for the quarks inside
the pion since they are not on shell and introduce a new parameter to
be determined. We get the parameter in this approach. If
assuming the expansion of in the series in Gegenbauer polynomials
, one can obtain its approximate expression which can be
determined by its first few moments.Comment: 12 pages, 3 figure
The canonical partition function for relativistic hadron gases
Particle production in high-energy collisions is often addressed within the
framework of the thermal (statistical) model. We present a method to calculate
the canonical partition function for the hadron resonance gas with exact
conservation of the baryon number, strangeness, electric charge, charmness and
bottomness. We derive an analytical expression for the partition function which
is represented as series of Bessel functions. Our results can be used directly
to analyze particle production yields in elementary and in heavy ion
collisions. We also quantify the importance of quantum statistics in the
calculations of the light particle multiplicities in the canonical thermal
model of the hadron resonance gas.Comment: 10 pages, 2 figures; submitted for publication in EPJ
Following Gluonic World Lines to Find the QCD Coupling in the Infrared
Using a parametrization of the Wilson loop with the minimal-area law, we
calculate the polarization operator of a valence gluon, which propagates in the
confining background. This enables us to obtain the infrared freezing (i.e.
finiteness) of the running strong coupling in the confinement phase, as well as
in the deconfinement phase up to the temperature of dimensional reduction. The
momentum scale defining the onset of freezing is found both analytically and
numerically. The nonperturbative contribution to the thrust variable,
originating from the freezing, makes the value of this variable closer to the
experimental one.Comment: 25 pages, 5 figure
Signatures of Thermal Dilepton Radiation at RHIC
The properties of thermal dilepton production from heavy-ion collisions in
the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3
GeV. Using an expanding thermal fireball to model the evolution through both
quark-gluon and hadronic phases various features of the spectra are addressed.
In the low-mass region, due to an expected large background, the focus is on
possible medium modifications of the narrow resonance structures from
and mesons, whereas in the intermediate-mass region the old idea of
identifying QGP radiation is reiterated including effects of chemical
under-saturation in the early stages of central Au+Au collisions.Comment: 17 pages ReVTeX including 16 figure
Exclusive Radiative B-Decays in the Light-Cone QCD Sum Rule Approach
We carry out a detailed study of exclusive radiative rare -decays in the
framework of the QCD sum rules on the light cone, which combines the
traditional QCD sum rule technique with the description of final state vector
mesons in terms of the light-cone wave functions of increasing twist. The
decays considered are: and the corresponding decays of the mesons, and . Based on our estimate of the transition
form factor F_1^{B \to K^*\pg}(0) =0.32\pm0.05, we find for the branching
ratio , which is in
agreement with the observed value measured by the CLEO collaboration. We
present detailed estimates for the ratios of the radiative decay form factors,
which are then used to predict the rates for the exclusive radiative B-decays
listed above. This in principle allows the extraction of the CKM matrix element
from the penguin-dominated CKM-suppressed radiative decays when they
are measured. We give a detailed discussion of the dependence of the form
factors on the -quark mass and on the momentum transfer, as well as their
interrelation with the CKM-suppressed semileptonic decay form factors in , which we also calculate in our approach.Comment: 32 pages, 10 uuencoded figures, LaTeX, preprint CERN-TH 7118/9
- âŠ