403 research outputs found
Event-by-Event Search for Charged Neutral Fluctuations in Pb - Pb Collisions at 158-A-GeV
Results from the analysis of data obtained from the WA98 experiment at the
CERN SPS have been presented. Some events have been filtered which show photon
excess in limited zones within the overlap region of the charged
particle and photon multiplicity detectors.Comment: 6 pages, 4 figure
Coherence and Decoherence in Radiation off Colliding Heavy Ions
We discuss the kinetics of a disoriented chiral condensate, treated as an
open quantum system. We suggest that the problem is analogous to that of a
damped harmonic oscillator. Master equations are used to establish a hierarchy
of relevant time scales. Some phenomenological consequences are briefly
outlined.Comment: 15 latex pages, LPTHE Orsay 93/19, e-mail: [email protected]
Isospin Fluctuations from a Thermally Equilibrated Hadron Gas
Partition functions, multiplicity distributions, and isospin fluctuations are
calculated for canonical ensembles in which additive quantum numbers as well as
total isospin are strictly conserved. When properly accounting for
Bose-Einstein symmetrization, the multiplicity distributions of neutral pions
in a pion gas are significantly broader as compared to the non-degenerate case.
Inclusion of resonances compensates for this broadening effect. Recursion
relations are derived which allow calculation of exact results with modest
computer time.Comment: 10 pages, 5 figure
Model of Centauro and strangelet production in heavy ion collisions
We discuss the phenomenological model of Centauro event production in
relativistic nucleus-nucleus collisions. This model makes quantitative
predictions for kinematic observables, baryon number and mass of the Centauro
fireball and its decay products. Centauros decay mainly to nucleons, strange
hyperons and possibly strangelets. Simulations of Centauro events for the
CASTOR detector in Pb-Pb collisions at LHC energies are performed. The
signatures of these events are discussed in detail.Comment: 19 pages, LaTeX+revtex4, 14 eps-figures and 3 table
Effect of friction on disoriented chiral condensate formation
We have investigated the effect of friction on the DCC domain formation. We
solve the Newton equation of motion for the O(4) fields, with quenched initial
condition. The initial fields are randomly distributed in a Gaussian form. In
one dimensional expansion, on the average, large DCC domains can not be formed.
However, in some particular orbits, large instabilities may occur. This
possibility also greatly diminishes with the introduction of friction. But, if
the friction is large, the system may be overdamped and then, there is a
possibility of large DCC domain formation in some events.Comment: 9 pages, including figure
Threshold meson production and cosmic ray transport
An interesting accident of nature is that the peak of the cosmic ray
spectrum, for both protons and heavier nuclei, occurs near the pion production
threshold. The Boltzmann transport equation contains a term which is the cosmic
ray flux multiplied by the cross section. Therefore when considering pion and
kaon production from proton-proton reactions, small cross sections at low
energy can be as important as larger cross sections at higher energy. This is
also true for subthreshold kaon production in nuclear collisions, but not for
subthreshold pion production.Comment: 9 pages, 1 figur
Forming Disoriented Chiral Condensates through Fluctuations
Using the influence functional formalism, classical equations of motion for
the O(N) model are derived in the presence of a heat bath, in both the
symmetric phase as well as the phase of spontaneously broken symmetry. The heat
bath leads to dissipation and fluctuation terms in the classical equations of
motion, which are explicitly computed to lowest order in perturbation theory.
In the broken phase these terms are found to be large for the sigma field, even
at zero temperature, due to the decay process sigma -> pi pi, while they are
small for the pi fields at temperatures below T_c = 160 MeV. It is shown that
in large volumes the presence of dissipation and fluctuations suppresses the
formation of disoriented chiral condensates (DCC's). In small volumes, however,
fluctuations become sufficiently large to induce the formation of DCC's even if
chiral symmetry has not been restored in the initial stage of the system's
evolution.Comment: 34 pages, 11 figures, ReVTeX, eps-, aps-, psfig-style files require
Pseudovector vs. pseudoscalar coupling in one-boson exchange NN potentials
We examine the effects of pseudoscalar and pseudovector coupling of the pi
and eta mesons in one-boson exchange models of the NN interaction using two
approaches: time-ordered perturbation theory unitarized with the relativistic
Lippmann-Schwinger equation, and a reduced Bethe-Salpeter equation approach
using the Thompson equation. Contact terms in the one-boson exchange amplitudes
in time-ordered perturbation theory lead naturally to the introduction of
s-channel nucleonic cutoffs for the interaction, which strongly suppresses the
far off-shell behavior of the amplitudes in both approaches. Differences
between the resulting NN predictions of the various models are found to be
small, and particularly so when coupling constants of the other mesons are
readjusted within reasonable limits.Comment: 24 pages, 4 figure
Can Disordered Chiral Condensates Form? A Dynamical Perspective
We address the issue of whether a region of disordered chiral condensate
(DCC), in which the chiral condensate has components along the pion
directions, can form. We consider a system going through the chiral phase
transition either via a quench, or via relaxation of the high temperature phase
to the low temperature one within a given time scale (of order ).
We use a density matrix based formalism that takes both thermal and quantum
fluctuations into account non-perturbatively to argue that if the linear
sigma model is the correct way to model the situation in QCD, then it is very
unlikely at least in the Hartree approximation, that a large ()
DCC region will form. Typical sizes of such regions are
and the density of pions in such regions is at most of order . We end with some speculations on how large DCC regions may be
formed.Comment: 21 pages LATEX, 12 figures available upon request via regular mail,
PITT-94-0
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