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 $\eta-\phi$ 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 $\sim 1 \rm{fm/c}$). We use a density matrix based formalism that takes both thermal and quantum fluctuations into account non-perturbatively to argue that if the $O(4)$ 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 ($> 10\ \rm{fm}$) DCC region will form. Typical sizes of such regions are $\sim 1 -2 \ \rm{fm}$ and the density of pions in such regions is at most of order $\sim 0.2 / \rm{fm}^3$. 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