Ahmed Tounsi

Canonical description of strangeness enhancement from p–A to Pb–Pb collisions

Abstract We consider the production of strange particles in Pb–Pb and p–A collisions at the SPS energy reported by the WA97 experiment. We show that the observed enhancement of strange baryon and antibaryon yields in Pb–Pb collisions relative to p–Be and p–Pb can be explained in terms of the statistical model formulated in the canonical ensemble with respect to strangeness conservation. The importance and the role of strangeness under saturation is also discussed.

Evidence for a phase with high specific entropy in nuclear collisions

We determine the entropy per baryon content of the central reaction region in terms of the charged particle multiplicity. We study the consistency of our findings with recent data on strange anti-baryon production at 200 GeV A in S→A collisions (A∼200) assuming formation of a central fireball. Hadron gas models which do not invoke strong medium modifications of hadron masses do not provide enough entropy and are inconsistent with the combined experimental results. In contrast the quark-gluon plasma hypothesis explains them naturally. PACS 25.75.+r, 12.38.Mh Published in Phys. Rev. Letters 70 (1993) 3530–3533. PAR/LPTHE/92–37 TPR-92-39 AZPH-TH/92-30 September 30, 1992 Work supported by DFG, BMFT, GSI, and NATO, grant CRG 910991 Work supported by DOE, grant DE-FG02-92ER 40733, and NATO, grant CRG 910991 Unité associée au CNRS

Strangeness conservation in hot nuclear fireballs.

Within a thermal model generalized to allow for nonequilibrium strange particle abundances we study how the constraint that the balance of strangeness in a fireball is (nearly) zero impacts the allowable thermal fireball parameters. Using the latest data of the CERN-WA85 experiment for the case of 200[ital A] GeV S-[ital A] ([ital A][similar to]200) collisions we extract the values of the thermal parameters considering in detail the impact of hadronic resonance decays on the abundances and spectral form of strange baryons and antibaryons. Given these results and invoking further the observed charged particle multiplicities we are able to consider the (specific) entropy content of the fireball in order to understand the nature of the disagreement of the hadronic gas picture of the fireball with the experimental data.

Gluon production, cooling, and entropy in nuclear collisions

We study the cooling (heating) of a glue-parton gas due to production (destruction) of particles and determine the associated evolution of entropy. We incorporate sharing of the system energy among a changing number of particles. We find that the entropy of an evolving glue-parton gas hardly changes, once the initial thermalized state has been formed, despite a significant change in particle number and temperature.

Hot hadronic matter and strange anti-baryons

Abstract We demonstrate that both quark-gluon plasma (QGP) and hadronic gas (HG) models of the central fireball created in S → W collisions at 200 A GeV are possible sources of the recently observed strange (anti-)baryons. From the theoretical point of view, the HG interpretation we attempt remains more obscure because of the high fireball temperature required. The thermal properties of the fireball as determined by the particle ratios are natural for the QGP state. We show that the total particle multiplicity emerging from the central rapidity region allows to distinguish between the two scenarios.

STRANGENESS AND PARTICLE FREEZE-OUT IN NUCLEAR COLLISIONS AT 14.6 GeV A

Abstract We study the chemical conditions at freeze-out associated with the production of strange-particles in SiAu collisions at 14.6 GeV A. We obtain freeze-out chemical potentials and temperature, and determine the entropy as well as the final particle abundance. We also consider in detail the alternative evolution scenarios involving the hadronic gas and the deconfined phase.

Strange particle abundance in QGP formed in 200 GeV A nuclear collisions

We investigate the relative abundance of strange particles produced in nuclear collisions at the SPS energies (∼ 9 GeV A in CM frame) assuming that the central reaction fireball consists of quark-gluon plasma. We show that the total strangeness yield observed in Sulphur induced reactions is compatible with this picture.

Strange particle freeze-out

Abstract We reconsider thermal conditions of the central fireball presumed to be the source of abundantly produced strange (anti-) baryons in S → W collisions at 200 GeV A. We show that it is possible to completely fix the freeze-out temperature of strange particles in terms of the central rapidity kaon to Lambda particle abundance ratio at fixed, high transverse mass using a non-equilibrium hadronization model and the measured quark fugacities.

Canonical aspects of strangeness enhancement

Abstract Strangeness enhancement (SE) in heavy ion collisions can be understood in the statistical model on the basis of canonical suppression. In this formulation, SE is a consequence of the transition from canonical to the asymptotic grand canonical limit and is predicted to be a decreasing function of collision energy. This model predictions are consistent with the recent NA49 data on Λ enhancement at plab = 40, 80, 158GeV.

Canonical constraints on particle production

We discuss the statistical model description of particle production in heavy-ion collisions. We put particular emphasis on the interpretation of the enhancement of particles that carry a non-vanishing quantum number related to U(1) internal symmetry. We argue that the basic features of experimental data showing enhancement of strange mesons and baryons are well described by the model.