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Ahmed Tounsi

Publications and source records attributed to Ahmed Tounsi.

17 recordsLinked to original sources

Strange Particles from Dense Hadronic Matter

After a brief survey of the remarkable accomplishments of the current heavy ion collision experiments up to 200A GeV, we address in depth the role of strange particle production in the search for new phases of matter in these collisions. In particular, we show that the observed enhancement pattern of otherwise rarely produced multistrange antibaryons can be consistently explained assuming color deconfinement in a localized, rapidly disintegrating hadronic source. We develop the theoretical description of this source, and in particular study QCD based processes of strangeness production in the deconfined, thermal quark-gluon plasma phase, allowing for approach to chemical equilibrium and dynamical evolution. We also address thermal charm production. Using a rapid hadronization model we obtain final state particle yields, providing detailed theoretical predictions about strange particle spectra and yields as function of heavy ion energy. Our presentation is comprehensive and self-contained: we introduce in considerable detail the procedures used in data interpretation, discuss the particular importance of selected experimental results and show how they impact the theoretical developments.

nucl-th

Strangeness Enhancement and Canonical Suppression

We demonstrate the essential role of canonical suppression in strangeness enhancement. The pattern of enhancement of strange and multistrange baryons observed by the WA97 collaboration can be understood on this basis. Besides, it is shown that in canonical approach strangeness enhancement is a decreasing function of collision energy. It is the largest at $\sqrt{s}=8.7$ GeV where the enhancement of $Ω,Ξ$ and $Λ$ is of the order of 100, 20 and 3 respectively.

hep-ph

Canonical Description of Strangeness Enhancement from p-A to Pb-Pb Collisions

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 canonical ensemble with respect to strangeness conservation. The importance and the role of strangeness under saturation is also discussed.

hep-ph

Free Energy of a Hot Quark-Gluon Plasma

Thermal (Th-)QCD properties appear to disagree with lattice (L-)QCD results, which suggests that Th-QCD does not have a valid perturbative expansion. However, we reproduce L-QCD using the first order $α_s$ corrections in Th-QCD.

hep-ph

Low-m_t charged pion asymmetry enhancement from hadronization of QGP

We show that in sudden hadronization of QGP a non-equilibrium value of the pion phase space occupancy parameter gamma_q>1 is expected in order to accommodate the entropy excess in QGP and the process of gluon fragmentation. When gamma_q is near to its maximum allowed value, pion overabundance is shown to arise at low $m_\bot$, where the charged pion asymmetry is also amplified. These effects are considered and we show that their magnitude suffices to explain pertinent experimental data obtained in 160-200AGeV Pb- and S- induced reactions.

nucl-th

Energy Dependence of Strange Particle Yields from a QGP-Fireball

We explore, as function of the collision energy and stopping in relativistic nuclear collisions, the production yields of strange particles, in particular strange antibaryons,assuming formation of a deconfined thermal QGP-fireball which undergoes a sudden hadronisation. The non-equilibrium freeze-out conditions are established and strange antibaryon excitation functions are shown to have characteristic features that should allow to discriminate between the QGP hypothesis and other reaction scenarios.

hep-ph

Formation and Evolution of the Quark-Gluon Plasma

Imposing an equilibrium between the thermal pressure of deconfined quarks and gluons and the dynamical compression pressure exercised by in-flowing nuclear matter, we study the initial thermal conditions reached in a quark-gluon plasma fireball formed in a relativistic heavy ion collision. We show that entropy is produced primarily in the pre-equilibrium stage of the reaction. We test our approach, comparing our results with the S-W/Pb collision results at 200 GeV A and find a surprising degree of agreement assuming about 50% stopping. We apply our method to a determination of the conditions in collisions of Au-Au at 11 GeV A and Pb-Pb at 157 GeV A, assuming full stopping of momentum, energy and baryon number. Our detailed results directly determine the spectral shape and abundance of (strange) hadrons and electromagnetic probes (photons, dileptons) produced in the collision, and we explore specific experimental consequences.

hep-ph

Strangeness and Particle Freeze-out in Nuclear Collisions at 14.6 Gev a

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.

hep-ph

QGP Formation and Strange Antibaryons

We analyze current experimental results and explore, as function of the collision energy and stopping in relativistic nuclear collisions, the production yields of strange antibaryons, assuming formation of a deconfined thermal QGP-fireball which undergoes a sudden hadronisation.

hep-ph

Strange anti-baryons - QGP versus HG

We study quark-gluon plasma (QGP) and hadronic gas (HG) models of the central fireball presumed to be the source of abundantly produced strange (anti-)baryons in S -> W collisions at 200 GeV A. We consider how multi-strange (anti-)baryon multiplicities depend on strangeness conservation and compare the HG and QGP fireball scenarios. We argue that the total particle multiplicity emerging from the central rapidity region as well as the variation of production rates with changes in the beam energy allows to distinguish between the two reaction scenarios.

hep-ph

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 production 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 changes in an insignificant range once the initial high temperature state has been formed, despite a great change in particle number and temperature.

hep-ph

Strange Particle Freeze-Out

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.

hep-ph

Hot hadronic matter and strange anti-baryons

We demonstrate that both quark-gluon plasma (QGP) and hadronic gas (HG) models of the central fireball created in S -> W collisions at 200 GeV A 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.

hep-ph

Strange Particle Abundance in QGP Formed in 200 GeV A Nuclear Collisions

We investigate here 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 in S-S, S-Ag and S-W reactions is compatible with this picture.

hep-ph

Strange Antibaryons from QGP

We study as function of the collision energy and stopping the thermal conditions reached in a quark-gluon plasma fireball formed in a relativistic heavy ion collision. We explore strange particle yields for the current round of Pb-Pb and Au-Au experiments.

hep-ph

Hadronic Signatures of Deconfinement in relativistic nuclear collisions

We describe the remarkable accomplishments of the current heavy ion Pb--Pb collision experiments involving strange particle production, carried out at 158A GeV at CERN--SPS. These experimental results, together with the earlier 200A GeV S-induced reactions, imply that, at central rapidity, a novel mechanism of strangeness production arises, accompanied by excess entropy formation. We argue that: These results are consistent with the formation of a space-time localized, highly excited, dense state of matter; The freeze-out properties of strange hadrons are suggestive of the formation of a color-deconfined, thermally and nearly chemically equilibrated phase, which provides at present the only comprehensive framework to describe all experimental data; The matter fireball is undergoing a transverse expansion with nearly the velocity of sound of relativistic matter; longitudinal expansion is not in the scaling regime. We present a first analysis of the recent Pb-Pb results and discuss several alternative reaction scenarios. We evaluate quantitatively strangeness production in the deconfined quark-gluon phase and obtain yields in agreement with the experimental observations made in 200A GeV S--W and 158A GeV Pb-Pb interactions. We also present a qualitative discussion of J/Psi results consistent with our understanding of strange particle results.

hep-ph

Strangeness in Pb-Pb Collisions at 158 A GeV

We study relative strange particle abundances measured in Pb-Pb 158 A GeV interactions. The thermal and chemical source parameters of these particles are determined under reaction scenario hypothesis invoking confined and deconfined hadronic matter.

hep-ph