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Transparency, mixing and reflection of initial flows in relativistic nuclear collisions

Marek Gazdzicki; Mark I. Gorenstein — Wed, 14 Jun 2006 13:00:43 +0200

We propose to use the hadron number fluctuations in the limited momentum regions to study the evolution of initial flows in high energy nuclear collisions. In this method by a proper preparation of a collision sample the projectile and target initial flows are marked in fluctuations in the number of colliding nucleons. We discuss three limiting cases of the evolution of flows, transparency, mixing and reflection, and present for them quantitative predictions obtained within several models. Finally, we apply the method to the NA49 results on fluctuations of the negatively charged hadron multiplicity in Pb+Pb interactions at 158A GeV and conclude that the data favor a hydrodynamical model with a significant degree of mixing of the initial flows at the early stage of collisions.

Energy and system size dependence of chemical freeze-out in relativistic nuclear collisions

Francesco Becattini; Jaakko Manninen; Marek Gazdzicki — Wed, 14 Jun 2006 12:55:58 +0200

We present a detailed study of chemical freeze-out in p-p, C-C, Si-Si and Pb-Pb collisions at beam momenta of 158A GeV as well as Pb-Pb collisions at beam momenta of 20A, 30A, 40A and 80A GeV. By analyzing hadronic multiplicities within the statistical hadronization model, we have studied the parameters of the source as a function of the number of the participating nucleons and the beam energy. We observe a nice smooth behaviour of temperature, baryon chemical potential and strangeness under-saturation parameter as a function of energy and nucleus size. Interpolating formulas are provided which allow to predict the chemical freeze-out parameters in central collisions at centre-of-mass energies > 4.5 GeV and for any colliding ions. Specific discrepancies between data and model emerge in particle ratios in Pb-Pb collisions at SPS between 20A and 40A GeV of beam energy which cannot be accounted for in the considered model schemes.

Eksperimental'nye dannye po srednemu cislu provzaimodejstvovavšich protonov v neuprugich i zentral'nych jadro-jadernych vzaimodejstvijach pri energij 3.6 GeV/nuklon

M. Kh. Anikina; A. U. Abdurachimov; S. A. Avramenko; et. al.; Marek Gazdzicki — Tue, 04 Oct 2005 09:00:28 +0200

Comment on 'Strangeness enhancement in p+A and S+A interactions at energies near 200 A GeV"

Marek Gazdzicki; Ulrich Heinz — Tue, 04 Oct 2005 08:51:17 +0200

We argue that the recent analysis of strangeness production in nuclear collisions at 200 A GeV/c performed by Topor Pop et al. is flawed. The conclusions are based on an erroneous interpretation of the data and the numerical model results. The term "strangeness enhancement" is used in a misleading way.

Pion and strangeness puzzles

Marek Gazdzicki — Tue, 04 Oct 2005 08:27:52 +0200

The dependence of pion and strangeness production on the number of participant nucleons and collision energy is discussed for central A+A collisions. A possible interpretation of the experimental results assuming transition to QGP is sketched within a simple statistical approach.

Strangeness in Nuclear Collisions

Marek Gazdzicki; Dieter Röhrich — Tue, 04 Oct 2005 08:26:39 +0200

Data on the mean multiplicity of strange hadrons produced in minimum bias proton--proton and central nucleus--nucleus collisions at momenta between 2.8 and 400 GeV/c per nucleon have been compiled. The multiplicities for nucleon--nucleon interactions were constructed. The ratios of strange particle multiplicity to participant nucleon as well as to pion multiplicity are larger for central nucleus--nucleus collisions than for nucleon--nucleon interactions at all studied energies. The data at AGS energies suggest that the latter ratio saturates with increasing masses of the colliding nuclei. The strangeness to pion multiplicity ratio observed in nucleon--nucleon interactions increases with collision energy in the whole energy range studied. A qualitatively different behaviour is observed for central nucleus--nucleus collisions: the ratio rapidly increases when going from Dubna to AGS energies and changes little between AGS and SPS energies. This change in the behaviour can be related to the increase in the entropy production observed in central nucleus-nucleus collisions at the same energy range. The results are interpreted within a statistical approach. They are consistent with the hypothesis that the Quark Gluon Plasma is created at SPS energies, the critical collision energy being between AGS and SPS energies.

Observation of deconfinement phase transition in nucleus-nucleus collisions

Marek Gazdzicki — Fri, 30 Sep 2005 09:51:45 +0200

The main results obtained within the energy scan program at the CERN SPS are presented. The anomalies in energy dependence of hadron production indicate that the onset of deconfinement phase transition is located at about 30 A GeV. For the first time we seem to have clear evidence for the existence of a deconfined state of matter in nature. PACS numbers: 24.85.+p

Pion Suppression in Nuclear Collisions

Marek Gazdzicki; Mark I. Gorenstein; Stanislaw Mrowczynski — Fri, 30 Sep 2005 09:26:24 +0200

The pion multiplicity per participating nucleon in central nucleus-nucleus collisions at the energies 2-15 A GeV is significantly smaller than in nucleon-nucleon interactions at the same collision energy. This effect of pion suppression is argued to appear due to the evolution of the system produced at the early stage of heavy-ion collisions towards a local thermodynamic equilibrium and further isentropic expansion.

Pion and Strangeness Production as Signals of QCD Phase Transition

Marek Gazdzicki — Fri, 30 Sep 2005 09:25:46 +0200

It is shown that data on pion and strangeness production in central nucleus-nucleus collisions are consistent with the hypothesis of a Quark Gluon Plasma formation between 15 A GeV/c (BNL AGS) and 160 A GeV/c (CERN SPS) collision energies. The experimental results interpreted in the framework of a statistical approach indicate that the effective number of degrees of freedom increases by a factor of about 3 in the course of the phase transition and that the plasma created at CERN SPS energy may have a temperature of about 280 MeV (energy density $\approx$ 10 GeV/fm^3). Experimental studies of central Pb+Pb collisions in the energy range 20-160 A GeV/c are urgently needed in order to localize the threshold energy, and study the properties of the QCD phase transition.

On chemical equilibrium in nuclear collisions

Francesco Becattini; Marek Gazdzicki; J. Sollfrank — Fri, 30 Sep 2005 08:55:49 +0200

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off-equilibrium version allowing for partial strangeness saturation is significantly better. The freeze-out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in e+e-, pp and p{\bar p} collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor gamma_S increases from about 0.45 for pp interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions. The quark strangeness suppression factor lambda_S is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system

On event-by-event fluctuations in nuclear collisions

Marek Gazdzicki; Andrei Leonidov; Gunther Roland — Fri, 30 Sep 2005 08:44:39 +0200

We demonstrate that a new type of analysis in heavy-ion collisions, based on an event-by-event analysis of the transverse momentum distribution, allows us to obtain information on secondary interactions and collective behaviour that is not available from the inclusive spectra. Using a random walk model as a simple phenomenological description of initial state scattering in collisions with heavy nuclei, we show that the event-by-event measurement allows a quantitative determination of this effect, well within the resolution achievable with the new generation of large acceptance hadron spectrometers. The preliminary data of the NA49 collaboration on transverse momentum fluctuations indicate qualitatively different behaviour than that obtained within the random walk model. The results are discussed in relation to the thermodynamic and hydrodynamic description of nuclear collisions.

A method to study "chemical" fluctuations in nucleus-nucleus collisions

Marek Gazdzicki — Fri, 30 Sep 2005 08:41:45 +0200

A method to study event-by-event fluctuations of the chemical (particle type) composition of the final state of high energy collisions is proposed.

Quark gluon plasma in a + a collisions at CERN SPS

Marek Gazdzicki — Fri, 30 Sep 2005 08:26:27 +0200

A brief review of a history of data collection and interpretation of the results on high energy A+A collisions is presented. Basic assumptions and main results of a statistical model of the early stage of the A+A collisions are discussed. It is concluded that a broad set of experimental data is in agreement with the hypothesis that QGP is created in central A+A (S+S and Pb+Pb) collisions at the SPS. Carefull experimental investigation of the A+A collisions in the energy region between top AGS and SPS energies is needed.

On the early stage of nucleus-nucleus collisions

Marek Gazdzicki; Mark I. Gorenstein — Fri, 30 Sep 2005 08:24:04 +0200

A statistical model of the early stage of central nucleus--nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus--nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event--by--event fluctuations. An attempt to understand the data on J/psi production in Pb+Pb collisions at the SPS within the same approach is presented.

On J/psi production in nuclear collisions

Marek Gazdzicki — Fri, 30 Sep 2005 08:16:58 +0200

Data on J/psi production in inelastic proton-proton, proton-nucleus and nucleus-nucleus interactions at 158 A GeV are analyzed and it is shown that the ratio of mean multiplicities of J/psi mesons and pions is the same for all these collisions. This observation is difficult to understand within current models of J/psi production in nuclear collisions based on the assumption of hard QCD creation of charm quarks.

Baryon number conservation and statistical production of antibaryons

Mark I. Gorenstein; Marek Gazdzicki; Walter Greiner — Thu, 29 Sep 2005 09:32:06 +0200

The statistical production of antibaryons is considered within the canonical ensemble formulation. We demonstrate that the antibaryon suppression in small systems due to the exact baryon number conservation is rather different in the baryon-free (B=0) and baryon-rich (B>1) systems. At constant values of temperature and baryon density in the baryon-rich systems the density of the produced antibaryons is only weakly dependent on the size of the system. For realistic hadronization conditions this dependence appears to be close to B/(B+1) which is in agreement with the preliminary data of the NA49 Collaboration for the antiproton/pion ratio in nucleus-nucleus collisions at the CERN SPS energies. However, a consistent picture of antibaryon production within the statistical hadronization model has not yet been achieved. This is because the condition of constant hadronization temperature in the baryon-free systems leads to a contradiction with the data on the antiproton/pion ratio in e+e- interactions.

Evidence for quark gluon plasma from hadron production in high energy nuclear collisions

Marek Gazdzicki — Thu, 29 Sep 2005 09:31:28 +0200

The experimental results on the pion, strangeness and J/psi production in high energy nuclear collisions are discussed. The anomalous energy dependence of pion and strangeness production is consistent with the hypothesis that a transition to a deconfined phase takes place between the top AGS (15 AGeV) and the SPS (200 AGeV) energies. The J/psi production systematics at the SPS can be understood assuming that the J/psi mesons are created at hadronization according to the available hadronic phase space. This new interpretation of the J/psi data allows one to establish a coherent picture of high energy nuclear collisions based on the statistical approaches of the collision early stage and hadronization. Surprisingly, the statistical model of strong interactions is successful even in the region reserved up to now for pQCD based models.

Evidence for statistical production of J/psi mesons in nuclear collisions at the CERN SPS

Marek Gazdzicki; Mark I. Gorenstein — Thu, 29 Sep 2005 09:26:52 +0200

The hypothesis of statistical production of J/psi mesons at hadronization is formulated and checked against experimental data. It explains in the natural way the observed scaling behavior of the J/psi to pion ratio at the CERN SPS energies. Using the multiplicities of J/psi and eta mesons the hadronization temperature T_H = 175 MeV is found, which agrees with the previous estimates of the temperature parameter based on the analysis of the hadron yield systematics.

Search for short lived particles in high multiplicity environment

Marek Gazdzicki; Waldemar Retyk; Jan Pluta — Thu, 29 Sep 2005 09:26:22 +0200

A method of statistical selection of short lived particles in high multiplicity nucleus-nucleus collisions is discussed.

Charm estimate from the dilepton spectra in nuclear collisions

Marek Gazdzicki; Mark I. Gorenstein — Thu, 29 Sep 2005 09:21:30 +0200

A validity of a recent estimate of an upper limit of charm production in central Pb+Pb collisions at 158 AGeV is critically discussed. Within a simple model we study properties of the background subtraction procedure used for an extraction of the charm signal from the analysis of dilepton spectra. We demonstrate that a production asymmetry between positively and negatively charged background muons and a large multiplicity of signal pairs leads to biased results. Therefore the applicability of this procedure for the analysis of nucleus-nucleus data should be reconsidered before final conclusions on the upper limit estimate of charm production could be drawn.

Comment on 'Comparison of strangeness production between A + A and p + p reactions from 2 to 160 A GeV', by J. C. Dunlop and C. A. Ogilvie

Marek Gazdzicki; Mark I. Gorenstein; Dieter Röhrich — Thu, 29 Sep 2005 08:55:54 +0200

A recent paper on energy dependence of strangeness production in A+A and p+p interactions written by Dunlop and Ogilvie (Phys. ReV. C61 031901(R) (2000) indicates that there is a significant misunderstanding about the concept of strangeness enhancement and its role as a signal of Quark Gluon Plasma creation. In this comment we will try to clarify some essential points. 25.75.Dw, 13.85.Ni, 21.65.+f

Highlights of CERN workshop on charm production in A + A collisions

Marek Gazdzicki — Thu, 29 Sep 2005 08:27:13 +0200

Models and experimental effort concerning open and hidden charm production in nuclear collisions discussed at Cern Workshop in December 99 are reviewed. The most recent development is also mentioned.

Comment on "Charged particle ratio fluctuation as a signal for quark-gluon plasma" and "Fluctuation probes of quark deconfinement"

Marek Gazdzicki; Stanislaw Mrowczynski — Thu, 29 Sep 2005 08:20:04 +0200

Charge fluctuations studied on event-by-event basis have been recently suggested to provide a signal of the equilibrium quark-gluon plasma produced in heavy-ion collisions at high energies. It is argued that the fluctuations generated at the early collision stage when the energy is released can fake the signal. PACS 25.75.-q, 12.38.Mh, 24.60.-k

Power law in hadron production

Marek Gazdzicki; Mark I. Gorenstein — Thu, 29 Sep 2005 08:19:07 +0200

In high energy p+p(bar) interactions the mean multiplicity and transverse mass spectra of neutral mesons from eta to Upsilon (m = 0.5 - 10 GeV/c^2) and the transverse mass spectra of pions (m_T > 1 GeV/c^2) reveal a remarkable behaviour: they follow, over more than 10 orders of magnitude, the power-law function:The parameters C and P are energy dependent, but similar for all mesons produced at the same collision energy. This scaling resembles that expected in the statistical description of hadron production: the parameter P plays the role of a temperature and the normalisation constant C is analogous to the system volume. The fundamental difference is, however, in the form of the distribution function. In order to reproduce the experimental results and preserve the basic structure of the statistical approach the Boltzmann factor e^(-E/T) appearing in standard statistical mechanics has to be substituted by a power-law factor (E/Lambda)^(-P).

Transverse momentum spectra of J/psi and psi prime mesons from quark gluon plasma hadronization in nuclear collisions

Kyrill A. Bugaev; Marek Gazdzicki; Mark I. Gorenstein — Tue, 27 Sep 2005 09:37:18 +0200

Recent results on transverse mass spectra of J/psi and psi prime mesons in central Pb+Pb collisions at 158 AGeV are considered. It is shown that those results support a hypothesis of statistical production of charmonia at hadronization and suggest the early thermal freeze-out of J/psi and psi prime mesons. Based on this approach the collective transverse velocity of hadronizing quark gluon plasma is estimated to be \approx 0.2. Predictions for transverse mass spectra of hidden and open charm mesons at SPS and RHIC are discussed.