Signatures of the deconfinement transition in nucleus-nucleus collisions at the CERN SPS

The energy dependence of hadron production in relativistic nucleus-nucleus collisions reveals anomalies-kink, horn, and step. They have been predicted as the signals of the deconfinement phase transition and observed recently by the NA49 Collaboration in Pb + Pb collisions at the CERN SPS. This indicates the onset of deconfinement in nucleus-nucleus collisions at about 30 A GeV. The text was submitted by the author in English.     

Analysis of variances of quasirapidities in collisions of gold nuclei with track-emulsion nuclei

A new method of an analysis of variances was developed for studying n-particle correlations of quasirapidities in nucleus-nucleus collisions for a large constant number n of particles. Formulas that generalize the results of the respective analysis to various values of n were derived. Calculations on the basis of simple models indicate that the method is applicable, at least for n ?? 100. Quasirapidity correlations statistically significant at a level of 36 standard deviations were discovered in collisions between gold nuclei and track-emulsion nuclei at an energy of 10.6 GeV per nucleon. The experimental data obtained in our present study are contrasted against the theory of nucleus-nucleus collisions.     

Charm and bottom production at RHIC

The current status of charm and bottom production measurements at RHIC is summarized. Heavy-flavor data from p+p collisions at provide a crucial testing ground for perturbative QCD calculations and serve as a baseline for measurements in nucleus-nucleus collisions. Due to their large mass heavy quarks are produced in hard parton scattering processes in the earliest phase of a nucleus-nucleus collisions such that they can be used to probe the hot and dense medium that is formed in the course of Au+Au collisions at RHIC.     

Baseline cold matter effects on J/ω production in AA collisions at RHIC

We present baseline calculations of initial-state shadowing and finalstate absorption effects on J/Ψ production in nucleus-nucleus collisions at the Relativistic Heavy Ion Collider. We show predictions for Au+Au and Cu+Cu collisions at √S _NN =200 GeV and Cu+Cu collisions at √S _NN =62 GeV as a function of the rapidity, y, and the number of binary nucleon-nucleon collisions, N _coll.     

Resolution of theK-factor ambiguity in QCD

The previously presented relations between inclusive cross sections for nucleus-nucleus, nucleon-nucleus and nucleon-nucleon scatterings with largep _T production were generalized to include effects of large momentum tail of nucleons inside the nucleus. Simple numerical estimations show that these effects are likely to dominate the inclusive cross section for nucleus-nucleus collisions at very largep _T.     

Transverse momentum spectrum of mesons produced in high energy nucleus-nucleus collisions

The momentum of all charged mesons produced in 19 high energy, high multiplicity nucleus-nucleus collisions of cosmic rays in emulsion have been measured. The transverse momentum distribution differs qualitatively from proton-proton collisions at comparable energies. One possible interpretation is in terms of a transverse explosion with effective temperature T=78 MeV and transverse flow velocity υ=0.61c.     

Quark-gluon plasma signatures in nucleus-nucleus collisions at CERN SPS

Two signatures of the quark-gluon plasma—strangeness ‘enhancement’ and J/ψ ‘suppression’—in nucleus-nucleus collisions at the SPS energies are critically discussed.     

The horn, kink and step, dale: from few GeV to few TeV

Rich experimental data have been collected in heavy-ion collisions at high energies to study the properties of strongly interacting matter. As the theory of strong interactions, QCD, predicts asymptotic freedom, the created matter at sufficiently high temperature and density will be dominated by a state of quasi-free quarks and gluons referred to as the Quark-Qluon Plasma (QGP). Experimental signals for the onset of the QGP creation (the onset of the deconfinement) have been predicted within the statistical model for the early stage of nucleus-nucleus collisions. In this model the existence of two different phases is assumed: confined mater and the QGP, as well as a first order phase transition between them. Until recently, these predictions were confirmed only by the NA49 experiment at the CERN SPS. In this report recent results from STAR at RHIC/BNL and from ALICE at LHC/CERN, related to the onset of deconfinement, will be compared to published results from NA49.     

Solitons in nucleus-nucleus collisions near the speed of sound

The equations of hydrodynamics as applied to nucleus-nucleus collisions admit a soliton solution at energies slightly exceeding the velocity of sound (10 MeV/A < E? 30 MeV/A). Possible phenomenological signatures of this effect are mentioned.     

Strange baryon production in heavy ion collisions

The rapidity distribution of Λ and ?Λ produced in nucleus-nucleus collisions at CERN energies is studied in the framework of an independent string model — with quark-antiquark as well as diquark-antidiquark pairs in the nucleon sea. It is shown that, besides the Λ-?Λ pair production resulting from the fragmentation of sea diquarks, final state interactions of co-moving secondaries π + N → K + Λ and ?N → K → ?Λ are needed in order to reproduce the data. Predictions for Pb-Pb collisions are presented.     

Strange mesons in dense nuclear matter

Experimental data on the production of kaons and antikaons in heavy-ion collisions at relativistic energies are presented and discussed with respect to in-medium effects. The K ^?/K⁺ ratios measured in nucleus-nucleus collisions are 1–2 orders of magnitude larger than in proton-proton collisions. The azimuthal angle distributions of K ⁺ mesons indicate a repulsive kaon-nucleon potential. Microscopic transport calculations consistently explain both the yields and the emission patterns of kaons and antikaons when assuming that their properties are modified in dense nuclear matter. The K ⁺ production excitation functions measured in light and heavy collision systems provide evidence for a soft nuclear equation-of-state.     

ICPAQGP2001: Conference summary

I review recent progress in ultrarelativistic nucleus-nucleus collisions, and the connection of this field to modern QCD theory of deconfinement and/or chiral symmetry restoration. The talks at this Conference have shown a convergence of data and theory as far as the CERN SPS investigations at √s = 17 GeV are concerned; the parton-hadron phase boundary seems now located atT = 170 ± 10 MeV. New data from RHIC and direct photon production results from CERN have been shown that point out the field’s future direction: analysis of partonic matter atT > 200 MeV. Astrophysics analysis was shown to be linked crucially to further theoretical progress with non-perturbative QCD.     

An “effective-A” dependence of cross sections in ultra-relativistic heavy-ion collisions and its interference with quark-gluon plasma signatures

It is shown that in ultra-relativistic nucleus-nucleus collisions, as a consequence of the geometry of the collision, there is a dependence of the total and differential cross sections on effective atomic mass numbers of the target and beam nuclei. The general expressions of the effective atomic mass numbers are deduced and they are computed for 0+Cu, O+U, S+U, Si+Au and U+U collisions. This particular A-dependence of the cross sections can have a strong effect on the production ratio of two particles of different species. As a consequence it might simulate a signature of quark-gluon plasma which is expected to be given by the yields ratio of two different particles. It is shown that this effect can explain quantitatively the K/π ratios measured in Si+Au and S+W collisions as well as the decrease of the J/ψ production relative to Drell-Yan lepton pairs measured in O+Cu, O+U and S+U collisions. The need of precise measurements of the powers α and α(x_F, P_T) in inclusive nuleon-nuleus interactions for the understanding of particle production in nucleus-nucleus collisions is stressed. Expectations for future experiments are presented.     

Rescattering production of dileptons in nucleus-nucleus collisions

We present a space-time formulation for the rescattering of hard partons produced in primary QCD subcollisions within a nucleus-nucleus collision. As an application we calculate the rescattering rate for dilepton production in 100 GeV/A+100 GeV/A uranium-uranium collisions.     

A microscopic calculation of fragment formation in nucleus-nucleus collisions

A microscopic model using effective and free nucleon-nucleon scattering cross sections is used to calculate the yields of projectile-like fragments from nucleus-nucleus collisions from 20 MeV/A to 2 GeV/A. Good agreement with reaction cross-section and fragment cross-section measurements is obtained. The enhanced yields of neutron-rich fragments observed experimentally at low beam energies from collisions of projectiles with heavy targets are reproduced somewhat better by the inclusion of a neutron-rich surface on the heavy-target nuclei. Each fragment mass is produced in a strongly localized region of the distance of closest approach between the colliding nuclei; lighter fragments come from small distances and the heavier ones from more peripheral collisions.     

Compressed baryonic matter—experiments at GSI and FAIR

Experiments on strangeness production in nucleus-nucleus collisions at SIS energies address fundamental aspects of modern nuclear physics: the determination of the nuclear equation-of-state at high baryon densities and the properties of hadrons in dense nuclear matter. Experimental data and theoretical results will be reviewed. The planned “Compressed Baryonic Matter” (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) aims at the exploration of the QCD phase diagram at the highest baryon densities. This includes the search for the deconfinement and chiral phase transition and for the QCD critical endpoint. Possible observables and the experimental setup will be discussed. The text was submitted by the author in English.     

Energy flow in ultra-relativistic nucleus-nucleus collisions and comparison between models

We calculate the cross sections of transverse and neutral energy production in the nucleus-nucleus collisions using the additive quark model (AQM) and the wounded nucleon model (WNM). In the case of α?α collisions, the experimental data of the transverse energy (E _T ) distribution favor the WNM, whereas those of the total neutral energy and the transverse neutral energy distributions favor the AQM. We predict theE _T distributions in the heavy ion collisions using these two models. The AQM and the WNM give the different results.     

Kaon production in hadronic matter

Subthreshold kaon production has been studied in symmetric nucleus-nucleus collisions as a function of the nucleus mass, beam energy and centrality. In Au+Au collsions at 1 AGeV theK ⁺ multiplicity increases more than linearly with increasing number of participating nucleons. Transport calculations have to assume a soft equation of state in order to reproduce the data. The in-mediumK ^? cross section measured in Ni+Ni collisions is enhanced by about a factor of 7 as compared to the free cross section when using theK ⁺ cross section at equivalent beam energies as a normalization.     

Target mass dependence and intranuclear cascade in high energy nucleus-nucleus collisions

Intranuclear cascade, one of the most important non-linear effects in high energy nucleus-nucleus collisions, is investigated, its influence on the target mass dependence of the distributions of final state particles is discussed. Using hydrodynamical model to treat non-linear effects, the main features of the rapidity distributions and their dependence on the target mass are naturally explained. The results of numerical calculation for¹⁶O-A central collisions at 60 and 200A GeV agree with the existing data.