Strange particle production in relativistic collisions of heavy ions

This review examines data on strange particle production in Pb + Pb collisions obtained in heavy ion experiments at CERN SPS. The yields of K, Λ, Ξ and Ω are considered, as well as their rapidity and transverse mass distributions, depending on the centrality of Pb + Pb collisions at SPS energies. Experimental results are compared with predictions of the statistical nuclear fireball and microscopic parton -string models. We discuss in detail the experimentally observed effect of strangeness enhancement in nucleus-nucleus collisions as compared to nucleon-nuclear interactions and its interpretation within the framework of various theoretical models. Also, we analyze the behavior of hyperon yields and nuclear modification factors in passing from SPS to RHIC energies.     

Lead beam experiments at the CERN SPS

The acceleration in 1986 of¹⁶O beams to 200 GeV/nucleon at the CERN SPS created a new frontier of particle and nuclear physics, namely the study of high energy density systems with hundreds of quarks and gluons created in the central collisions of nuclei with heavy targets. In order to produce the largest piece of “quark matter”, beams as heavy as²⁰⁸Pb are needed. The Lead-Injector Project that would provide them is presented. Possible experimental approaches to extract the physics from collisions with thousands of produced particles are discussed.     

How dense does parton matter get in Pb + Pb collisions at the CERN SPS?

We examine the qualitative features of parton production through materialization in heavy-ion collisions within perturbative QCD, and estimate the magnitude of the resulting parton density created during the early stage of the collisions. The implications for “anomalous”J/? suppression observed in Pb+Pb collisions at the CERN SPS are discussed. We argue that theA-dependence of absorption ofJ/? by (partonic) comovers is steeper than assumed in most phenomenological models, because the absorption process is dominated by quasi-perturbative QCD interactions. Our argument is supported by results recently obtained in the framework of the parton cascade model. We predict significant “anomalous” suppression for Pb+Pb collisions at the CERN-SPS, but not for S+U collisions.     

Constraints for the nuclear sea quark distributions from the Drell–Yan process at the SPS

Nuclear modifications to the Drell–Yan dilepton production cross sections in and collisions in the leading twist approximation are caused by nuclear effects in the parton distributions of bound nucleons. For non-isoscalar nuclei, isospin corrections must also be considered. We calculate these effects for and Pb+Pb collisions at CERN SPS energies. Our goal is to place constraints on nuclear effects in sea quark distributions in the region . We show that the net nuclear corrections remain small for collisions at GeV. However, in collisions at GeV, effects of are predicted at large M. The data collected by the NA50 collaboration could thus be used to constrain the nuclear effects in the sea quark distributions in the region of the EMC effect, . Received: 6 October 2000 / Revised version: 2 July 2001 / Published online: 19 September 2001     

Wigner functions in high energy physics

Recent developments are (meta)reviewed in the applications of Wigner functions to describe the observed single-particle spectra and two-particle Bose-Einstein (or Hanbury Brown-Twiss) correlations in high energy particle and nuclear physics, with examples from hadron-proton and Pb+Pb collisions at CERN SPS.     

Fission and fragmentation of 208Pb nuclei in collisions with gold nuclei at an energy of 158 GeV per nucleon

The fission and fragmentation of ultrarelativistic ²⁰⁸Pb nuclei in collisionswith gold nuclei were studied by using a beam from the SPS accelerator at CERN at an energy of 158 GeV per nucleon. The detectors of the target area of the NA45/CERES spectrometer were used in respective measurements. The value obtained for the fission cross section is 301 ± 44 mb, where about 77% of events stem from the electromagnetic interaction of colliding nuclei, while the remaining part is the contribution of peripheral nuclear interactions. The spallation of lead nuclei that involves the formation of heavy fragments occurs only in collisions where the impact parameter satisfies the condition b > 10 fm. A complete disintegration of lead nuclei to intermediate-mass fragments and light particles is observed in some peripheral collisions.     

Simple models of the contribution of intermediate stage gluons to J/ψ suppression at the CERN SPS

We construct three simple scenarios of the time dependence of density of intermediate stage gluons in nuclear collisions in the CERN SPS energy range. Gluons with energy of about 0.6–1.0 GeV are assumed to be produced in nucleon-nucleon collisions in a Glauber type model. The rate of gluon production is given by the parameter n _g/nn equal to the average number of gluons produced per nucleon-nucleon collision. The value of this parameter determines the behaviour of the gas of gluons. The number of gluons increases due to gluon branching and processes like g+g → g+g+g and decreases due to the hadronization. Gluons are assumed to be able to dissociate J/ψ in g+J/ψ collisions, the dissociation cross-section σ _gγ is taken as a free parameter. In the first scenario, the energy density of the gas of gluons never reaches the critical energy density ε _c ≈ 0.7 GeV/fm³ and gluons rapidly hadronize. In the second scenario, the critical energy density is reached but the system of gluons is unable to reach thermalization. In the third scenario gluons reach thermalization and the thermalized system suppresses J/ψ by the Matsui-Satz mechanism. The third scenario under the assumption of a small value of σ _gψ is able to describe qualitatively the data on J/ψ suppression in Pb?Pb interactions obtained by the NA50 Collaboration. Other scenarios have problems with getting the rather abrupt onset of J/ψ suppression.     

Onset of deconfinement and critical point: NA49 and NA61/SHINE at the CERN SPS

This paper is dedicated to the memory of József Zimányi one of the founders of the experiment NA49 at the CERN SPS. Firstly, the paper summarizes the main results of NA49 concerning observation of the onset of deconfinement in central Pb+Pb collisions at the low SPS energies. Secondly, it sketches the physics program of NA61 at the CERN SPS, the successor of NA49, which in particular aims to discover the critical point of strongly interacting matter. Finaly, a brief review of the future experimental programs in the CERN SPS energy range is given.     

Charged particle spectra in16O induced nuclear collisions at the CERN SPS

Charged particle multiplicity distributions and pseudo-rapidity density distributions from¹⁶O induced nuclear collisions at 200 and 60A GeV are presented. The data were taken, using a minimumbias trigger, with the WA80 set-up at the CERN SPS. In this presentation we focus on how the yield of charged particles depends on the projectile energy, the mass number of the target nucleus and the energy measured in a uranium-plastic sampling calorimeter covering angles close to zero degrees. The data are compared to simulations from the event generator FRITIOF.     

超高能诱发核反应的国际合作实验研究进展

综述超高能诱发核反应的国际合作实验进展及相关的重离子物理研究现状.内容包括:1.有关夸克物质(夸克胶子等离子体QGP)的理论预言.2.超高能重离子碰撞的实验:(1)BNL/AGS能区的固定靶实验回顾;(2)CERN/SPS能区的固定靶实验回顾;(3)BNL/RHIC能区的对撞实验现状;(4)CERN/LHC能区的对撞实验现状.     

Nonadiabatic corrections to the adiabatic Efimov potential

Abstact: The composition of forward-going projectile spectator matter in fixed-target Pb+Pb collisions at 158 A · GeV at the CERN SPS has been studied as a function of centrality. The data were measured with the NA49 veto calorimeter. We observe that forward-going spectator matter in central collisions consists of 9 neutrons, 7 protons, and half a deuteron on average. At large impact parameters most spectator nucleons are bound in fragments. The relative resolution of the average impact parameter derived from the measurement of spectator neutrons is roughly 19% in the range from zero to half maximum impact parameters. Received: 16 February 1998 / Revised version: 30 March 1998     

Strange meson enhancement in PbPb collisions

The NA44 Collaboration has measured yields and differential distributions of K⁺, K⁻, π⁺, π⁻ in transverse kinetic energy and rapidity, around the center-of-mass rapidity in 158 A GeV/c Pb+Pb collisions at the CERN SPS. A considerable enhancement of K⁺ production per π is observed, as compared to p+p collisions at this energy. To illustrate the importance of secondary hadron rescattering as an enhancement mechanism, we compare strangeness production at the SPS and AGS with predictions of the transport model RQMD.     

Search for deconfinement in NA49 at the CERN SPS

Experiment NA49 at the CERN SPS uses a large acceptance detector for a systematic study of particle yields and correlations in nucleus-nucleus, nucleon-nucleus and nucleon-nucleon collisions. Preliminary results for Pb+Pb collisions at 40, 80 and 158 A⋅GeV beam energy shown and compared to measurements at lower and higher energies.     

Study of A+A collisions in NA49 at the CERN SPS

The NA49 experiment performed an energy scan of particle production in Pb+Pb collisions through the CERN SPS range. Observed features in the energy dependence of pion and strangeness production can so far only be described by a model assuming deconfinement to set in at the early stage of the reaction at the lower SPS energies.     

On chemical equilibrium in nuclear collisions

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 chemical 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 , and collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor increases from about 0.45 for interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor 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. Received: 31 October 1997 / Published online: 26 February 1998     

Particle production in Pb+Pb collisions at the SPS

Experiment NA49 at the CERN SPS employs a large acceptance detector to study particle spectra, yields and correlations in nucleus-nucleus, nucleon-nucleus and nucleon-nucleon collisions. Preliminary results on pion, kaon, A and ā production in central Pb+Pb collisions at 40, 80 and 158 A·GeV beam energy are shown and compared to measurements at lower and higher energies.     

Charmonium production in Pb−Pb collisions at 158 <Emphasis Type="Italic">A</Emphasis> GeV

The NA50 experiment at CERN SPS studies charmonia and Drell-Yan production in the μ⁺ μ^? decay channel at 158A GeV. The data collected in Pb?Pb collisions during 1995, 1996 and 1998 run periods show that the J/ψ is anomalously suppressed in central collisions. This observed suppression pattern can be considered as a strong indication of the production of Quark-Gluon Plasma.     

Pseudorapidity distributions of charged particles as a function of centrality in Pb–Pb collisions at 158 and 40 GeV per nucleon incident energy

The charged particle distributions dN_ch/dη have been measured by the NA50 experiment in Pb–Pb collisions at the CERN SPS. Measurements have been done at incident energies of 158 and 40 GeV per nucleon over a broad impact parameter range. Results obtained with two independent centrality estimators, namely the neutral transverse energy E_T and the forward energy E_ZDC, are reported.     

Event-by-event fluctuations of average transverse momentum in central Pb+Pb collisions at 158 GeV per nucleon

We present first data on event-by-event fluctuations in the average transverse momentum of charged particles produced in Pb+Pb collisions at the CERN SPS. This measurement provides previously unavailable information allowing sensitive tests of microscopic and thermodynamic collision models and to search for fluctuations expected to occur in the vicinity of the predicted QCD phase transition. We find that the observed variance of the event-by-event average transverse momentum is consistent with independent particle production modified by the known two-particle correlations due to quantum statistics and final state interactions and folded with the resolution of the NA49 apparatus. For two specific models of non-statistical fluctuations in transverse momentum limits are derived in terms of fluctuation amplitude. We show that a significant part of the parameter space for a model of isospin fluctuations predicted as a consequence of chiral symmetry restoration in a non-equilibrium scenario is excluded by our measurement.     

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.