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.     

Selective observation of continuum transitions in the Pb+Pb quasimolecule

Cross sections for doubleK shell excitation and for the production of continuum x-rays in coincidence withK x-rays from 4.8 MeV/amu²⁰⁸Pb +²⁰⁸Pb collisions have been measured. A simple statistical model is presented to explain double vacancy contributions to the coincidence cross sections and the relative contributions of MO x-ray,K x-ray cascade and MO x-ray,K x-ray double vacancy coincidences are evaluated. These latter yields are found to be significant enough to make extraction of the cascade contribution to the measured coincidence yields problematic.     

Strangeness enhancement in sulphur—Nucleus collisions at 200 A GeV/c

The NA35 experiment used several independent methods to determine the strange particle production in p+S and S+A collisions. The different techniques show consistent results. Strangeness conservation in full phase space is used as an additional check of the consistency of the data. On the base of the analysis in full phase space it could be shown that strangeness conservation is fullfilled. The NA35K _S ⁰ in S+S and S+Ag are consistent with the NA44 results forK ⁺ andK ^?. The results of the NA36 collaboration for S+Pb collisions were extrapolated to full phase space. The comparison with the NA35 results shows more than two times lower yields. The ratio of Λ to $\bar \Lambda $ at midrapidity of NA36 is inconsistent with the high baryon density determind by NA35. The strange particle production is compared to the abundance of non strange particles, especially negatively charged pions which are measured in full phase space in the same experiment. A clear enhanced strange hadron production relative to σ^? is observed in S+Ag collisions compared to p+S reactions at the same energy. TheK _S ⁰ multiplicity in full phase space per negative hadron (h ^?) in S+S, S+Ag and Pb+Pb is enhanced by about a factor 1.6 compared to N+N and p+S collisions. The NA36 result for theK _S ⁰ multiplicity perh ^? in S+Pb is below the N+N value.     

Indications for the onset of deconfinement in Pb+Pb collisions at the CERN SPS

Particle production in central Pb+Pb collisions was studied with the NA49 large acceptance spectrometer at the CERN SPS at beam energies of 20, 30, 40, 80 and 158 GeV per nucleon. A change of the energy dependence is observed around 30A GeV for the yields of pions and strange particles as well as for the shapes of the transverse mass spectra. The energy dependence of fluctuations and correlations shows no structure. At present only a reaction scenario with onset of deconfinement is able to reproduce the measurements.     

φ production in In–In collisions at 158 A GeV

The NA60 experiment has measured muon pair production in In–In collisions at 158 AGeV at the CERN SPS. This paper presents a high statistics measurement of φ→μ μ meson production. Differential spectra, yields, mass and width are measured as a function of centrality and compared to previous measurements in other colliding systems at the same energy. The width of the rapidity distribution is found to be constant as a function of centrality, compatible with previous results. The decay muon polar angle distribution is measured in several reference frames. No evidence of polarization is found as a function of transverse momentum and centrality. The analysis of the p _T spectra shows that the φ has a small radial flow, implying a weak coupling to the medium. The T _eff parameter measured in In–In collisions suggests that the high value observed in Pb–Pb in the kaon channel is difficult to reconcile with radial flow alone. The absolute yield is compared to results in Pb–Pb collisions: though significantly smaller than measured by NA50 in the muon channel, it is found to exceed the NA49 and CERES data in the kaon channel at any centrality. The mass and width are found to be compatible with the PDG values at any centrality and at any p _T : no evidence for in-medium modifications is observed.     

On direct photon production in Pb+Pb and Au+Au collisions

We attempt in this paper to deal with some aspects of the direct photon production phenomenon mainly in the Pb+Pb and Au+Au collisions on the basis of a model for production of the same particle species in PP collisions. The results have, thereafter, been converted to those for a few AB/AA interactions through an appropriate mechanism. Comparison of the theoretical results obtained on the basis of model-based calculations with measured data on the relevant observables leads to a fair agreement between them. This reflects modestly the fair achievements attained by the models that are made use of in the present work.     

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.     

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.     

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.     

X-ray study of the deep inelastic reactions 1,160-MeV136Xe+Pb and136Xe+232Th

The spectra ofK x-rays emitted by the target-like reaction products in the deep inelastic collisions 1,160-MeV¹³⁶Xe+Pb and¹³⁶Xe+²³²Th were investigated. Target-like x-ray emission probabilities per projectile-like and per target-like product were determined and compared with theoretical predictions of atomic vacancy production in the higher-Z particle (1s }-vacancy production). It is shown that a large fraction of the x-rays is produced by internal conversion ofγ-rays in the higher-Z reaction product. Measurement of target-like x-rays in coincidence with projectile-like and with target-like reaction products allowed the determination of the average survival probability against fission and of theZ-distribution of the highly excited target-like nuclei as a function of theQ value (total kinetic energy loss). The observed average survival probabilities agree qualitatively with theoretical predictions of Wilcke et al. Nuclearreaction-time effects on the 1s σ-vacancy production in these collisions could not be demonstrated with any certainty.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

LowpT phenomena in A+A and p+A collisions at mid-rapidity

CERN experiment NA44 measures pion and kaon distributions atpT=0–320 MeV/c at midrapidity in p+A and A+A collisions at 450 and 200A GeV/c, respectively. Pion production from p+Pb and A+A is slightly enhanced at lowpT relative to p+Be, but less than observed at target rapidity. The lowpT behavior can be understood in the context of the RQMD model as arising from baryonic resonance decays.     

PbK-shell ionization probability in high-energy Xe+Pb collisions

Probabilities for PbK-shell ionization in 5.9-, 7.2-, and 8.5-MeV/a.m.u. Xe+Pb collisions for impact parameters between 10 and 60 fm are reported. The measured probabilities are compared with theories of 1sσ-vacancy production.     

Low p T phenomena in A+A and p+A collisions at mid-rapidity

CERN experiment NA44 measures pion and kaon distributions at p _T = 0–320 MeV/c at midrapidity in p+A and A+A collisions at 450 and 200A GeV/c, respectively. Pion production from p+Pb and A+A is slightly enhanced at low p _T relative to p+Be, but less than observed at target rapidity. The low p _T behavior can be understood in the context of the RQMD model as arising from baryonic resonance decays.     

Kaon interferometry in heavy-ion collisions at the CERN SPS

K⁺K⁺ and K^–K^– correlations from S+Pb collisions at 200 GeV/c per nucleon and K⁺K⁺ correlations from p+Pb collisions at 450 GeV/c per nucleon, are presented as measured by the focusing spectrometer of the NA44 experiment at CERN. Multidimensional fits are performed in order to characterize the kaon-emission volume, which is found to be smaller than the pion-emission volume.     

Azimuthal Distributions of K + Mesons in Heavy-Ion Collisions

Strange mesons are considered to be sensitive to in-medium modifications. Theory predicts a repulsive K ⁺ N potential and an attractive K ^? N potential in dense matter. A repulsive K ⁺ N potential would repel the K ⁺ mesons from the bulk of the nucleons and therefore cause a preferred out-of-plane emission of K ⁺ mesons at midrapidity and a directed flow opposite to the nucleons at target and projectile rapidity. One of observables to probe in-medium effects is the azimuthal emission pattern of K ⁺ mesons in heavy ion collisions. KaoS collaboration has measured the azimuthal distributions of K ⁺ mesons in Au + Au reactions at 1.5 A GeV and Ni + Ni reactions at 1.93 A GeV. Data show that K ⁺ mesons exhibit a pronounced enhancement at ${\phi = {90}^\circ}$ , i.e. perpendicular to the reaction plane. The data have also been fitted using the first two components of a Fourier series to get the directed flow v ₁ and elliptic flow v ₂. We used the quantum molecular dynamics model based on the covariant kaon dynamics to simulate the Au + Au collisions at 1.5 A GeV and the Ni + Ni collisions at 1.93 A GeV, to analyze the azimuthal distributions of K ⁺ mesons, and to calculate v ₁ and v ₂ of K ⁺ mesons. Calculated results with a repulsive in-medium K ⁺ N potential can reasonably describe the features of KaoS data. This indicates that the azimuthal distribution is one of sensitive probes to extract information on in-medium properties at high densities.     

K + production in proton-nucleus collision

We present a dynamical study ofK ⁺ production in proton-nucleus collisions from 1.2 to 2.5 GeV bombarding energy. The evolution of the proton-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type. We incorporate all known sources forK ⁺ production and study their momentum and angular distributions, and the excitation function. We show that at lower energies (E _b<1.5 GeV) theNΔ andNN* channels dominate the kaon yield for light systems. At higher bombarding energies the directNN channel accounts for almost the whole cross section.     

Two-kaon correlations in central Pb+Pb collisions at 158 a GeV/c

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta approximately 0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p(T) data, while a two-dimensional analysis was used for the higher p(T) data. We find that the source-size parameters are consistent with the m(T) scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.     

Mid-rapidity ϕ meson production at \sqrt {s_{NN} } = 200 GeV Au+Au and pp collisions from STAR

We present the results for the measurement of ? meson production in $\sqrt {s_{NN} } = 200$ GeV Au+Au and pp collisions at the Relativistic Heavy Ion Collider (RHIC). Using the event mixing technique, spectra and yields are obrained from the ?→K⁺K^? decay channel for five centrality bins in Au+Au collisions and in pp collisions. We observe that the spectrum shape in Au+Au collisions depends weakly on the centrality and the shape of the spectrum in pp collisions is significantly different from that in Au+Au collisions.     

The effect of Lorentz-like force on collective flows of <Emphasis Type="Italic">K</Emphasis><Superscript>+</Superscript> in Au+Au collisions at 1.5 GeV/nucleon

Producing kaon mesons in heavy-ion collisions at beam energies below their threshold energy is an important way to investigate the properties of dense nuclear matter. In this study, based on the newly updated version of the ultrarelativistic quantum molecular dynamics model, we introduce the kaon-nucleon (KN) potential, including both the scalar and vector (also dubbed Lorentz-like) aspects. We revisit the influence of the KN potential on the collective flow of K⁺ mesons produced in Au+Au collisions at E_lab = 1.5 GeV/nucleon and find that the contribution of the newly included Lorentz-like force is very important, particulary for describing the directed flow of K⁺. Finally, the corresponding KaoS data of both directed and elliptic flows can be simultaneously reproduced well.