Search

A measurement of strange baryon and antibaryon production in Pb-Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 A GeV/c beam momentum. Results on $\Lambda$, $\Xi$ and $\Omega$ hyperon yields at mid-rapidity in the most central 53% of Pb-Pb collisions at 40 A GeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range. The $\Lambda$ and $\Xi^-$ yields per unit rapidity stay roughly constant while those of $\Omega^-$, $\bar\Lambda$, $\bar\Xi^+$ and $\bar\Omega^+$ increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in $\sqrt{s_{NN}}$ = 130 GeV Au-Au collisions at RHIC., Comment: 9 pages, 4 figures, 3 tables, accepted for publication in Physics Letters B

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 ng/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 σgγ is taken as a free parameter. In the first scenario, the energy density of the gas of gluons never reaches the critical energy density ec ≈ 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 σgψ 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.

We study a very simple model of correlations and intermittency of identical final state pions in hadronic collisions. Final state pions are either products of resonance decays or they are “directly” produced. The “direct” production is simulated by an immediate decay of a resonance. For “direct pions” forming about a half of final state pions and for formation times of resonances less than 0.5fin/c we get density of sources which via Hanbury-Brown and Twiss interference leads to correlations of two identical pions consistent with recent data and shows intermittency patterns for the second factorial moment. The essential ingredient of the scheme is the combination of pions from resonance decays and direct pions. Due to life-times of resonances taken from experiment, pions from resonance decays are responsible for short-range correlations in the longitudinal momentum, whereas directly produced pions, due to their fast production, dominate in the region of longitudinal momentum difference of the order of 100 MeV/c. The combination of both can give an approximate scaling leading to intermittency.

In an attempt to disentangle the effects of nuclear geometry from those of nuclear geometry we study dependence of longitudinal radius on nucleon numbers of colliding nuclei within a simple model of multiple nucleon- nucleon collisions.We discuss two approaches to resulting simple formula.In the former the data up to S-Au can be described by nuclear geometry at the price of two parameters,but R_L for Pb-Pb is significantly larger than geometry can accomodate, indicating an onset of a new dynamics. In the latter even collisions induced by lighter ions give evidence of some collective behaviour and the increase of R_L for Pb-Pb indicates again the onset of a new dynamics., Comment: 10 pages in TeX, 1 Fig. as a ps-file Preceding version contained in one place an erroneous factor of 2,which propagated to some other places.Conclusions remain unchanged. Comenius University Bratislava preprint, April 1996

We study the effect of the photon formation length on the interference of two bremsstrahlung photons in the simplest case of radiation emitted by only two charged particles. Most of interference patterns obtained can be interpreted in terms of the photon formation length. After having analyzed the simplest situations we discuss the interference of bremsstrahlung photons in more realistic and more complicated situations. A possibility of studying the effect in synchrotron radiation is briefly sketched.

Hanbury-Brown and Twiss correlations of bremsstrahlung photons produced in hadronic collisions at very high energies show intermittent features. The phenomenon is due to large logitudinal dimensions of the space-time evolution of hadronic collisions and to the fact that the rapidity of a bremsstrahlung photon need not be close to the rapidity of the charged particle radiating the photon.

The dimension of the region emitting particles in nuclear collisions can be determined by Hanbury-Brown and Twiss (HBT) correlations of identical particles. In thiss note we demonstrate that this dimension as determined by the HBT correlation of bremsstrahlung photons is larger than the actual one due to the photon formation length. Qualitatively r2 → r2 + a/ω2 where the last term is due to the formation length being proportional to 1/ω, where ω is the energy of the photon. This phenomenon permits to “see” experimentally the photon formation length.

The new results on hyperon production in Pb-Pb collisions at 158 A GeV/c and 40 A GeV/c are presented. Publisher Summary This chapter discusses the yields of hyperon in Pb–Pb collisions from the NA57 experiment. The aim of NA57 was to study the dependence of the strangeness enhancement on the interaction volume and the collision energy per incoming nucleon. The centrality range of the NA57 experiment extended down to a limit of about 50 wounded nucleons. The experiment collected data using 158 and 40 A GeV/c beams at the CERN SPS. There was indication of an increase of the enhancements with the centrality of the collisions. The data were corrected for acceptance and efficiency losses by calculating weight for each particle with a GEANT-based Monte-Carlo simulation.

It is pointed out that accurate information about shapes of dilepton spectra in central, intermediate and peripheral S-W ion collisions at high energy may provide a convincing evidence of the formation of matter with high density. In principle, it is also possible to obtain information about the nature of this matter but the evidence is less convincing.

The formation of quark-gluon plasma in heavy ion collisions should manifest itself by the increase of the ratio of medium to large mass dileptons, the former being produced predominantly in the QGP and the mixed phase whereas the latter are produced mostly by the Drell-Yan process. The increase of this ratio should be visible either when comparing collisions of lighter with heavier ions, e.g. O+O with Pb+Pb or when comparing low and large transverse energy events in heavy ion e.g. Pb+Pb collisions.

The dependence of dilepton production on total transverse energy in nuclear collisions is proposed as a signature of quark- gluon plasma formation. In particular the ratio of dilepton production in central (large transverse energy) to that in peripheral (low transverse energy) gives information about the presence of hydrodynamical space-time evolution of the quark-gluon plasma or of a mixed phase.

The multiplicity of charged particles in the central rapidity region has been measured by the NA57 experiment in Pb--Pb collisions at the CERN SPS at two beam momenta: 158 A GeV/{\it c} and 40 A GeV/{\it c}. The value of $dN_{ch}/d\eta$ at the maximum has been determined and its behaviour as a function of centrality has been studied in the centrality range covered by NA57 (about 50% of the inelastic cross section). The multiplicity increases approximately logarithmically with the centre of mass energy., Comment: 19 pages, 8 figures

Enhancements of strange baryon and anti-baryon yields were first observed in Pb–Pb interactions at 160 A GeV/c by WA97. The aim of the NA57 experiment is to investigate how these enhancements behave as a function of energy, and over a broader centrality range than that which was available with WA97. This paper presents a comparison of the hyperon yields in Pb–Pb interactions at 40 A GeV/c and 160 A GeV/c. Preliminary signals for the p-Be data at 40 A GeV/c are also shown.

The NA57 experiment has measured strange baryon and antibaryon production in Pb-Pb collisions at 40 A GeV/c and 160 A GeV/c beam momenta. This presentation covers strangeness enhancement and transverse spectra from the 160 A GeV/c data, and energy dependence of the particle yields. Enhancement factors increase with increasing strangeness content of the particle, when production yields from Pb-Pb collisions are compared with those observed in p-Be and p-Pb interactions. The transverse mass spectra have been analysed both with exponential fits and using a transverse flow model.

We present a few estimates of energy densities reached in heavy-ion collisions at the CERN SPS. The estimates are based on data and models of proton-nucleus and nucleus-nucleus interactions. In all of these estimates the maximum energy density in central Pb+Pb interactions is larger than the critical energy density of about 0.7 GeV/fm^3 following from lattice gauge theory computations. In estimates which we consider as realistic the maximum energy density is about twice the critical value. In this way our analysis gives some support to claims that deconfined matter has been produced at the CERN SPS. Any definite statement requires a deeper understanding of formation times of partons and hadrons in nuclear collisions. We also compare our results with implicit energy estimates contained in earlier models of anomalous J/psi suppression in nuclear collisions., Comment: 19 pages, 5 eps figures included, three cosmetic changes, published version

We propose a simple Glauber-type mechanism for suppression of jet production up to transverse momenta of about 10 GeV/c at RHIC. For processes in this kinematic region, the formation time is smaller than the interval between two successive hard partonic collisions and the subsequent collision influences the jet production. Number of jets then roughly scales with the number of participants. Proportionality to the number of binary collisions is recovered for very high transverse momenta. The model predicts suppression of jet production in d+Au collisions at RHIC., Comment: 11 pages, 4 figures included via epsfig, note and references about related literature added

We propose an alternative way of looking at data on anomalous J/psi suppression. The proposed method is in principle equivalent to the one used by the NA50 Collaboration, but it permits to visualize separate contributions of individual processes responsible for the disintegration of J/psi's produced by a hard process in nuclear collisions. The method can be used provided that the time sequence of contributing mechanisms is known or assumed. It offers an alternative graphical presentation of the onset of anomalous J/psi suppression in Pb-Pb interactions observed by the NA50 Collaboration at the CERN SPS and might contribute to explain why different mechanisms, such as J/psi suppression by the Quark-Gluon Plasma and by co-movers in the Dual Parton Model or in Monte Carlo microscopic approaches, all lead to an approximate description of anomalous J/psi suppression., 15 pages 5 figures included via epsfig

The enhanced suppression of $J/\psi$ production at large $x_F$ in $pA$ collisions is studied in the framework of gluon depletion at large $x_1$. The nonperturbative process that modifies the gluon distribution as the gluons propagate in nuclear matter is described by an evolution equation with a kernal to be determined by phenomenology. With nuclear shadowing and anti-shadowing taken into account, the effect on the gluon distribution is shown to be a depletion in excess of 40% at $x_1 \approx 0.8$ for $A > 100$. There is a small amount of enhancement of the gluon distribution at small $x_1$, but it does not lead to any contradiction with the existing data on $J/\psi$ suppression in the central region. Extentions to $\psi^{\prime}$ suppression and $AB$ collisions are also investigated in the framework of gluon redistribution.

The NA57 experiment has been designed to study the onset of enhanced production of strange baryons and anti-baryons in Pb-Pb collisions with respect to p-Be collisions. Results from 40 and 160 A GeV/c Pb-Pb data will be shown. The main focus is on the checks of the NA57 analysis chain.

The procedure employed by the WA97 and NA57 experiments to determine the number of wounded nucleons in different Pb + Pb centrality classes is discussed. We will also compare different centrality scales and their influence on the measurement of the centrality dependence of strange particle yields.

We point out that data on the onset of anomalous J/psi suppression as a function of nucleon numbers A and B could provide information on the dynamics of nuclear interactions. In particular the models of anomalous J/psi suppression by Blaizot and Ollitrault (BO) and by Kharzeev, Lourenco, Nardi and Satz (KLNS) are based on different assumptions on the dynamics of nuclear collisions and lead to different predictions of the dependence of the onset of anomalous J/psi suppression on nucleon numbers of colliding nuclei. The data on this onset as function of A and B could become a tool for the study of the dynamics of nuclear collisions and bring further evidence on J/psi suppression by new form of hadronic matter, possibly Quark-Gluon Plasma. In particular we propose to study J/\psi suppression in A+Pb interaction with nucleon number of A between 58 and 73 or a bit higher., Comment: 10 pages, 5 Figures

A simulation study of a silicon pixel multiplicity detector proposed for the NA57 experiment at CERN SPS has been carried out to evaluate its capability to measure both the total multiplicity and the net baryon content created in Pb+Pb collisions at 40 and 160 GeV/ c per nucleon in order to get information on the specific entropy produced in the reaction. A method of charge assignment is proposed and discussed in detail and the results of the simulation are presented. A benchmark of the method using computer-generated events is also performed.

The nonlinear effect of gluon depletion in the collision of large nuclei can be large. It is due to multiple scatterings among comoving partons initiated by primary scattering of partons in the colliding nuclei. The effect can give rise to substantial suppression of $J/\psi$ production in very large nuclei, even if the linear depletion effect is insignificant for the collisions of nuclei of smaller sizes. This mechanism offers a natural explanation of the enhanced suppression in the Pb-Pb data recently observed by NA50., Comment: 6 pages + 2 figures (in ps file), LaTex, submitted to Phys. Rev. Lett

We point out that the recent data obtained by the NA35 collaboration at the CERN SPS on E T distributions in 16 O + Pb collisions can be understood within the wounded nucleon model with a simple geometrical calculation of the number of interacting nucleons in 16 O. Although the large E T trigger increases the average E T per final state hadron, we are unable to explain in this way the observed rather large value of this quantity.

Qualitative features of the preliminary data on large transverse-energy proton-Pb collisions at 200 GeV c obtained by the HELIOS collaboration at CERN are reproduced by a simple model in which the large transverse energy is built up by a large amount of soft final-state hadrons.

Basic assumption of multistring models can be tested by measuring correlations of total transverse energiesET andE′T deposited in two adjacent rapidity intervals. A high positive correlation ofET andE′T is caused by the fact that largeET is most likely connected with a large numbers of strings and this implies in turn high probability for largeE′T. We present specific predictions which could test the multistring model as applied to the recent results of the HELIOS collaboration at CERN onp-Pb interactions.

We present a set of semiquantitative predictions for the behaviour ofJ/Ψ suppression in heavy ion collisions as a function of the total transverse energy, nucleon numbers of colliding ions and theJ/Ψ transverse momentum. These predictions are based on the assumption that all ofJ/Ψ suppression is due toJ/Ψ and χ disintegrations in collisions with hadrons in a dense hadronic gas formed in the heavy ion collisions. The hydrodynamic evolution of the hadron gas is taken into account. The onset of the quarkgluon plasma formation should manifest itself by clear qualitative deviations fromJ/Ψ suppression calculated in this way, in particular as thresholds or kinks in theET-dependence, or as a threshold forJ/Ψ suppression as a function of the atomic number of the target, expected a Cu or below.