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

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.

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.

We present a quark-parton model for reactions π − p → μ + μ − + hadrons and π − p → hadrons. The model can give detailed prescriptions about relationships between the two reactions. In particular it predicts that the difference of charge distributions d Q /d y in the two reactions will give a direct and unambiguous determination of the charge of the up-quark.

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.

The expression for the Drell-Yan cross section is presented which takes into account the geometry of heavy ion collisions. The Drell-Yan cross section is shown to be proportional to the transverse energy produced within a fixed rapidity interval. A dependence steeper than linear of the cross section for production of dileptons with masses larger than about 2 GeV on the transverse energy in a given rapidity bin would be a clear-cut signal for the formation of the quark-gluon plasma.

We argue that J ψ suppression observed recently in high energy oxygen-uranium collisions by the NA38 Collaboration at CERN need not imply that the quark-gluon plasma was produced. A considerable part of the effect may be caused by the disintegration of J ψ 's in collisions with dense hadronic matter (below the phase transition into quark-gluon plasma) produced simultaneously with J ψ in heavy ion interactions.