B. Cheynis

Observation of a threshold effect in the anomalous J/ψ suppression

Abstract We report on a search for a phase transition from ordinary nuclear matter to a state of deconfined quarks and gluons as predicted by lattice QCD calculations. A new measurement of charmonium production in Pb-Pb interactions at 158 GeV/ c per nucleon agrees with our previous results and confirms the anomalous J/ ψ suppression we had already observed on a significantly smaller data sample. New event selection and analysis techniques show that, for peripheral collisions, the J/ ψ cross-section per nucleon-nucleon collision agrees with the precise suppression pattern inferred from a wide range of measurements extending from p-p up to S-U collisions. As the collisions become more central, the Pb-Pb cross-section exhibits a clear departure from this normal behaviour. The onset of the anomalous J/ ψ suppression reported here is the first clear observation of a threshold effect in heavy ion collisions and can be considered as a strong indication of the production of a deconfined quark-gluon phase in central Pb-Pb collisions.

Transverse momentum distributions of J/ψ, ψ′, Drell–Yan and continuum dimuons produced in Pb–Pb interactions at the SPS

Muon pairs produced in Pb-Pb interactions at 158~GeV/

The particle detector array AMPHORA

c

Cross talk and diaphony in neutron detectors

per nucleon are used to study the transverse momentum distributions of the \jpsi, \psip\ and dimuons in the mass continuum. In particular, the dependence of these distributions on the centrality of the Pb-Pb collision is investigated in detail.

Intermediate mass muon pair continuum in Pb-Pb collisions at 158 GeV c

Abstract A 4π-multidetector consisting of 140 CsI(Tl) detectors has been designed and built to be used for detecting and identifying either light charged particles (with a detection threshold of about 0.4 MeV/nucleon) and neutrons or light heavy ions and light charged particles, in the range of energies available at SARA (from 10 to 40 MeV/nucleon). The choice of the scintillator material is discussed and a description of the mechanical and electronic design is given. First experimental results obtained with this device are presented.

Transverse momentum distribution of J/Ψ produced in PbPb and p-A interactions at the CERN SPS

Abstract A neutron which enters a detector can be scattered into a neighbouring detector. If the neutron is registered in each detector the effect will be calle

α-Emission temperatures, coulomb instabilities and nuclear cracking

The NA50 experiment measures, at CERN SPS, the dimuon production in PbPb collisions at 158 GeV/c incident momentum. In this paper an analysis of the intermediate mass region (1.5⩽Mμμ⩽2.5 GeV/c2) is presented. Known dimuon sources in this interval of the spectrum are the e.m. annihilation of qq pairs (i.e. Drell-Yan) and the semileptonic decay of charmed hadrons. The contribution of these processes is calculated by means of a fit to the measured invariant mass distribution (for Drell-Yan) and of a linear extrapolation of p-A experimental results obtained by NA38 (for charm). It will be shown that known sources do not account for all of the dimuon yield measured by NA50, i.e. an excess is observed. The size of the excess measured in PbPb is larger than the one seen by NA38 in SU collisions. The centrality dependence of this effect, both in SU and PbPb, is also discussed.

The NA50 segmented target and vertex recognition system

Abstract Muon pairs produced in Pb Pb interactions at 158 GeV/c per nucleon and in p-A interactions at 400 GeV/c, together with older S-U results obtained at 200 GeV/c are used to study the transverse momentum and transverse mass distributions of the J/Ψ.

Measurement of the production of high-

Abstract The α-emission temperatures of medium mass nuclei with masses near 140, determined at excitation energies from 2 to 4.3 MeV/nucleon, suggest a rapid increase in the heat capacity of the composite system consistent with model predictions of the onset of a cracking phase transition or fragmentation. The limiting value of the temperature, near 6 MeV, provides evidence for a soft nuclear equation of state ( K ⋍ 220 MeV ) at these high excitation energies.

p_{\rm T}

The NA50 segmented target and vertex recognition system is described. The segmented target consists of 7 sub-targets of 1–2 mm thickness. The vertex recognition system used to determine the sub-target where an interaction has occured is based upon quartz elements which produce Cerenkov light when traversed by charged particles from the interaction. The geometrical arrangement of the quartz elements has been optimized for vertex recognition in 208PbPb collisions at 158 GeV/nucleon. A simple algorithm provides a vertex recognition efficiency of better than 85% for dimuon trigger events collected with a 1 mm sub-target set-up. A method for recognizing interactions of projectile fragments (nuclei and/or groups of nucleons) is presented. The segmented target allows a large target thickness which together with a high beam intensity (≈ 107 ions/s) enables high statistics measurements.