R. Arnaldi

First Measurement of the

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

\rho

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.

Spectral Function in High-Energy Nuclear Collisions

Abstract.The production of

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

{\rm J}/\psi

Charmonium production and nuclear absorption in p-A interactions at 450 GeV

and

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

\psi^\prime

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

charmonium states in proton-nucleus interactions has been investigated by the NA50 experiment, at the CERN SPS. High statistics data sets were collected with collisions induced by 450 GeV protons incident on Be, Al, Cu, Ag and W targets. The

J/ψ production in p-A and A-A collisions at fixed target experiments

{\rm J}/\psi

A low-resistivity RPC for the ALICE dimuon arm

and

A dual threshold technique to improve the time resolution of resistive plate chambers in streamer mode

\psi^\prime