The Heavy Ion group of the Weizmann Institute is engaged in the study of relativistic
nuclear collisions in the framework of the CERES experiment at CERN.
The main motivation is the study of hadronic matter under extreme conditions of density and temperature. Under these conditions nuclear matter is predicted to undergo a phase transition to a deconfined state, commonly referred to as a quark-gluon plasma, where quarks and gluons are free to move over a large volume compared to the typical size of hadrons. A second phase transition, that of chiral symmetry restoration, where masses drop to xero, is predicted by QCD lattice calculations to take place under similar or even identical conditions.
CERES is an experiment dedicated to the measurement of low-mass (m < 1 GeV/c2) e+e- pairs and direct photons emitted in ultra-relativistic heavy ion collisions. These observables are considered useful probes of the dynamics of the collisions and in particular the early stages where the conjectured phase transition(s) are expected to take place.
CERES has completed a systematic physics programme with p, S and Pb beams, including the measurements of electron pairs in p-Be (a very good approximation to p-p collisions) and p-Au collisions at 450 GeV, S-Au collisions at 200 GeV/nucleon and Pb-Au collisions at 158 GeV/nucleon. This systematic approach has revealed a very interesting result: the observation of a strong enhancement of low-mass electron pairs in S-Au and Pb-Au collisions suggesting the onset of a new source, beyond the mere superposition of p-p collisions.
These observations have triggered an enormous attention and a huge theoretical acitivity. The characteristic features of the excess - its onset at a mass mee~ 2 m, its extension to the low-mass region below and around the rho-meson, and the possibility of a quadratic dependence with multiplicity - suggest that the excess is due to the two-pion annihilation channel, + - > e+e-. Theoretical calculations show indeed that a large fraction of the excess originates from this channel providing first evidence of thermal radiation emitted by the high density hadronic matter formed in these collisions. However, in order to fully account for the excess, the models invoke also in-medium modifications of the vector mesons and in particular a decrease of the -meson mass, as a precursor of chiral symmetry restoration.
The exciting possibility that first hints of chiral symmetry restoration might have been observed in the S and Pb data calls for a vigorous continuation of the experimental programme. In the short range, we are engaged now in a major upgrade of the CERES apparatus to achieve a mass resolution m/m = 1% at m = 1 GeV/c2. With this resolution, which is of the order of the natural line width of the -meson, we plan to directly measure the yield of all three vector mesons , , and including any possible changes in their properties. The observation of mass shifts or increased widths for the , , and -mesons will provide compelling evidence for the scenarios invoking chiral symmetry restoration. We plan therefore to repeat the measurements of low-mass pairs in Pb-Au collisions at 158 GeV/nucleon with the upgraded spectrometer. In addition to that, we plan to run the upgraded experiment at a lower beam energy, ~ 30 - 40 GeV/nucleon in order to study the dependence of the effect on the baryon density.
In a longer time scale, we plan to extend the measurements of lepton pairs in the framework of the PHENIX experiment, one of the two major experiments approved to run at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory to study Au-Au collisions at a center-of-mass energy of 200 GeV/nucleon. RHIC offers much better conditions to detect the phase transitions and to measure the properties of the deconfined state (much larger energy densities, much larger volumes and much longer-lived system). The starting up of RHIC is expected in the second half of 1999, well matched to the completion of the CERES experimental programme by the year 2000.
The PHENIX detector is designed to measure all aspects of the Au-Au collisions at RHIC, and it is particularly focussed in the measurement of electron pairs, muon pairs, and photons in the different parts of the detector. The Weizmann Institute Heavy-Ion group joined the PHENIX collaboration in 1997 and it took upon itself the responsibility for the design and construction of the first or innermost set of pad-chambers, called PC1 which are an essential element of the global tracking system of the PHENIX detector. Once the construction effort appraches completion, our main interest will be in developing a scheme to upgrade the baseline detector of PHENIX to allow the detection of low-mass electron pairs (which cannot be done in the present configuration of the detector). In view of the CERES results and in view of the fact that PHENIX is the only experiment at RHIC devoted to the measurement of lepton pairs, it is imperative for PHENIX to develop such a scheme. Several groups within PHENIX are interested and have been working in the past on this important issue and we plan to work with them in this challenging development project.
In our participation in the PHENIX experiment we foresee a particularly strong collaboration with the Stony Brook group of the PHENIX Collaboration.
[Back to main CERES page.]