Department of Particle Physics and Astrophysics

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Itzhak Tserruya

Professor Emeritus
Location:Edna and K.B. Weissman Building of Physical Sciences, Room 1102

The CERES and PHENIX Experiments

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 and the PHENIX experimental the RHIC (Relativistic Heavy-Ion Collider) accelerator at BNL. 

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 into a new state of matter, the 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 zero, is predicted to take place under similar or even identical conditions.

CERES is an experiment dedicated to the measurement of low-mass (m < 1.5 GeV2) 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, 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 A GeV and Pb-Au collisions at 158 A GeV.   A very interesting result has emerged from this  study, namely the observation of a strong enhancement of low-mass pairs in S and Pb induced  collisions, whereas no enhancement was observed in p-Be and p-Au collisions. The onset of the excess, starting at a mass of m ~ 2mπ and indications of a quadratic dependence of the yield with the event multiplicity, suggest the opening of the two-pion annihilation channel (π+π- →  e+e- ). This would then be the first evidence of thermal radiation emitted by dense hadronic matter formed in relativistic heavy-ion collisions. Furthermore, the CERES results seem to hint at in-medium modifications  of the vector meson properties, in particular of the r meson as precursor of chiral symmetry restoration. These results have prompted us to engage in a major upgrade of the CERES apparatus with the addition of a radial TPC behind the double RICH spectrometer. The goal is to considerably improve the mass resolution of the  spectrometer such that possible mass shifts or changes in width of the vector mesons ρ,  ω and Φ can be directly observed, thereby seeking direct evidence  for the scenarios invoking in-medium modifications of the vector meson properties.

The group is also participating in the PHENIX experiment, one of the two major experiments approved to run at RHIC, scheduled to start operations in the year 2000. RHIC will allow the study of Au-Au collisions at a center-of-mass energy of 200 GeV per nucleon, offering much better conditions to detect the phase transitions and to measure the properties of the deconfined state. The PHENIX detector is designed to measure all aspects of the Au-Au collisions at RHIC, and is particularly focused on the measurement of high-mass (m > 1 GeV/c2) electron and muon pairs, and photons  in the different parts of the detector.  The Weizmann Institute Heavy-Ion group 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.  In addition to that, the group is interested in  the development of an upgrade scheme of the baseline PHENIX detector of  to allow  the detection of low-mass electron pairs.