# Etude de la suppression de la résonance $J/\psi$ en fonction de la centralité de la collision mesurée à l'aide du calorimètre électromagnétique dans les réactions Pb-Pb

Abstract : Theoretical predictions of a new state of matter, the gluon quark plasma, led the CERN to conduct a vast experimental research campaign to prove its existence. Collisions of heavy ions can lead to this phase transition. Among the proposed signatures, experiment NA50 studies the production of the J/psi relative to DRELL-YAN. The J/psi is a ccbar resonance which undergoes strong interaction and modifications of properties of the nuclear medium. DRELL-YAN, a purely electromagnetic process, serves as a reference.
Experiment NA38 measured a normal suppression of J/psi production depending on the average length of crossed matter. In its prolongation, experiment NA50 highlighted in 1995, 1996 and 1998 an "abnormal" J/psi suppression in Pb-Pb collisions at 158 GeV/nucleon. This analysis is carried out according to the centrality of the collision measured by the electromagnetic calorimeter. Information from this detector makes it possible to calculate neutral transverse energy connected to the average length of crossed matter.
In 2000, conditions of optimal data acquisitions in Pb-Pb collisions made it possible to analyze data for very peripheral collisions. Systematic sources of error were studied, which made it possible to obtain a good compatibility between the various analyses of the Pb-Pb system. Peripheral collisions are compatible with the normal principle of absorption.
In conclusion, we highlighted a suppression of ~31% in J/psi production relative to normal absorption. This change of regime takes place at ~30 GeV in transverse energy (E_T) and is due to a modication of nuclear properties of the reactional medium. Several possible scenarios have been advanced to explain this phenomenon.
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Submitted on : Tuesday, September 16, 2003 - 2:10:37 PM
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• HAL Id : tel-00003380, version 1

### Citation

Carl Castanier. Etude de la suppression de la résonance $J/\psi$ en fonction de la centralité de la collision mesurée à l'aide du calorimètre électromagnétique dans les réactions Pb-Pb. Physique Nucléaire Théorique [nucl-th]. Université Blaise Pascal - Clermont-Ferrand II, 2003. Français. ⟨tel-00003380⟩

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