document: Production of charmonium states J/ψ and ψ′ in nucleus–nucleus collisions has been studied at CERN SPS over the previous 15 years by the NA38 and NA50 Collaborations. This experimental program was mainly motivated by the suggestion [1] to use the J/ψ as a probe of the state of matter created at the early stage of the collision. The original picture [1] (see also [2] for a modern review) assumes that charmonia are created exclusively at the initial stage of the reaction in primary nucleon–nucleon collisions. During the subsequent evolution of the system, the number of hidden charm mesons is reduced because of: (a) absorption of pre-resonance charmonium states by nuclear nucleons (normal nuclear suppression), (b) interactions of charmonia with secondary hadrons (comovers), (c) dissociation of cc̄ bound states in deconfined medium (anomalous suppression). It was found [3] that J/ψ suppression with respect to Drell–Yan muon pairs measured in proton–nucleus and nucleus–nucleus collisions with light projectiles can be explained by the so-called “normal” (due to sweeping nucleons) nuclear suppression alone. In contrast, the NA50 experiment with a heavy projectile and target (Pb+Pb) revealed essentially stronger J/ψ suppression for central collisions [4–7]. This anomalous J/ψ suppression was attributed to formation of quark–gluon plasma (QGP) [7], but a comover scenario cannot be excluded [8].
doc_bio_tags: O O B O O O B O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O B O O O O O O O O O O O B I I O O O O O O O O O O B I I O O O O O O O O B O O B I B I I O O O B O O B B O O O B O O O O O B I O O O O O O O O O O B I I O O B O B I O B I O O O O O O O O O B I B I O O O O B I O O B I I I O O O O O O O B I O O O O O O O O B I I I B O O O B I O O O O
extractive_keyphrases: anomalous suppression cc̄ central collisions charmonia charmonium comovers comover scenario drell–yan muon pairs experimental program formation of quark–gluon plasma hadrons heavy projectile and target hidden charm mesons light projectiles na50 experiment normal nuclear suppression nuclear nucleons nuclear suppression nucleus–nucleus collisions original picture primary nucleon–nucleon collisions proton–nucleus qgp sweeping nucleons ψ′
abstractive_keyphrases: absorption of pre-resonance charmonium states by nuclear nucleons charmonium states dissociation of cc̄ bound states in deconfined medium interactions of charmonia with secondary hadrons j/ψ j/ψ suppression nucleon nucleus pb+pb production of charmonium states j/ψ and ψ′ in nucleus–nucleus collisions quark–gluon plasma subsequent evolution of the system, use the j/ψ as a probe of the state of matter created at the early stage of the collision