A New Approach of J/ψ suppression in pA and AA Collisions

When the cross section of J/ψ production is considered varying with the energy of the nucleon-nucleon interaction the production of J/ψ in pA and AA collisions has been studied using FRITIOF Model. The calculation shows that the cross section of J/ψ production “per nucleon-nucleon collision” decreases with increasing mass number and centrality as a consequence of continuous energy loss of the projectile nucleons to the target nucleons in their successive binary nucleon-nucleon collisions. We have compared our model predictions with the experimental data of J/ψ production. PACS number: 25.75.-q [email protected] [email protected] [email protected] Suppression of J/ψ production in high energy heavy ion collisions was proposed as an effective signature of QGP formation ten years ago [1]. The ensuing experimental data confirmed a significant suppression of J/ψ production in both pA and AA collisions [2]. However, alternative explanations of the J/ψ suppression exist based on the absorption of J/ψ in nuclear matter [3]. The overall set of extensive data collected and analysed by NA38 seems to support the absorption mechanism in p+Btarget, O+Btarget and S+U collisions. Nevertheless a rather large absorption cross section (∼ 6.2 mb) has to be used in order to fit the experimental data, about three times larger than the total J/ψ-N cross section from the EMC Collaboration [4]. Such a difference was already noted long time ago in [5]. Recent calculations based on the colour octet model show that this phenomenological cross section could be understood by the absorption of pre-resonance states (ccg) in nuclear matter [6]. Other sources of J/ψ suppression in nuclear collisions, such as the interaction of J/ψ particles with the produced mesons (called comover) [7], gluon shadowing in nuclei, intrinsic charm component, energy degradation of produced cc̄ pair, etc. [8], have been introduced besides the absorption due to the J/ψ-N interaction to explain the data. We in this paper propose a simple model to investigate J/ψ suppression in pA and AA collisions focusing on the decrease of the cross section of J/ψ production with increasing mass number and centrality due to the continuous energy loss of the projectile nucleons to the target nucleons in their successive binary nucleon-nucleon collisions, not on its later absorption in nuclear matter. From the calculations of this model, we conclude that the absorption of J/ψ particles in nuclear matter is only part of the sources of J/ψ suppression seen in pA and AA collisions so far. Since the probability for a nucleon-nucleon collision leading to J/ψ production is very small it is generally accepted that multiple J/ψ production processes in pA and AA collisions can be neglected. The studies of J/ψ suppression based on the absorption mechanism actually assume that the probability of J/ψ production “per nucleon-nucleon collision” is the same independent of the masses of the colliding nuclei at a given energy. However, each binary nucleon-nucleon collision experienced by a projectile nucleon on its way out of the target in pA and AA collisions may not be the same if the projectile nucleon loses a fraction of its energy in each binary collision to the target nucleon such that the cms energy of each binary collision of this incoming nucleon with a target nucleon is different. For the cross sections of the hard QCD parton-parton scatterings, which is responsible for J/ψ production, increase with increasing energy it is conceivable that the probability of J/ψ production would also depend on the energy of a nucleon-nucleon collision in a similar way. It is confirmed both theoretically and experimentally that J/ψ photoproduction cross section exhibits a strong threshold behaviour at the low energy region and then increases with energy at the higher energy region [9]. In a participant-spectator model of nucleus-nucleus collisions, like FRITIOF, each projectile nucleon may collide several times with the target nucleons. If momentum transfers are assumed to take place in each binary nucleon-nucleon collision, then the cms energy of these binary collisions will decrease with time, thereby the probability to produce J/ψ in each binary collision going down. We in this paper have calculated how the cross section of J/ψ production varies with increasing mass number of colliding nuclei and centrality using FRITIOF model. The calculations show that J/ψ production is suppressed in pA and AA collisions in comparison with the nucleon-nucleon collision, and the larger the mass number of colliding nuclei and the centrality, the greater the suppression, as a result that probability of finding a QCD hard process in a binary nucleon-nucleon decreases with increasing mass number of colliding nuclei and centrality. Taking into account the J/ψ absorption by J/ψ-N interaction our results are in good agreement with experimental data with the exception