The anomalous suppression of J/[psi] production in Pb-Pb collisions at 158 GeV observed by the NA50 Collaboration can be explained as due to the occurrence of a new phase of strong J/[psi] absorption, which sets in when the local energy density exceeds about 3.4 GeV/fm[sup 3]. The peculiar behavior of the[psi][sup']/[psi] ratio in[ital p]-A and nucleus-nucleus collisions can be understood as due to approximately equal[psi]-N and[psi][sup']-N absorption cross sections, but greater absorption cross sections for[psi][sup'] than J/[psi] with regard to absorption by soft particles and matter in the new phase.

The authors review the search for the quark-gluon plasma using the signal of the suppression of J/[psi] production in high-energy heavy-ion collisions. Recent anomalous J/[psi] suppression in high-energy Pb-Pb collisions observed by the NA50 Collaboration are examined and compared with earlier results from pA and nucleus-nucleus collisions with heavy ions of smaller mass numbers. The anomalous suppression of J/[psi] production in Pb-Pb collisions can be explained as due to the occurrence of a new phase of strong J/[psi] absorption, which sets in when the number of nucleon-nucleon collisions at a spatial point exceeds about 4 and corresponds to a local energy density of about 3.4 GeV/fm3.

The level of 'anomalous' charmonium suppression in high-energy heavy-ion collisions and its interpretation as a signal of quark-gluon plasma formation requires a robust understanding of charmonium production and absorption in proton-nucleus collisions. In a previous study we have shown that, contrary to common belief, the so-called J/[psi] 'absorption cross section', [sigma]{sub abs}{sup J/[psi]}, is not a 'universal constant' but, rather, an effective parameter that depends very significantly on the charmonium rapidity and on the collision energy. Here we present ugraded Glauber calculations with the EPS09 parameterization of nuclear modifications of the parton densities. We confirm that the effective 'absorption cross section' depends on the J/{psi} kinematics and the collision energy. We also make further steps towards understanding the physics of the mechanisms behind the observed 'cold nuclear matter' effects.

J/psi and psi' produced in high-energy heavy-ion collisions are absorbed by their collisions with nucleons and produced soft particles, leading to two distinct absorption mechanisms. The signature of absorption by produced soft particles, as revealed by psi' production data, consists of a gap and a change of the slope in going from the pA line to the nucleus-nucleus line when we make a semi-log plot of the survival probability as a function of the path length. Using this signature, we find from the J/psi production data in pA, O-Cu, O-U, and S-U collisions that the degree of J/psi absorption by produced soft particles is small and cannot account for the J/psi data in Pb-Pb collisions. The anomalous suppression of J/psi production in Pb-Pb collisions can be explained as due to the occurrence of a new phase of strong J/psi absorption, which sets in when the local energy density exceeds about 3.4 GeV/fm3. To probe the chemical content of the new phase, we propose to study the abundance of open-charm mesons and charm hyperons which depends sensitively on the quark chemical potential.

13. High-energy heavy-ion collisions and quark-gluon plasma. 13.1. Nuclear stopping power and the baryon content. 13.2. Bjorken's estimate of the initial energy density in high-energy nucleus-nucleus collisions. 13.3. Hydrodynamics of the quark-gluon plasma -- 14. Signatures for the quark-gluon plasma (I). 14.1. Dilepton production in the quark-gluon plasma. 14.2. Dilepton production from other processes. 14.3. Spectrum of dileptons -- 15. Signatures for the quark-gluon plasma (II). 15.1. Debye screening in the quark-gluon plasma. 15.2. J/[symbol] suppression in the quark-gluon plasma. 15.3. Experimental information on J/[symbol] production and J/[symbol] suppression. 15.4. J/[symbol] suppression in hadron environment. 15.5. Transverse momentum distribution of J/[symbol] particles -- 16. Signatures for the quark-gluon plasma (III). 16.1. Photon production in the quark-gluon plasma. 16.2. Photon production by quark-antiquark annihilation. 16.3. Photon production by q[symbol] annihilation in the plasma. 16.4. Photon production by the Compton process. 16.5. Photon production in the plasma due to the Compton process. 16.6. Photon production by hadrons. 16.7. Photon production by parton collisions. 16.8. Experimental information on photon production -- 17. Signatures for the quark-gluon plasma (IV). 17.1. The Hanbury-Brown-Twiss effect of intensity interferometry. 17.2. Pion momentum correlations from chaotic sources. 17.3. Coherent and partially coherent sources. 17.4. Experimental information on pion interferometry -- 18. Signatures for the quark-gluon plasma (V). 18.1. Strangeness content in matter at thermal and chemical equilibrium. 18.2. Rate of approach to chemical equilibrium in a quark-gluon plasma. 18.3. Experimental information on strangeness production -- 19. Summary

The observed features of[psi]' to J/[psi] suppression in pA and nucleus-nucleus collisions can be explained in terms of a two-component absorption model. For the hard component of the absorption due to the interaction of the produced c[bar c] systems with baryons at high relative energies, the absorption cross sections are insensitive to the radii of the c[bar c] systems, as described by the Additive Quark Model. For the soft component due to the low energy c[bar c] interactions with soft particles produced by other baryon-baryon collisions, the absorption cross sections are greater for[psi]' than for J/[psi], because the breakup threshold for[psi]' is much smaller than for[psi].

ABSTRACT: Mesons composed of heavy quark-antiquark pairs, known as quarkonia, provide the only direct probe of the screening length in the deconfined state of quarks and gluons, known as the quark gluon plasma (QGP), which is believed to be produced in high energy heavy ion collisions. However, the observation of suppression of quarkonia production in heavy ion collisions at high energies is complicated by the modification of quarkonia production in normal nuclear matter. Measuring the modification of quarkonia production due to the effects of normal nuclear matter, often termed cold nuclear matter (CNM) effects, provide a critical baseline for understanding the properties of the QGP. Measurements of CNM effects on quarkonia production are also interesting in their own right, and can be measured independently in proton-nucleus (p+A) collisions. The modification of quarkonia production in p+A collisions provides insight into quarkonia production mechanisms unavailable through the study of proton-proton collisions alone. The study of quarkonia production in p+A collisions over a wide range of kinematic variables can also provide constraints on the modification of parton distribution functions in nuclei. In order to quantify the CNM effects present at the Relativistic Heavy Ion Collider (RHIC), the PHENIX experiment has recorded data on d+Au collisions at \sqsn=200 GeV. The analysis of J/psi→e+e- and psi'→e+e- production from that data set is presented here. Both J/psi and psi' production are found to be suppressed in d+Au relative to p+p collisions, with the suppression increasing for collisions with small impact parameters. The psi' production is found to be much more suppressed than J/psi production, a result which is unexpected based on measurements at lower collision energy and present theoretical pictures. A parametrization of the J/psi modification measured by PHENIX in terms of two CNM effects is also presented. One is the nuclear breakup of the forming quarkonium state through collisions with nucleons during the d+Au collision. The other is the modification of the gluon distribution in the Au nucleus. It is found that the two effects can be separated due to the very different impact parameter dependencies. A strongly non-linear geometric dependence on the modification of the gluon distribution function is observed, with the modification found to be concentrated near the center of the Au nucleus. This parametrization is also used to estimate the modification of J/psi production in Au+Au collisions due to CNM effects. This modification is compared to PHENIX measurements of J/psi production in Au+Au collisions. Suppression of J/psi production in Au+Au collisions beyond CNM effects is observed. This excess suppression is interpreted as suppression of J/psi production due to the formation of a QGP.