J/ψ production in PHENIX

Heavy quarkonia production is expected to be sensitive to the formation of a quark gluon plasma (QGP). The PHENIX experiment has measured J/ψ production at √ sNN = 200 GeV in Au+Au and Cu+Cu collisions, as well as in reference p+p and d+Au runs. J/ψ’s were measured both at mid (|y| < 0.35) and forward (1.2 < |y| < 2.2) rapidity. In this letter, we present the A+A preliminary results and compare them to normal cold nuclear matter expectations derived from PHENIX d+Au and p+p measurements as well as to theoretical models including various effects (color screening, recombination, sequential melting...). PACS. PACS-key discribing text of that key – PACS-key discribing text of that key PHENIX is one of the experiments located at the Relativistic Heavy Ion Collider (RHIC) of Brookhaven National Laboratory. It has the capability of measuring quarkonia through their dilepton decay in four spectrometers: two central arms covering the mid-rapidity region of |η| < 0.35 and twice π/2 in azimuth, and two forward muon arms covering the full azimuth and 1.2 < |η| < 2.2 in pseudorapidity. Electrons are identified in the central arms by their Čerenkov rings and by matching the momentum of charged particles reconstructed in drift chambers with the energy deposited in an electromagnetic calorimeter. In the forward arms, muons are selected by an absorber and identified by the depth they reach in a succession of proportional counters staggered with steel walls. The event vertex and centrality are measured by beam-beam counters situated at 3 < |η| < 3.9. For A+A collisions, the centrality measurement is further refined by the use of two zero degree calorimeters located downstream the beams. A detailed description of the PHENIX apparatus can be found in [1]. This letter is organized as follows: we first set up the references by looking at d+Au data (section 1) and compare the cold nuclear matter effects derived from it to A+A data (section 2). In section 3, we compare our results to what was observed at the CERN SPS. We then review in section 4 the state of the art of the possible explanations of our observed suppression. Kinematic distributions could help distinguishing between models and we present our rapidity (section 5) and mean squared transverse momentum (section 6) before to conclude in section 7. Note that all A+A J/ψ data shown below are PHENIX preliminary. Send offprint requests to: 1 J/ψ production in d+Au collisions The nuclear modification factor RAB for any A+B collision type is defined as the J/ψ yield observed in these collisions, divided by the yield measured in a p+p run and scaled by the average number of nucleon-nucleon collisions 〈Ncoll〉 extracted from a Glauber model: RAB = dN J/ψ AB 〈Ncoll〉 × dN pp (1) Departure from RAB = 1 implies some nuclear modifications of the produced J/ψ’s. The PHENIX collaboration has measured the J/ψ nuclear modification factor for d+Au collisions [2]. Its centrality dependence is shown on figure 1 for our three rapidity ranges. In d+Au collisions at 200 GeV, J/ψ’s in our three rapidity ranges probe the following momentum fractions x of gluons in the gold nucleus (neglecting the emitted gluon): 0.05 to 0.14 (upper panel, negative rapidity, gold-going side), 0.011 to 0.022 (midrapidity) and 0.0014 to 0.0047 (lower panel, positive rapidity, deuteron-going side). The difference observed between forward (lower panel) and backward (upper panel) yields indicate that shadowing and/or anti-shadowing are at play. These effects are modifications of the parton distribution functions in nuclei with respect to the usual functions in free nucleons. In particular, one expect a depletion (shadowing) at low x due to overlapping and recombining partons (gluons in the case of J/ψ). The strength of gluon shadowing is not heavily constrained by theory and models predictions [3,4,5] differ by a factor of three. Comparison with Vogt’s theoretical predictions [6] shows that a modest amount of absorption (0 to 3 mb), added 2 Raphaël Granier de Cassagnac, for the PHENIX collaboration: J/ψ production in PHENIX 0 4 8 12 16 20 Number of Collisions 0 0.4 0.8 μμ (Y = 1.8) μμ MB (Y = 1.8) 0.4 0.8 R dA ee (Y = 0) ee MB (Y = 0) 0.4 0.8 1.2 μμ (Y = -1.7) μμ MB (Y = -1.7) R dA PHENIX nucl-ex/0507032 Fig. 1. J/ψ nuclear modification factor RdA as a function of centrality (given here by the number of collisions Ncoll) for backward (top, y = −1.7) mid (middle, y = 0) and forward (bottom, y = 1.8) rapidities. The histograms are the distributions of Ncoll for each of our four centrality classes. Theoretical curves are from Vogt [6] and take EKS shadowing and 0, 1, 2 or 3 mb normal absorption cross section (from top solid to bottom dot-dashed). to a modest amount of shadowing such as the one provided by the Eskola-Kolhinen-Salgado (EKS [3]) scheme, can describe both the rapidity and centrality dependencies, as shown on figure 1. 2 Normal nuclear matter effects in A+A To extrapolate the effects seen in d+Au to A+A collisions, one has to rely on a model. Together with our Au+Au (∼ 0.2 nb) and Cu+Cu (∼ 3 nb) measurements [7], Au+Au predictions from such a cold nuclear matter model [8] are presented on figure 2, as a function of centrality (displayed here as the number of participants). They account for shadowing (following the EKS prescription) and nuclear absorption (1 mb for solid lines and 3 mb for dashed lines). Predictions are shown for two rapidity values (y = 0 and y = 2) corresponding to our two rapidity regions. The difference between the 1 mb and the 3 mb curves give an idea of our current uncertainty on the cold nuclear matter effects affecting the J/ψ production in A+A collisions. However, in both rapidity ranges, our most central Au+Au measurements depart from the predictions made with the stronger nuclear effects (3 mb), suggesting that other suppression mechanisms are involved, namely, an anomalous suppression. part N 0 50 100 150 200 250 300 350 400 A A R 0 0.2 0.4 0.6 0.8 1 1.2 =3mb) abs σ R. Vogt, nucl-th/0507027 (y=2, =1mb) abs σ R. Vogt, nucl-th/0507027 (y=2, =3mb) abs σ R. Vogt, nucl-th/0507027 (y=0, =1mb) abs σ R. Vogt, nucl-th/0507027 (y=0,