J/ψ Measurements in PHENIX
By Tony Frawley
At high enough temperature and density, when the average
distance between quarks and gluons in nuclear matter becomes
smaller than the average size of a hadron, the color force that
holds hadrons together is screened by nearby quarks and gluons
in the nuclear matter. Deconfinement occurs when this screening
becomes so strong that the light quark hadrons become unbound.
Two decades ago, Matsui and Satz proposed verifying deconfinement directly by looking for the disappearance of the J/ψ in central heavy ion collisions. The J/ψ is a (charm, anti-charm) meson that can decay into an electron positron pair, or a muon anti-muon pair, so it is easy to reconstruct experimentally. But most importantly, creating charm quarks requires so much energy that they are produced only in the initial nucleon-nucleon collisions as the beam particles pass through each other. So there is a fixed number of charm anti-charm pairs created in the initial collision, a small fraction of which will normally form J/ψ. Matsui and Satz proposed that if deconfined matter was created in a collision, it would prevent charm anti-charm pairs binding into a J/ψ , suppressing J/ψ production.
Things have become more complicated since then. First, Lattice Gauge calculations now indicate that the J/ψ remains bound up to 1.5 to 2 times the deconfinement temperature (the J/ψ is very small in radius, and would reasonably require a higher density to become unbound due to screening). The maximum temperature reached in central Au+Au collisions at RHIC is thought to be ~ 2 times the transition temperature, so it is now not clear if the J/psi is expected to melt at RHIC. To make things more complicated, some higher states that feed down to the J/ψ are expected to melt just above the deconfinement temperature. Second, the large charm quark production cross section at RHIC leads to predictions that J/ψ will be formed by random coalescence of unrelated charm pairs in central Au+Au collisions, even if the initial group of forming J/ψ is destroyed in deconfined matter. And finally, modifications of gluon densities at low momentum-fraction in heavy nuclei are expected to start to be significant at RHIC and perhaps modify the initial charm anti-charm production cross sections. Nevertheless, studying J/ψ production promises to teach us a great deal about the conditions in deconfined matter.
PHENIX can reconstruct the J/ψ at mid rapidity using electron positron pairs in the central arms, or at forward and backward rapidity using muon anti-muon pairs in the muon arms. Since 2001, a coordinated set of measurements has been made that were aimed at characterizing the effect of the nuclear medium on J/ψ production. The baseline J/ψ production rate has been measured in pp collisions in RHIC runs 2-6. Effects due to having a nucleus in the initial state, called "cold nuclear matter effects", were studied using d+Au collisions in Run 3. And the effects of adding hot nuclear matter in the final state were studied in Cu+Cu collisions in Run 5, and Au+Au collisions in Runs 2, 4 and 7.
Figure 1: J/ψ rapidity distribution from Run 5 pp data. The curves were used in the estimation of the total cross section. (enlarge)
The rapidity distribution in pp collisions from Run 5 is shown in the first figure. This is the measured reference for J/ψ production in a vacuum. To quantify effects due to the presence of nuclear matter, heavy ion data are binned in collision centrality - characterized by the number of participating nucleons. The cross section in each centrality bin is divided by the estimated number of nucleon-nucleon collisions, then by the pp cross section. This quantity, the nuclear modification factor RAA, would be unity if there were no effects due to the presence of nuclear matter, since J/ψ production is a "hard" process that should scale with the number of nucleon-nucleon collisions.
Figure 2: RAA for J/ψ in d+Au collisions versus rapidity. (enlarge)
The RAA for d+Au collisions, using the Run 3 pp reference, is shown in the second figure as a function of rapidity. The asymmetry between the Au-going direction and the d-going direction for the d+Au data in the theory is due to gluon saturation effects in the entrance channel. There is also an ad-hoc J/ψ breakup cross section to account for collisions between the forming J/ψ and nucleons in the colliding nuclei that have still to pass through the production point. The figure shows that the data are not precise enough to determine the breakup cross section at RHIC very well, although they did eliminate some models.
The third figure shows the Au+Au RAA from Run 4, using the Run 5 pp reference, as a function of collision centrality. The most striking feature is that the RAA value is smaller at forward rapidity than at mid rapidity for more central collisions. Because the effective energy density is largest at mid rapidity, this seems to eliminate any model in which J/ψ suppression simply increases with energy density. The observed behavior could result from a large coalescence contribution, which would be strongest at mid rapidity. But there are still large uncertainties in the baseline cold nuclear matter calculations, due to the lack of very precise d+Au data. These uncertainties make it impossible so far to rule out other explanations, such as a lower initial production rate of J/ψ at forward rapidity due to shadowing effects (the Color Glass Condensate model, for example, would suggest this). The need for much higher statistics d+Au data is clear, and PHENIX has requested a d+Au run for Run 8 that would increase the J/ψ yield by a factor of 20 over Run 3.
Figure 3: RAA for J/ψ in Au+Au collisions versus average number of participating nucleons in the collision. (enlarge)
In the near future PHENIX will publish two new papers on J/ψ production. One will contain the J/ψ results from Run 5 Cu+Cu data, the other will present a re-analysis of Run 3 d+Au data combined with the use of the more precise Run 5 pp reference when making the RAA. The Run 7 Au+Au data set will provide an increase of almost a factor of four in Au+Au J/ψ yield, and the data were collected with a new reaction plane detector in PHENIX that greatly enhances the reaction plane resolution. As a result, it is hoped that Run 7 will produce the first J/ψ elliptic flow measurements, expected to shed light on the importance of random coalescence of charm ant-charm pairs in J/ψ production at RHIC.
References:
PHENIX collaboration, "J/ψ Production in p+p Collisions at &sqrt(s) = 200 GeV" Phys. Rev. Lett. 98:232002, 2007
PHENIX collaboration, "J/ψ Production in Au+Au Collisions at &sqrt(sNN) = 200 GeV" Phys. Rev. Lett. 98:232301, 2007
PHENIX collaboration, "J/ψ Production and Nuclear Effects for d+Au and p+p Collisions at &sqrt(sNN) = 200 GeV" Phys. Rev. Lett 96, 012304 (2006)
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