Physics, P-DO
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J/ψ and Charm Quark Production Measurements with the PHENIX Detector at RHIC
P.L. McGaughey, J.G. Boissevain, M.L. Brooks, G.J. Kunde, D.M. Lee, M.J. Leitch, M.X. Liu, B. Norman, W.E. Sondheim, H.W. van Hecke (P-25), J.P. Sullivan (ISR-1) IntroductionThe Relativistic Heavy-Ion Collider (RHIC) facility at Brookhaven National Laboratory was designed to collide counter-rotating beams of gold atoms with velocities near the speed of light. When two gold atoms suffer a head-on collision at these high speeds, it is expected that a new phase of matter may be formed. This new type of matter, called the quark-gluon plasma (QGP), consists of quarks and gluons that are no longer bound inside the protons and neutrons contained in the gold atoms. Plasma formation can be understood by an analogy to boiling water. As liquid water heats, it begins to boil, evaporating to form a gas. Likewise, protons and neutrons heat up and begin to evaporate quarks and gluons. The PHENIX Experiment and QGP FormationThe PHENIX experiment1 at RHIC is designed to search for signatures of QGP formation. Matsui and Satz2 have proposed that the J/ψ particle, which consists of a charm and anticharm quark bound together, is not formed in the presence of a QGP. Due to the relatively large size of the J/ψ, the attractive force between the two charm quarks can be greatly reduced by interference with the strong fields associated with the light quarks and gluons liberated in the plasma. This behavior is analogous to the Debye screening process in chemical solutions and atomic plasmas. The result is that the two charm quarks are unlikely to form a bound state. Instead, they combine individually with light quarks to form a pair of D mesons. Therefore, an experimental signature of QGP formation is the observation of a strong suppression of J/ψ production in central gold-gold collisions, relative to peripheral collisions. An increase in D meson production is also expected. However, because the total yield of D mesons is much larger than for the J/ψ, the latter effect will probably be too small to observe.
The PHENIX experiment, shown under construction in Figure 1, consists of two central arms and two muon arms. Each muon arm consists of a tracking system, which measures the momentum vector of each muon, together with a muon identifier, which separates muons from other particles and provides the muon trigger for PHENIX. Our team led the design, construction, and operation of the muon tracking system over a period of more than ten years. Staff from Oak Ridge National Laboratory led the muon identification system effort. We also pioneered the use of cathode-strip-chamber technology for the high-precision tracking chambers used to accurately measure the muon trajectories and momentum. The muon arms measure the decay J/ψ → μ+μ- over the rapidity range of 1.2 < |η| < 2.4, while D mesons are detected via the semileptonic decay D → μ + X.
Experimental Results
The ratio between the deuterium-gold and proton-proton J/ψ data versus rapidity is given in Figure 4. While the ratio is near one at backward (negative) rapidity, a significant suppression is observed at forward (positive) rapidity. The latter region corresponds to low-Bjorken-x (fraction of the proton momentum) values for the gluons in deuterium, where we expect the gluons to be suppressed, due to the presence of the color glass condensate4 or as a result of other models of gluon shadowing. Also shown in Figure 4 are theoretical predictions of shadowing from Vogt5 and Kopeliovich6. The data favor less shadowing than in the Kopeliovich model, but the uncertainties are large. Also of interest are the transverse momentum distributions of the J/ψ. The data are consistent with nuclear suppression at low pT and enhancement at high pT, similar to that seen previously by E866.7 This behavior, which is often referred to as the Cronin effect8, comes about from the multiple scattering of particles as they propagate through a nucleus, leading to an average increase in their transverse momentum. PHENIX has the ability to measure the centrality of individual ion-ion collisions, where centrality is a measure of the degree of overlap of the two ions. Using the beam-beam Cerenkov counters located at small angles with respect to the two ion beams, the yield of produced particles is determined, which is directly correlated with the collision centrality. The centrality dependence of the J/ψ yield can then be computed. Shadowing of the gluons in gold is again observed, consistent with theory, with little centrality dependence. However, a strong increase with centrality is observed for the yield of J/ψ produced at backward rapidity, which is inconsistent with theory. Open charm particles (D mesons) have been measured by both the PHENIX9 and the STAR10 experiments at central rapidity. Data is available from proton-proton and deuteron-gold collisions. Both experiments report total cross sections in reasonable agreement with predictions of perturbative QCD. The deuteron-gold data show no strong nuclear dependence, which is not surprising as these data do not correspond to the shadowing region and have poor statistics at high pT. Open charm data from the muon arms are presently under analysis and will be able to address both the shadowing and high-pT regions.
ConclusionThe PHENIX muon arm physics program is well underway after more than 10 years of design, construction, and installation. Both muon arms are completely functional and working within design specifications. The first measurements of J/ψ production in proton-proton and deuteron-gold collisions at RHIC energies have been presented and are providing important insight into the nature of gluon shadowing and the Cronin effect. An excellent set of data from gold-gold collisions has been recorded and the analysis is well underway. Within the near future, we should be able to determine if the J/ψ signal is suppressed enough in these collisions to indicate the formation of a quark-gluon plasma. References
AcknowledgmentThis work was supported by the U.S. DOE and includes contributions from many members of the PHENIX Collaboration. For further information, contact Patrick McGaughey, 505-667-1594, plm@lanl.gov.
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In order to quantify J/ψ suppression in the QGP, it is critical to understand J/ψ production in collisions where the QGP is not formed. Examples of these are proton-proton and proton-gold collisions at similar energies. At RHIC energies, the production of charm quarks is dominated by the gluon fusion process g + g → c + c–. Thus, charm production is sensitive to the gluonic content of the colliding nuclei. It’s also known that J/ψ production is somewhat suppressed by the ordinary nuclear medium, relative to the proton, and that the level of suppression is dependent upon the momentum vector of the J/ψ. These nuclear effects have been characterized as initial-state effects, which refer to the modification of J/ψ formation in a nucleus, and final-state effects relevant to the propagation of the J/ψ through the nucleus. Initial-state effects can arise from changes in the gluon momentum distributions in a nucleus. Final-state effects include multiple scattering, energy loss, and dissociation of the J/ψ. It’s important to measure open charm (D mesons) to help separate initial- from final-state effects, because D mesons suffer different final-state effects than the J/ψ.
A schematic of the operation of a muon arm is given in Figure 2. A muon originating from the intersection point of the two gold beams (left side of drawing) travels through the steel of the central magnet into the muon lampshade magnet. Particles other than muons generally interact in the steel and are stopped there. Three stations of cathode strip chambers (vertical blue bars on the drawing) inside the magnet accurately measure the trajectory of each muon. Due to the presence of a large magnetic field inside the magnet, a muon follows a curved trajectory. By measuring this curvature, we determine both the muon’s charge and momentum. After exiting the muon magnet, the muon encounters alternating layers of steel and Iarocci tube detectors in the muon identifier, which remove any remaining backgrounds due to particles other than muons. The depth of penetration is a rough measure of the muon’s energy, which we use to provide a trigger to PHENIX, requesting that the data for this beam crossing be recorded.
PHENIX has recently measured proton-proton, deuterium-gold3, and gold-gold collisions at energies of 100 GeV per nucleon. Preliminary results for the J/ψ differential cross section versus rapidity from proton-proton collisions are shown in Figure 3. The shape of differential cross section is consistent with predictions of perturbative quantum chromodynamics (QCD), based upon gluon fusion diagrams. The total cross section times branching ratio is 159 nb ± 8.5% (fit uncertainty) ± 12.3% (systematic uncertainty). These proton-proton J/ψ data serve as the baseline for extracting nuclear effects from the deuterium-gold data.
