Search for the QCD Critical Point at RHIC: First Experience with Au-Au Collisions at 9.2 GeV
By Tapan Nayak
In thermodynamics, a critical point occurs where the first-order transition between two phases of matter ceases to exist. On one side of the point, the two phases are distinct and the transition from one phase to the other is sharp and typically characterized by a latent heat. On the other side of the point, the two phases can co-exist and the transition from one to the other is a smooth crossover. Theoretical models suggest that the QCD phase diagram has such a critical point separating the two phases: the hadron gas, in which quarks are confined and the quark-gluon plasma (QGP). A recent review of the theoretical work may be found in Ref. . A schematic QCD phase diagram is shown in Figure 1 as a function of temperature (T) and the baryon chemical potential (μB) which is related to the density of baryons in the system. Lattice QCD models predict a smooth crossover at high T and small μB while there are arguments for a first order transition at smaller T and larger μB. The exact location of the critical point is not known yet, but various models suggest that it might be within the reach of heavy-ion experiments.
The QCD phase structure can be mapped by studying heavy-ion collisions at different energies. A range of values of μB can be probed by varying the center of mass energy of the colliding nuclei. Systems with low μB values are created in collisions at high energies, for example at the higher RHIC energies. As the beam energy is decreased, the resulting systems have larger values of μB. A new RHIC program has recently started to search for the QCD critical point by making a detailed energy scan  using Au-Au collisions at nucleon-nucleon center of mass energies between 5 GeV and 20 GeV, corresponding to μB values from about 100 MeV to 500 MeV. The initial data run of the energy scan at RHIC is proposed to take place during 2009-2010 (Run-10).
An engineering study was undertaken by C-AD and the RHIC experiments at the end of this most recent run (Run-8). It is a major challenge to inject and collide ions at the low end of the desired energy range, since that is well below the design RHIC injection energy which corresponds to center of mass energy of 19.6 GeV . During this study, the goal of the C-AD accelerator physicists was to demonstrate the injection and collision of Au ions at center of mass energies of 9.2 GeV and 5 GeV. Both the STAR and PHENIX collaborations took active roles in establishing that collisions took place within their detector acceptances. The primary goal was trigger testing but there was also a desire to perform preliminary physics studies. Within a few hours of the initial setup for the beam at 9.2 GeV, the trigger counters of STAR started ticking with Au ions circulating and colliding. The peak luminosity at this energy was determined to be 3.2x1023 cm-2s-1 and an average luminosity of 1.5x1023 cm-2s-1. These values are about 1000 times smaller than those of Au-Au collisions at full RHIC energy, but sufficient for the physics we are after. These luminosity values are expected to improve by a factor of 4-8 for future runs. After completing all of their tests and providing a brief period for the experiments to collect a small data sample, C-AD staff performed detailed beam studies of a 2.5 GeV Au beam injected and circulated in one of the rings. These machine configurations were well outside the original design parameters of the machine. Injecting, cogging, and colliding beams as well as delivering physics quality conditions at these low energies represent yet another major accomplishment for the C-AD personnel.
Although the primary goal of the engineering run was to perform machine studies, both experiments attempted to take data. Analysis is ongoing, but it appears that the STAR detector accumulated roughly 5-10 thousand physics-quality events for the 9.2 GeV collisions. Figure 2 shows the display of tracks in the STAR TPC for two events. The reconstructed tracks clearly show the position of the vertex in the event display and will allow a precise extraction of the vertex position in off-line analysis. Figure 3 shows a preliminary distribution of the vertex spread in the beam (z) direction. Detailed analysis is in progress to establish the beam characteristics more precisely and also to perform a variety of physics analyses. The STAR measures of various trigger rates will be combined with information from the accelerator physicists to establish a much more reliable estimate of the rates to be expected in the future energy scan.
The capability of the STAR detector, including the upgrade of the Time-of-Flight (TOF) system, to perform a search for the critical point has been covered in RHIC News previously . The counters which will be used for the TOF start time were included in one of the triggers during this year's test and were found to have very good efficiency. The lessons learned from this short beam time will go a long way in preparing both C-AD and the experiments for the planned energy scan in the search of QCD critical point.
 M. A. Stephanov, hep-lat/0701002
 Workshop on "Can we Discover the QCD Critical Point at RHIC", March 9-10, 2006; https://www.bnl.gov/riken/QCDRhic/
 T. Satogata, RHIC Low Energy Operations, RHIC News, July 31, 2007; http://www.bnl.gov/rhic/news/073107/story3.asp
 P. Sorensen, The Critical Point Search and STAR Time-of-Flight upgrade, RHIC News, July 31, 2007; http://www.bnl.gov/rhic/news/073107/story2.asp