One of the main aim of high energy heavy-ion collisions is to map the QCD phase diagram. The goal being to locate the QCD phase boundary (separating matter with hadronic degrees of freedom from matter with quark gluon degrees of freedom) and the QCD critical point (where the first order phase transition ends). The phase diagram is plotted as temperature versus baryon chemical potential. These quantities can be changed by varying the colliding beam energy to map the phase diagram. The temperature and baryon chemical potential can be measured from the produced particle spectra and ratios. Then one looks for signatures for different phases and for the QCD critical point. In addition STAR also would like to study the beam energy which corresponds to onset of several interesting observations seen at top RHIC energy (Au+Au 200 GeV) : Number of constituent quark scaling of
In order to achieve the above goals STAR has proposed a beam energy scan program at RHIC spanning beam energies from 5 GeV to 50 GeV. As a first step towards achieving this goal, recently STAR collected data from a test run for Au+Au collisions at 9.2 GeV. The events for this test run was collected at a rate of 0.7 Hz. The first results were presented at the SQM2008. Here we discuss only a small subset of the results.
The first figure shows the strangeness enhancement through the Kaon to pion ratio as a function of colliding beam energy. The ratios seem to be consistent with the established beam energy trend. The 9.2 GeV data are from 3000 events only, higher statistics in future will make significant qualitative improvement over the existing SPS results. The second figure shows how the 9.2 GeV elliptic flow parameter falls into the beam energy systematic. The STAR data at 9.2 GeV denoted by the "star" symbol nicely fits into the observed beam energy dependence. The third figure shows how the HBT radii for new 9.2 GeV data fits into the beam energy dependence. The Rout/Rside at 9.2 GeV is close to unity. This ratio is expected to give information about emission duration of the source. A value much larger than unity would indicate formation of a partonic phase in the collisions.
These interesting results demonstrates STAR experiments preparedness for the proposed beam energy scan program at RHIC. It is worth mentioning that STAR being a collider experiment has uniform acceptance across all beam energies for produced particles and a variation in particle density at mid rapidity that is reasonably small for various colliding beam energies at RHIC. These features are very different from that in fixed target experiments. STAR also has an excellent particle identification capability from its Time Projection Chamber, which will be further enhanced with inclusion of the full Time Of Flight detector by 2010.
The RHIC beam energy scan program will provide an unique opportunity to study the QCD phase diagram. It provides a great opportunity to understand the fundamental theory governing the interactions of quarks and gluons, the building blocks of most of the matter we see. It will help us to put actual experimentally measured points on the phase diagram, which so far has only one physical point corresponding to the ordinary nuclear matter. For more results at 9.2 GeV please refer to the STAR presentation at SQM2008 .
The experience of data taking with Au+Au 9.2 GeV collisions can also be found in this RHIC news.