The Critical Point Search and the STAR Time-of-Flight Upgrade
By Paul Sorensen
The predicted map of nuclear matter contains many exotic and varied regions. The now familiar nucleus was discovered by Ernst Rutherford in March 1911. The nucleus was later found to be a bundle of protons and neutrons which in turn are bundles of three quarks bound together by gluons. The normal nucleus represents a small point on the map of nuclear matter. In other regions, we expect to find phases of matter such as a soup of unbound quarks and gluons (quark-gluon plasma), various arrangements of color super-conducting phases, and even a controversial phase of matter that could involve quarks confined into nearly massless bundles. The borders between these regions are phase transitions. The transition from one region to another can be smooth or abrupt.
Experiments at RHIC have found evidence that when they collide nuclei together with very high energy, a strongly coupled quark-gluon plasma is created. The discovery of this phase was listed as the top physics story of 2005 by the American Institute of Physics and was widely covered in the popular press. Computer simulations carried out to solve the equations governing quarks and gluons demonstrate that for the top RHIC energies used, the transition into and out of that phase is a smooth crossover. For lower energies, however, the transition is thought to be abrupt. The point where the transition changes from smooth to abrupt is an important landmark called the critical point. For nearly a decade, experiments have searched for signals in their detectors that might indicate that the matter created in nuclear collisions existed near this critical point. Those signals typically involve abrupt changes in measured quantities and large variations in the final states, even when the collision conditions are kept fixed.
Exactly one century after Rutherfords discovery of the nucleus, physicists at RHIC will conduct an energy scan to map-out a large region of the nuclear matter phase diagram, and the RHIC facility just happens to be ideally suited to locate the critical point. Figure 1 shows the region of the map that will be explored. The figure also shows the most advanced theoretical estimates for where the critical point might lie. While the higher energy collisions at RHIC have already explored the region near the left axis of the plot, the low energy scan will extend the coverage to include the region where the critical point is most likely located. It is then up to the experimentalists at RHIC to gather the data necessary to determine whether the critical point exists in that region; and if so, where.
The STAR detector was designed to measure critical point signatures. Its cylindrical layout allows the detection of particles coming out at all transverse angles. Fluctuation in the relative number of different kinds of particles is thought to be one of the more important signatures. To ensure that the necessary information is recorded for each collision, the STAR collaboration is upgrading its particle identification capabilities. The final pieces of a Time-of-Flight (TOF) detector will be put in place in time for the low energy scan (download pdf). Table 1 shows the TOF installation schedule. The TOF detector uses state-of-the-art multi-gap resistive plate chamber (MRPC) technology to measure the time it takes a particle to reach the detector. These chambers consist of stacks of thin glass separated by fishing line. Each TOF module is approximately 6 cm by 20 cm and has 6 readout pads. When installation is complete, there will be more than 23,000 readout pads covering the outer barrel of the STAR detectors main tracking detector, the Time-Projection-Chamber (TPC). When a particle's time-of-flight information is correlated with its momentum determined by its trajectory in the TPC, the particle's mass can be calculated. Figure 2 shows how the TOF detector will be able to separate the different kinds of particles into identified groups: mainly pions, kaons and protons. This information will be used to measure event-to-event fluctuations in particle ratios.
Previously, the NA49 collaboration at the CERN laboratory in Geneva Switzerland observed a sharp peak in the ratio of kaons over pions. To investigate whether that peak is related to the location of the critical point, subsequent measurements have been made of event-by-event fluctuations of the kaon to pion ratio. Though, tantalizing, those measurements are still inconclusive. This is partly because available detector technology, as well as the configuration of the particle accelerator used to scan different collision energies, limits accuracy. At CERN, a beam of nuclei was smashed into a fixed target. To conduct an energy scan, the energy of the incoming beam was varied. As the energy changed, the produced particles ended up in different parts of the detector. Comparing one energy to another was therefore difficult. The RHIC facility collides two counter-rotating beams of nuclei so that higher energies can be reached. In this case, the position where most of the produced particles strike the detector doesn't change as the beam energy is changed. This is a strong advantage for detectors at RHIC in the critical point search.
Figure 3 shows the dynamical fluctuations of the kaon to pion ratio measured at a variety of energies by NA49 and by STAR. Also included are estimates of the statistical uncertainty in those measurements for 100 thousand events detected by the STAR experiment with and without a TOF detector. Gathering that number of events requires approximately 12 hours of data taking at each energy. The TOF detector reduces the statistical uncertainty by a factor of two. The effect of accurate particle identification on systematic uncertainties, although difficult to quantify, may be even more important. Confusing one particle type for another can lead to apparent fluctuations in the ratios. A TOF detector covering the whole STAR acceptance will significantly reduce this confusion.
The STAR detector has strong potential to discover the location of the critical point during the upcoming RHIC energy scan. The energy scan will encompass a region of the nuclear matter map that contains the most advanced estimates for the location of this point. Rutherford's discovery of the nucleus in 1911 relied on state-of-the art technology developed by Hans Geiger to count alpha particles. The state-of-the-art technology at the heart of STAR's Time-of-Flight upgrade significantly increases the possibility that STAR will locate that other landmark on the nuclear matter map: the critical point.