Relativistic Heavy Ion Collision Studies at RHIC and the LHC

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

This is a three-year program funded by the NSF primarily to carry out experiments with relativistic heavy ion-collisions, i.e. head-on collisions of heavy nuclei such as gold near the speed of light. The principal investigators are T. J. Humanic and M. A. Lisa of the Department of Physics at The Ohio State University. The main goal of these experiments is to obtain evidence that collisions of this type produce a phase transition from normal nuclear matter into a new phase of matter made up of a large volume of free quarks and gluons (the constituents of protons and neutrons) called Quark Matter. This phase transition has been predicted by quantum chromodynamics (QCD), which is the theory that describes the behaviour of quarks and gluons in nature. Thus far, there have been no disagreements between the predictions of QCD and experiment, so finding Quark Matter would be a major test of this theory. Quark matter is of additional interest since it is thought to have been the state of the universe several microseconds after the Big Bang and may exist today in nature at the centers of neutron stars. Relativistic heavy ion collisions could provide a method to produce Quark Matter in the laboratory since it is necessary to create a very dense and hot region within nuclear matter in order for the initiation of this phase transition. In addition, because heavy ions are relatively large compared with the size of a proton the collision is extended in time, hopefully allowing the system enough time to reach equilibrium so that the Quark Matter state is well developed. Thus, to study this phenomenon one prefers heavy ion collisions at the highest possible energies and with the heaviest ions available. In the present proposal heavy ion collisions will be studied with gold on gold collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) and lead on lead collisions at the Large Hadron Collider (LHC) at CERN in Geneva. RHIC, which started delivering beams in Summer 2000, is the highest energy heavy ion accelerator now in operation. In order to look for signatures that a phase transition to Quark Matter has taken place, one must study the properties of the particles produced in the collisions, such as particle momenta and multiplicities of each type that can be detected (e.g. pions, kaons, protons, and more exotic particles). This will be done at RHIC using the large acceptance detectors of the STAR experiment, which can sample most of the interesting particles per collision. The information obtained in this way can be used to determine, for example, the physical size of the interaction region (using two-particle interferometry), the temperatures reached in the collisions (from particle momentum distributions) and exotic particle production such as strange baryons. In addition, preparations will be made to study Pb+Pb collisions at even higher energies at the LHC with the ALICE experiment. The LHC will provide beams 30 times higher in center-of-mass energy than RHIC, i.e. cosmic ray energies, starting in 2007. Thus, the energy dependence of Quark Matter production can be studies and the search for as yet unpredicted phenomena can also be pursued. Besides the intellectual merit described above, there are broader impacts to society of this proposal in the areas of education, technology and computing. Since undergraduate students, graduate students and postdoctoral researchers will play key roles in this project, training will be provided to these groups in how to carry out research in large collaborations and in general problem solving skills. Participation in particle detector development and testing will enhance new technologies which can be adapted to both commercial and government use. Since this research requires large-scale computing in order to acquire and analyze data on the teraflop an terabyte scale, new methods in information science, such as computing grid systems, will be developed which can enhance the computing power of society as a whole.