The challenge both to theorists and experimentalists is to determine whether or not such a plasma has been formed. So far, no convincing evidence of the phase transition has been seen in fixed-target interactions at the Brookhaven Alternating Gradient Synchrotron (gold beams up to 10.6 GeV/nucleon) and the CERN Super Proton Synchrotron (lead beams up to 160 GeV/nucleon) although some tantalizing hints of dense matter have been seen, particularly in "hard" probes of the collision such as J/psi and psi' production (bound states of charm-anticharm quarks) and lepton pair production at invariant masses greater than 1.5 GeV/c2.
For example, much intense interest has surrounded the J/psi data from the CERN SPS because fewer J/psi's are detected than are expected to be produced in the collision. This suppression was predicted to be an effect of quark-gluon plasma production. However similar suppression has also been observed in proton-nucleus interactions at these and higher energies. Models of J/psi interactions with hadrons, based on the understanding of proton-nucleus collisions, can explain the observed J/psi suppression up to sulphur-uranium interactions without quark-gluon plasma production. The most recent data from lead-lead interations seems to indicate more suppression than expected by hadronic sources. The interpretation of this data remains quite controversial, illustrating the importance of understanding the systematics of J/psi production in hadron-hadron, hadron-nucleus, and nucleus-nucleus interactions over a range of energies.
Because the interpretation of data from these heavy-ion collisions is difficult and depends on our still-limited understanding of nuclear effects in hadron-nucleus interactions, I have chosen to study high-momentum transfer interactions in protons and nuclei in an attempt to understand the "normal" background to these processes in nuclear collisions. While rare at current energies, these hard probes such as J/psi's, heavy quarks (charm and bottom) hand high mass lepton pairs are produced early in the collision and thus feel the effects of the hot interior. Their production is calculated in perturbative quantum chromodynamics (QCD), providing a firm theoretical foundation from which to build. Inside the nucleus or the quark-gluon plasma, the hard probes may reinteract before escaping or their production may be affected by the presence of the medium even before the hard collision that forms them. I have studied these nuclear effects at current energies and have also made predictions for their baseline production in collisions at the forthcoming heavy-ion colliders, the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven (center-of-mass energy of 200 GeV/nucleon pair, corresponding to 21 TeV/nucleon in fixed-target interactions) and the Large Hadron Collider (LHC) at CERN (center-of-mass energy of 5.5 TeV/nucleon pair or 16000 TeV/nucleon with a fixed target) where hard QCD production will not necessarily be rare. For example at the LHC, around 400 pairs of charmed quarks will be produced in a typical lead-lead collision, a substantial number indeed.
My interest in QCD has influenced my research in many ways. In addition to my work on ultrarelativistic heavy-ion collisions, I have also studied higher-order corrections to heavy quark (charm, bottom, and top) production to understand how well we can predict the heavy quark production rate at high energies. I am also interested in how fluctuations of hadronic wavefunctions can lead to enhanced forward heavy quark production. These fluctuations can produce fast J/psi's as well as explain differences in charmed and anticharmed particle production.
The ultrarelativistic heavy-ion field is quickly emerging and will mature with the advent of RHIC and LHC. RHIC is expected to begin colliding nuclear beams in April 1999 while the LHC is not expected to begin heavy-ion studies until 2005. Predictions for RHIC will soon be subjected to the humbling light of experimental evidence while new physics learned at RHIC can be used to hone predictions for the LHC. This is an especially exciting time in high-energy nuclear physics with many possibilities for bright students to make an impact.
I am based primarily at Lawrence Berkeley National Laboratory (LBNL) in nearby Berkeley where the nuclear theory group specializes in this field and where there is also a large experimental effort at all available energies. Thus in addition to being part of the local nuclear experimental group at Davis, students have the opportunity to take advantage of the strong program at LBNL.
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