First RHIC Results Highlight 2000 DNP Meeting

Conference Experience for Undergraduates at DNP 2000 Peter Rosen, DOE's Associate Director for High Energy and Nuclear Physics, gathers around student poster presentations with participants of this year's third annual CEU held at the Fall 2000 DNP meeting in Williamsburg, Virginia. The goal of the CEU program is to provide a "capstone" conference experience for undergraduate students by giving them the opportunity to present their research to the larger professional community and to one another. (Photo courtesy of Tom Clark)

The first results from RHIC, and reports of a natural endpoint to the RP process took center stage at the annual fall meeting of the APS Division of Nuclear Physics (DNP), held October 4-7 in Williamsburg, VA.

Searching for QGPs at RHIC

The search for the QCD phase transition has been the quest of ultra-relativistic heavy ion physics for the past two decades, according to Axel Drees of the State University of New York at Stony Brook, who kicked off a Wednesday morning session on the topic. "The hope is to establish the existence of the transition and by studying it in detail, to unveil fundamental puzzles of QCD, in particular confinement and chiral symmetry breaking," he says. Two recent major developments in particular are expected to herald a new era in the field: the completion of the experimental programs at CERN's SPS and Brookhaven National Laboratory's AGS, and the successful first run of BNL's new Relativistic Heavy Ion Collider (RHIC).

Drees opened the session with a summary of results obtained from previous experiments at CERN and BNL, which he maintains support researchers' hopes that the higher energies possible with RHIC (about 25 TeV) will be sufficient to create in the laboratory the conditions which must have existed about 2 microseconds after the Big Bang. Specifically, measurements of the energy emitted from Au-Au and Pb-Pb collisions over the last two decades indicate that if the ions collide head-on, energies are reached which should be sufficient to free the quarks and gluons normally confined in nuclear matter and form a quark-gluon plasma (QGP), a new state of strongly interacting matter. The key question now is whether the system spends some time in a QGP-like phase, and several characteristic features of such a state are believed to have been observed at both CERN and BNL.

"Clearly the evidence remains circumstantial," says Drees. "But even assuming a QGP-like state is formed, we know little about its properties." He adds that only limited progress can be expected from further experiments at the SPS because the energies aren't high enough to give a clear signature for the QGP-like state, which is just above the critical temperature and cools down rapidly. These limitations can be overcome at RHIC, where the Au ions collide at much larger energies: "The initial temperature should be higher and the system should spend a longer time in the deconfined phase," says Drees. "I have no doubt that RHIC will revolutionize our understanding of QCD over the next decade." In fact, he reports that the first published data from the PHOBOS collaboration at RHIC indicate that the initial energy density is increased by at least a factor of 1.5, which would correspond to an initial temperature above 250 MeV.

Rapid Proton Capture Process in Accreting Neuton Stars

The rapid proton capture (rp) process is the dominant reaction sequence in both X-ray bursters and X-ray pulsars, types of neutron stars found in X-ray binary systems that accrete matter from the envelope of their companion star. The process was first identified in 1981, and since then its natural endpoint has been an open question, with the latest calculations finding reaction networks ending at Yttrium and Tin. More recently, Hendrick Schatz of Michigan State University, and his colleagues performed the first calculations of the rp process beyond tin, and discovered that a natural endpoint of the process does, in fact exist: namely, the low binding energy of the proton-rich Tellurium isotopes leads to the formation of an Sn-Sb-Te cycle. According to Schatz, this cycle is a very effective barrier for the rp process because it prevents the synthesis of heavier nuclei in explosive and steady state burning on the surface of accreting neutron stars for all model parameters.

The result is significant, says Schatz, because in X-ray bursts with large amounts of hydrogen available at ignition, the Sn-Sb-Te cycle can operate at the end of the burst, leading to late time helium production, which in turn results in additional seed nuclei for the rp process and an increase in energy production and fuel consumption. In addition, this cycle provides an important constraint for the crust composition of accreting neutron stars, limiting it to light p nuclei. According to Schatz, knowledge of the neutron star crust composition is crucial to resolve such open questions as the evolution of magnetic fields; the possibility of gravitational wave emission from a deformed, rotating neutron star; or the possibility of distinguishing neutron stars and black holes by observing thermal radiation from the star crust during the off-state in transient systems.

The Future of Nuclear Physics

The DNP meeting also featured a special plenary session on future directions in nuclear physics. Walter Henning of the University of Frankfurt in Germany summarized the status of rare isotope physics, in which intense beams of unstable short-lived nuclei are providing scientists with new opportunities for studying the nuclear many-body system and fundamental symmetries and interactions, as well as answering some vital questions in nuclear astrophysics. Wick Haxton of the University of Washington's Institute for Nuclear Theory discussed numerous remaining opportunities for testing the predictions of the standard model using electroweak interactions, and for seeking signs of new physics beyond the standard model. Finally, Argonne National Laboratory's Donald Geesaman reported that several new experimental tools have been developed over the last decade that hold considerable promise for resolving some of the outstanding issues in spin-flavor physics, including the quantitative contribution of the glue to the nucleon's spin, which is still under debate.