Solar Neutrinos, Latest RHIC Data Highlight DNP Meeting

The solution to the solar neutrino problem and new data from RHIC experiments were among the highlights of the 2002 fall meeting of the APS Division of Nuclear Physics, held October 9-12 at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University in East Lansing, MI. The regular meeting was preceded by two all-day parallel workshops, one on the future of gamma-ray spectroscopy and the other on nuclear astrophysics at the limits of stability.

The so-called solar neutrino problem-that the measured flux of electron neutrinos is only about one- third as large as predicted by theory -has puzzled scientists for decades (see the Nobel Prize article on this page), but recent results from the Sudbury Neutrino Observatory (SNO) have demonstrated for the first time that the missing two-thirds of the predicted flux arrives at the detectors as mu and tau neutrinos. These results provide strong evidence for neutrino mass and mixing, according to Fermilab's John Beacom, and also have far-reaching implications, from the solar core temperature to models of neutrino mass to the lepton number of the universe. The upcoming low-energy solar neutrino experiments and the KamLAND reactor antineutrino experiment in Japan will play a very important role in exploring the remaining unresolved questions surrounding solar neutrinos.

John Harris of Yale University reported on new results on collisions of ultra-relativistic heavy nuclei at RHIC, in which nuclear matter is compressed and heated to energy densities where it is predicted to melt into a plasma of deconfined quarks and gluons. In his talk, Harris focused on new evidence for hard scattering, as well as jet quenching as a tool to probe the quagmire and understand its properties.

Another RHIC researcher, Brookhaven's Julia Velkovska, reported the first evidence for high-transverse- momentum (p_T) suppression of pions and inclusive hadrons discovered in central Au+Au collisions at the facility. Unexpectedly, protons and antiprotons remain unsuppressed and exceed the pion yields. Velkovska described one possible explanation that strong radial expansion in the system boosts the transverse momenta of heavier particles into the high p_T region.

Argonne National Laboratory's John Arrington addressed the question of whether ordinary nuclei contain exotic states of matter. Recent data from experiments at Jefferson Laboratory have enabled Arrington's group to begin to map out the strength of two-nucleon correlations in nuclei, and he believes that upcoming experiments should enable them to isolate the presence of multi-nucleon correlations. Such correlations help describe nuclear structure and represent high-density "droplets" of hadronic matter. While there have been hints of non-hadronic structure in nuclei, Arrington believes that future measurements will enable them to measure directly the quark distributions of high-density configurations in nuclei. A modified quark structure in these close-packed nucleons would provide a clear signature of exotic components to the structure of nuclei.

The advent of a new generation of radioactive beam facilities, as well as advances in the theoretical understanding of unstable nuclei, has enabled scientists to begin to delineate the nuclear processes that govern such stellar explosions as supernovae, novae and x-ray bursts, which are closely related to fundamental questions regarding the origin and fate of the elements. NSCL's Hendrik Schatz described recent calculations on the rp process and neutron star crust processes, and also summarized experimental data from NSCL and the GSI facility in Darmstadt, Germany.