Chiral Perturbation Theory, Discrete Symmetries Highlight 1997 Nuclear Division Meeting
A town meeting was also held on Tuesday afternoon to provide an opportunity for a large segment of the nuclear science community to contribute to the ongoing discussion regarding future challenges and priorities for the field. On hand to guide the discussion were Peter Rosen, newly appointed associate director of the DOE's Office of High Energy and Nuclear Physics, and Robert Eisenstein, the NSF's assistant director for mathematical and physical sciences. In addition, three parallel workshops were presented prior to, but in conjunction with, the DNP meeting on Sunday, October 5: one on discrete symmetries, another on electromagnetic dynamics of mesons and nucleons, and the third on radioactive beams and nuclear astrophysics.
Along with a short scientific biography of Wu, the plenary session that kicked off the conference included a presentation by Carl Wieman of the University of Colorado on his recent measurements of atomic parity violation, which has now been observed in numerous atoms such as cesium, particularly the first measurement of a nuclear anapole moment. Brookhaven's Maurice Goldhaber discussed various aspects of the study of neutrinos emitted in weak decay processes, for which work Wu is chiefly known. There is an ongoing investigation in this area of research by the Japan-USA Kamiokande collaboration of atmospheric and solar neutrinos.
Vincent Yuan of Los Alamos National Laboratory closed the session with a talk on compound-nuclear neutron resonances for fundamental and applied physics. According to Yuan, experiments using epithermal neutrons interacting with compound-nuclear resonances serve a wide range of scientific applications. For example, transmission changes correlated to the polarization reversal in incident neutrons have been used to study parity violation in the compound nucleus for many different targets. Neutron resonances can also be used to determine the polarization of neutron beams. Finally, the motion of target atoms results in an observed temperature-dependent Doppler broadening of resonance line widths, which can be used to determine temperatures on a fast time scale of one microsecond or less.
Chiral Perturbation Theory
Several speakers at a Tuesday morning session concentrated on recent developments on applying chiral perturbation theory (CPT) to study nucleon and hyperon properties such as masses, magnetic moments, and weak decays, as well as photo- and electro-production of pions on nucleons near thresholds. For example, according to Norm Kolb of the Saskatchewan Accelerator Laboratory, threshold photoproduction of pions off nucleons is one of the few low-energy phenomena for which QCD-based effective field theories can be formulated and tested. He has used model-independent low-energy theorems, as well as CPT, to predict the leading order terms of the s-wave electric dipole amplitude at threshold for these reactions, which are in good agreement with recent experimental measurements. In addition, measurements of neutral-pion production from the deuteron may soon provide pioneering tests of CPT in a nuclear system.
Electroproduction of pions on nucleons near threshold is one of the experiments for which CPT can make clear predictions, according to Henk Blok of Amsterdam's Vrije Universiteit, who also spoke at the session. In particular, this process represents a very sensitive test of various ingredients of the calculations, since the model-independent term due to the charge of the pion is absent. Data on the proton have been taken at the NIKHEF facility in Amsterdam and at the MAMI facility in Mainz. In both cases, Blok reported, the scattered electron and the residual proton were detected in two high-resolution magnetic spectrometers, which can measure the full angular distribution of electroproduction of pions on nucleons in just a few settings.
During a Wednesday afternoon session, Shelley Page of the University of Manitoba described an experiment currently underway at TRIUMF which will provide unique information on the weak nucleon-nucleon interaction from a measurement of parity violation in proton-proton scattering at 221 MeV. According to Page, the beam energy is chosen to isolate a single partial wave contribution to the parity-violating asymmetry, thus providing the cleanest possible interpretation of the results, and the first direct measurement of the weak meson-nucleon coupling constant. A second long data run has just been completed at TRIUMF. Because the measurements are sensitive to a wide variety of systemic errors which must be monitored and controlled, the current major focus is on reducing system errors.
Weak Interactions and Spin Structure
Type II supernova and neutron star-star neutron mergers are candidate sites for the production of certain heavy nuclei, according to Gail McLaughlin of INT in Seattle, who spoke on Monday afternoon. In these environments, nucleosynthesis takes place in the presence of a neutrino flux. Depending on the intensity of the flux, neutrino interactions with heavy nuclei can play an important role in determining final abundances. Thus, knowledge of the relevant weak interaction cross sections is an important ingredient in determining the astrophysical site and conditions during which this nucleosynthesis occurs. In addition, nuclear scattering can impact the nuclear flow during nucleosynthesis, and the neutrino flux experienced by a nucleus depends on the hydrodynamic conditions. Therefore, said McLaughlin, "The study of neutrino scattering in a rapidly expanding environment typical of the post-core-bounce supernova helps to discriminate between various outflow scenarios."
The "Other" Giant Dipole Resonance
On Tuesday afternoon, Umesh Garg of the University of Notre Dame reported on an investigation of the ISoscalar Giant Dipole Resonance (ISGDR). An exotic "squeezing" mode of collective nuclear vibration, this resonance is best described as "a hydrodynamical density oscillation in which the volume of the nucleus remains constant and the state can be visualized in the form of a compression wave oscillating back and forth through the nucleus," said Garg, comparing the pressure wave to a sound wave. Since the excitation energy of the ISGDR is directly related to the nuclear compressibility, his team used the K600 spectrometer at the Indiana University Cyclotron Facility to measure inelastic scattering of 200 MeV particles at 0 degrees, where the angular distribution of the ISGDR could be clearly distinguished from that of the nearby high-energy octupole resonance. They also employed the difference-of-spectra technique pioneered in the study of the giant monopole resonance to obtain the clearest evidence yet for the ISGDR.
Intermediate Energy Hadron Probes
Cooler storage rings with internal targets offer excellent possibilities to study threshold production of mesons, according to Hans Calen of Sweden's Uppsala University, who spoke at a Wednesday morning session. The experimental program at the CELSIUS ring of the Svedberg Laboratory in Uppsala has lately focused on n meson production in proton-proton and proton-deuteron collisions, as well as in proton-neutron collisions using deuterium as a target. For these studies, his team developed a detector setup with the capability to detect forward-moving charged particles together with photons from neutral meson decays. It also allows for a kinematically complete determination of the meson production events in most cases, and has enabled measurements very close to threshold.
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