APS News

Strangelet Searches, Spin Effects, QCD Field Theory Highlight 1996 DNP Meeting

Recent studies of quantum chaos in mesoscopic systems, and effective field theory were among the topics featured at the annual fall meeting of the APS Division of Nuclear Physics (DNP), held 2-5 October 1996 at the Massachusetts Institute of Technology in Cambridge, Massachusetts. The meeting consisted of six invited sessions, including a plenary session on basic research in nuclear physics, five mini-symposia, and 20 contributed sessions. A town meeting was also held on -Friday 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.

Effective Field Theory

Although traditional nuclear structure calculations have benefitted from new methods and increased computer power, they have lacked direct input from quantum chromodynamics (QCD), the basic theory of strong interactions. According to Richard Furnstahl of Ohio State University, who spoke at a Friday morning session, "Effective Field Theory provides a framework for connecting the energy scales and degrees of freedom appropriate for nuclear structure with those in the underlying QCD." He has found that, for heavier nuclei, this framework provides new insight into issues of nucleon compositeness, vacuum contributions and extrapolations to high density, for example. David Kaplan (University of Washington), who spoke at the same session, has found effective field theory techniques to be powerful tools for theoretical descriptions of nucleon-nucleon scattering.

Chiral dynamics is an effective low-energy field theory of QCD which provides a framework to make rigorous and model-independent predictions at the confinement scale. However, in a Saturday morning session, Michael Frank of the Institute for Nuclear Theory discussed how the advent of a new generation of accelerators, such as Thomas Jefferson National Laboratory, has enlarged the energy domain of nuclear physics beyond the scale for which such low-energy effective theories are valid. As a possible solution, he suggested developing and exploring an effective field theory of subhadronic degrees of freedom which maintains the global symmetries of QCD and reproduces chiral perturbation theory in the appropriate limit. He has used such a theory to calculate low-energy chiral coefficients and hadronic form factors, for example.

Searching for Strangelets

On Saturday morning, Huan Huang of the University of California at Los Angeles reported on recent progress in the search for strange quark matter and other exotic forms of matter at Brookhaven's Alternating Gradient Synchrotron. "Heavy ion collisions at the BNL-AGS are characterized by formations of high baryon density and yields of large strangeness, in which quark gluon plasma (QGP) may be formed due to fluctuations," he said, adding that strangelets are predicted to be possible remnants of this QGP formation. At the same session, Thomas Glasmacher of Michigan State University's National Superconducting Cyclotron Laboratory described a new technique using fast radioactive ion beams to investigate the evolution of nuclear shell structure.

Measuring Spin Observables

Polarized targets internal to electron storage rings represent a unique opportunity for the measurement of spin observables in electro-nuclear physics, according to Massimiliano Ferro-Luzzi of The Netherlands' NIKHEF facility. The technique has several advantages - including parity of the target species, high polarization, clean recoil hadron detection, and the ability to manipulate the target spin - which allow the study of the electromagnetic structure of the nucleon and light nuclei with high statistical and systematic precision. Ferro-Luzzi reported on measurements obtained from two recent experiments scattering unpolarized electrons from tensor polarized helium-2 and helium-3 targets. A third experiment is underway using a polarized electron beam and interal targets, in order to study simultaneously several channels over a broad kinematical range.

Quantum Chaos in Mesoscopic Systems

Recently there has been considerable interest in the transport properties of mesoscopic devices such as quantum dots: isolated regions of a few microns or less in length to which several hundred electrons are confined. While previous studies have focused on disordered systems, where the elastic scattering length is small compared to the size of the dot, recent advances in nanostructure technology allowing the fabrication of ballistic dots that are smaller than the elastic path of the electron. According to Yoram Alhassid of Yale University's Center for Theoretical Physics, because of the irregularities of the dot's shape, the electron dynamics are chaotic in nature, and the universal features of the conductance fluctuations are consistent with quantum chaos theory.

Attention is also shifting from studies of open dots - characterized by many overlapping resonances - to closed dots which are weakly coupled to the external leads via tunnel barriers. "In this regime, a single electron resonance whose energy is closest to the Fermi energy dominates the conductance, and it is thus possible to probe the chaoticity of the electronic wave functions," said Alhassid. He has developed a theory for the statistical properties of the conductance peaks using random matrix theory, and his predictions have recently been experimentally confirmed. In the same session, Argonne National Laboratory's John Schiffer described new simulations of the behavior of cold confined ions, which revealed that when both spherical and spheroidal ion clouds are cooled they form ordered structures that exhibit classical shells with magic numbers.

Mini-Symposia and Workshops

Speakers at the Thursday afternoon and Friday morning symposia on giant resonances described recent accomplishments, problems, and experimental challenges in giant resonance research, covering such topics as giant monopole resonance in cold and hot nuclei, multiphonons, the giant dipole resonance in hot nuclei, and the possibilities and promises of using unstable particle beams. Friday afternoon featured a mini-symposium on the phenomenon of "identical bands" in nuclei, whereby rotational cascades in different nuclei exhibit very similar transition energies and/or moments of inertia. Speakers at Saturday's symposia described recent investigations of a "caloric curve" of nuclear matter indicating a possible low-density phase transition, as well as of weakly bound halo nuclei.

Prior to, but in conjunction with, the DNP meeting, two workshops were held on Sunday. The first, focusing on the quark-gluon structure of the nucleon, covered such aspects as electric form factors of the neutron, quark seas and strangeness in the nucleon, and using lattice QCD to model nucleon structure. The second workshop featured talks on collective effects and the quark gluon plasma in heavy ion collisions, and disoriented chiral condensates. Speakers discussed multistrange baryons, signatures for quark gluon plasma, and recent experimental results at chiral phase transitions.


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Editor: Barrett H. Ripin