Physics Division 1996 to 1998
THEORETICAL AND COMPUTATIONAL PHYSICS
OVERVIEW
Since its inception, the ORNL Theory Group has been an international
leader in the development and application of innovative computer
simulations. Large-scale computer
codes developed at ORNL as part of Grand challenge initiatives have
shaped the development of atomic, nuclear, and astrophysics programs
and continue to be used by the physics community. Presently the
group operates a sixteen-processor Cray J90 for development work
and extensively uses massively parallel supercomputers. Work in
atomic and nuclear physics has developed new initiatives and programs in astrophysics that complement the on-going nuclear astrophysics programs in the Physics Division.
ATOMIC PHYSICS
The Physics Division's theoretical research in atomic physics fulfills
three basic missions: broad fundamental studies, work enabling energy science, and improved atomic-and-molecular-physics-based astrophysics. Close coordination and collaboration with other Physics Division activities, particularly including the atomic experimental programs and other researchers on a national and international basis, is vigorous.
The basic energy science component of this work includes elements such as the study of the interaction of matter with intense electromagnetic fields, collisions of atomic particles with solids, surfaces, atoms, and molecules, and the development of new mathematical and computational approaches for these investigations. Recent advances include the development of a new hyperspherical hidden-crossing approach to study ion– and electron–atom collisions, the elucidation of the manipulation
of highly excited atoms using designer laser pulses, and the development of an ab initio transport theory of atomic states traversing solids and surfaces. These, and other advances made by the group, improve the availability of theoretical and computational tools available and utilize them to provide new, basic understanding of atomic phenomena.
Work is also proceeding in the development of new computational techniques and the production of needed information in support of fusion energy sciences. Recent accomplishments in this area include the production and analysis of a huge database of atomic and molecular collision data pertaining to the transport properties of low-temperature plasmas. In addition, as an outgrowth of the plasma science-supporting atomic physics activities, a new program in atomic astrophysics has been formed through initial ORNL Seed Money funding and subsequent grants from NASA. (Click
here to view the atomic astrophysics research.) This new and novel program seeks to improve the understanding of diverse astrophysical phenomena such as supernovae and novae ejecta, x-ray emission from gas giant planets and comets, emissions from interstellar clouds, and the atomic and molecular chemistry of the early universe and of protogalaxy formation.
NUCLEAR PHYSICS AND ASTROPHYSICS
The nuclear theory program comprises basic research in the areas of
nuclear structure,
astrophysics, relativistic heavy-ion physics, and hadron
spectroscopy. The structure and astrophysics programs provide support for the experimental programs within the Physics Division, notably the research on high-spin states, nuclei far from stability, and properties of nuclei for astrophysics simulations. The focus of the
astrophysics research is the study of stellar explosions and
nucleosynthesis. Part of this research is to ascertain the nova
ignition mechanism and to tie this effort to nuclear physics measurements that will be made at the Holifield Radioactive Ion Beam Facility. Our research on hadrons is concerned with the structure of hybrid and exotic mesons and their decays. This work is strongly tied to the experimental programs at CEBAF and at the AGS. The research on relativistic heavy ions is closely connected to the PHENIX
detector at RHIC. The Physics Division has responsibility
for the construction and operation of PHENIX, and our efforts in
studying the J/psi and electromagnetic probes has been a major
source of support for the collaboration.
The underlying goal of our research in nuclear structure is to achieve a basic understanding of the structure and dynamics of complex nuclei. Progress in nuclear physics has been made traditionally through an interplay between theory and experiment, and this is a guiding principle in our approach. We have
developed a variety of mean-field and shell-model techniques, and
have applied these to the study of nuclei far from the valley of beta
stability. The structure of these nuclei and their excitation properties are not well known. Much of this work is based on Hartree–Fock
or Hartree–Fock–Bogoliubov calculations with effective forces. A notable success is our study of two-proton separation energies for proton-rich light mass nuclei. Here, mean-field calculations give a systematically good representation of the data. Also, masses, deformations, and
single-particle properties have been examined for nuclei with
neutron number N about 28. Such nuclei are precursors for the
nucleosynthesis of Ca–Ti–Cr isotopes. From our studies, we have predicted a deformation inversion near 44S. This result has recently been confirmed experimentally. In a very different approach, the interacting shell at finite temperature has been used with semi-realistic effective interactions to study Gamow–Teller transitions in nuclei near Fe. Electron capture in these nuclei play an important role in supernovae ignition and burning. The shell-model Monte Carlo method is remarkably successful in reproducing masses and transition strengths for these nuclei.
Understanding explosive nucleosynthesis is one of the major problems
facing astrophysics, and central to this is the basic understanding of
the explosion mechanism of supernovae. A key element of this
understanding and the principal focus of our research in supernovae
modeling is the correct treatment of neutrino transport
in these explosions. Neutrino transport is believed to be the primary
mode of energy transfer that is responsible for powering core collapse
supernovae. The ORNL codes developed by Mezzacappa and collaborators are unique in their propagation of neutrinos via an exact solution of the Boltzmann transport equations. Our simulations in one and two dimensions have shown that the proper treatment of neutrino transport substantially dampens convective energy transfer mechanisms and thereby inhibits the explosion. The astrophysical site of the r-process and the production of a large fraction of the trans-iron elements is still not certain; however, core collapse supernovae are possible
candidates. The study of stellar and explosive nucleosynthesis form an important part of our astrophysics research. The
production of Fe-group nuclei from Si
burning has been extensively studied by Hix and Thielemann under both
hydrostatic and explosive conditions. They have found that stopping
points in the reaction flow break up the reaction products into groups
of nuclei in quasi-equilibrium, coupled by slower reactions. This
observation opens the possibility of factoring the network equations
that govern the reaction and thus reducing the size of the
nucleosynthesis calculations without affecting its accuracy.
The work on relativistic heavy-ion collisions in support of the
PHENIX collaboration has been very fruitful. The study of the
J/psi, together with production and suppression mechanisms, has been
an important experimental effort at ORNL. Wong has extensively studied the problem and has developed a comprehensive picture of the
production and absorption applicable to both proton and heavy-ion
collisions. His analysis of recent CERN data on heavy-ion suppression
of the J/psi may indicate the possible formation of the quark-gluon
plasma.
The study of hadron spectroscopy may prove to be an exciting area of
high-energy nuclear physics. Our theoretical work modeling the
structure and reactions of mesons in terms of QCD has predicted a
number of new and fascinating structures, including both hybrid and
exotic mesons. The discovery of a 1-+ exotic meson earlier in 1997 near our predicted result has led to much experimental activity and the possible identification at CERN of two new exotic
mesons. We have also studied the 3P0 model of strong decays, and have calculated 374 two-body decay modes of all uu(bar) and
dd(bar) mesons.