What is the structure of the nucleon and how do its various interactions and excitations reveal the properties of its constituents? Moreover, how does the existence of these non-nucleonic particles affect the properties and structures observed in nuclei? These are among the questions which are currently being addressed in experimental and theoretical programs in nuclear and particle physics at the University of Kentucky.
Members of the theoretical particle physics group are undertaking studies of the strong, electromagnetic, and weak properties of elementary particles. They are investigating their internal structures, using both phenomenological models and field-theoretic approaches.
The quantum chromodynamics (QCD) theory of quarks and gluons for the strong interactions has achieved remarkable successes in recent years, and the group is involved in both perturbative and non-perturbative approaches. A large effort, based on lattice gauge Monte Carlo calculations using supercomputers, is leading to new insights about the structure of hadrons. Calculations of the electromagnetic and axial form factors of the proton, neutron, and other baryons, and of transition form factors between octet and decuplet baryons agree well with experiment and reveal, for example, the way in which quarks of different flavor in a proton contribute to its magnetic moment, thus uncovering deficiencies in models of hadronic structure. Furthermore, the relative importance of ``valence'', ``cloud'', and ``sea'' quarks is determined. In another direction, a new variational calculation of mesons consisting of a light and a very heavy quark provides an optimal way of studying B meson physics, which will be the best forum for testing the standard model.
It is believed that in the proper limit, the low-energy properties of QCD can be reproduced effectively by a field theory of mesons, in which the baryons emerge as topological solitons. The theory group has been exploring phenomenological aspects of a chiral soliton model to obtain static properties of nucleons and the phase shifts for pion-nucleon scattering.
The nuclear theory group is doing work in both low and intermediate energy. For many years the Skyrme effective nucleon-nucleon action has been used to describe nuclear systems. Despite its many successes, this force did not fit the basic compressibility of nuclear matter, and gave rather poor results for the electric and magnetic excitations of nuclei. The group at Kentucky has added density-dependent terms to this interaction which have led to a much improved description of the properties of both nuclear matter and the Hartree-Fock description of the ground states of nuclei. The resulting interaction leads to good agreement with experimental data for electromagnetic excitations, and the particle-particle interaction calculated from particle-hole plus core polarization contributions has been used in a fruitful study of nuclei with two nucleons outside a closed shell.
The recent availability of good experimental nucleon-nucleus polarization scattering data at intermediate energies has stimulated an analysis at UK of the relativistic optical model and the relativistic impulse approximation for scattering. Relativistic Brueckner-Hartree-Fock studies of the ground state properties of nuclear matter using realistic nucleon-nucleon interactions are underway. Effective interactions derived from nuclear matter calculations are being used in Dirac-Hartree-Fock studies of closed shell nuclei, and relativistic optical model potentials have been derived for use in intermediate energy nucleon-nucleus scattering. Differential cross sections, analyzing powers and spin rotation functions calculated in this model for 160--800 MeV proton scattering from Ca and Pb compare very well with experimental data.
The experimental program in medium energy nuclear physics seeks to understand the role of non-nucleonic particles, such as pions, deltas, quarks and gluons, in determining the observed properties of nucleon and other baryons. These studies involve measurements of electromagnetic, strong, and weak processes, and are carried out at large national and international accelerators throughout the world. The group has pion, muon, and neutron experiments in progress at the TRIUMF laboratory in Canada and the KEK laboratory in Japan, and is planning kaon and electron experiments at the Brookhaven National Laboratory and the CEBAF laboratory in the USA.
At the TRIUMF laboratory the experimental group is using a pion beam and a hydrogen target to produce the delta particle, the first excited state of the nucleon. The delta particle is very short-lived, and using massive photon detectors we are studying its decay via gamma-ray emission. The angular distribution of these gamma-rays reflect the structure of the delta particle, in particular, whether the particle is spherical or deformed. Measurement of the shape of particles such as the delta will provide a critical test of the theories of baryon structure in terms of their quark and gluon constituents.
Also at TRIUMF we are using muon beams to study the weak interaction of both the free and the bound proton. In particular we are measuring a component of the proton's weak force known as the weak pseudoscalar interaction which arises from the cloud of virtual pions surrounding the proton core. The strength of this interaction is a reflection of the size and range of the pion cloud surrounding the proton. Its measurement for the free proton is an important test of models of proton structure that use the languages of both mesons and baryons or quarks and gluons. Its measurement for the bound proton, from light to heavy nuclei, provides an instrument with which to study the pion cloud inside the atomic nucleus.
At the KEK laboratory in Japan using a 1 GeV neutron beam the most fundamental of nuclear reactions is being investigated, the capture of a neutron by a proton to form the simplest nucleus, the deuteron, through the emission of a gamma-ray. At these high energies the full richness of the deuteron structure, including its pion, delta, quark and gluon components, will be illuminated by this nuclear reaction. The measurement of the angular distribution of the gamma-rays emitted in the process will reflect the roles that these exotic components play in the deuteron nucleus.
We are planning new experiments to study the structure protons, neutrons, and other baryons. Using an electron beam at the CEBAF laboratory we will study the strange and anti-strange quark content of the proton, and using a kaon beam at Brookhaven National Laboratory we will study the structure of short-lived baryons containing a single strange quark. Both of these experiments will further challenge our models of the structure of baryons and mesons.
The intermediate energy group operates a detector laboratory in the Chemistry-Physics Building where students assemble and test the components required for each experiment. This includes the construction and testing of wire chambers, plastic scintillators, and semiconductor detectors. Graduate students are thus able to participate in the research of the group from the beginning of the design of an experiment, through the data collection at the accelerator laboratory, to the analysis and interpretation of the results.
Experimental studies of complex nuclear structures are the primary focus of the research carried out at the 7 MV accelerator, located in the Chemistry-Physics building. Other measurements include the nuclear reaction rates relevant to problems in nuclear astrophysics, such as neutron and other particle burning rates in massive stars and studies of the nuclear reactions important in the design of solar neutrino detectors.
Examining the gamma-ray decays of nuclear levels excited in neutron scattering reactions allows an identification of levels which exhibit the special collective character due to the participation of many nucleons in a coherent excitation. Harmonic excitations have been observed in tin nuclei which correspond to phonon excitations of the nuclear surface. Promising candidates for the elusive three phonon excitations have been identified. Strong collective levels have been observed to co-exist in nuclei along with much more complex excitations, without significant mixing occurring between the two classes of levels.
The neutron scattering program also studies collective nuclear excitations, through measurements of the spatial distributions of scattered neutrons. The scattering patterns yield clear signatures of those excited levels which correspond to oscillations of the nuclear surface, and provide information on the susceptibility of nuclei to deformation.
Faculty and Research Staff