Nuclear

Experimental research is carried out at the on-campus cyclotron (see image) and at accelerators around the world. Theoretical research encompasses low and high energy nuclear physics. A description of the cyclotron is included in a separate brochure describing the Cyclotron Insititute and its research programs. The Institute also houses several VAX computers, more than 50 PC's and workstations, a magnetic spectrograph, a 4-pi neutron calorimeter, a proton spectrometer, a 50 element barium fluoride detector array, a recoil mass spectrometer (MARS), a target-making laboratory, and a variety of detection systems. The facility will provide heavy ion and alpha beams up to 86 MeV/nucleon as well as proton (55-80 MeV) and polarized and unpolarized deuteron (160 MeV) beams.

The experimental nuclear physics program includes work in the general areas of nuclear astrophysics (Gagliardi, Tribble), fundamental interactions (Gagliardi, Hardy, Tribble), and giant resonances (Youngblood). Experiments based at the local cyclotron include measurements with both stable and radioactive beams. Outside facilities that are utilized by local faculty include ATLAS at Argonne National Laboratory, TRIUMF in Vancouver, British Columbia, Fermilab near Chicago, and RHIC on Long Island.

Nuclear Astrophysics: Pioneering new techniques have been developed at TAMU for measuring solar reaction rates relevant to stellar burning and nucleosynthesis. New results have been obtained for several reaction rates including measurements that have important implications for the solar neutrino puzzle (the new results provide an independent determination of the number of solar neutrinos that detectors such as Super Kamiokande and SNO should observe). The experiments utilize both stable and radioactive beams produced at the K500 cyclotron. Radioactive beams are purified with the recoil spectrometer MARS.

Nuclear Structure: Giant resonances in nuclei are collective excitations that provide much information about the properties of nuclei and nuclear interactions. One of the most interesting of these resonances, the giant monopole resonance, was discovered at Texas A&M University. Its properties have been determined for a wide range of nuclei during the past decade. This has provided important information about the nuclear equation of state and neutron stars. The techniques that were developed to find the monopole resonance are now being applied to other giant resonance excitations. Also experiments are underway to measure giant-resonance properties in radioactive nuclei

Fundamental Interactions: The introduction of the Standard Model (SM) in the late 1960 s has had a profound influence on experimental nuclear and particle physics. Four nearly four decades, physicists have been testing the validity of this quark- and lepton- based picture. To date, we have found no definitive evidence of any failure in the model. However, measurements in nuclear beta decay have provided some of the most precise information available to date on the possible existence of new physics beyond the Standard Model. A series of measurements of Ft values in various nuclei which, according to the SM should all give the same result, are shown in this figure. A detailed examination of the measurements shows that they do indeed give consistent results. However, when the average Ft value from these same measurments is used to test unitarity of the SM (see results) the sum of the three CKM matrix elements differs from 1 by more than two standard deviations. The unitarity test is one of the few tantalizing indications of a break-down of the Standard Model. TAMU faculty are working on experiments both at the cyclotron and at other accelerator labs to make this important test even more definitive. The MARS recoil spectrometer coupled with the K500 cyclotron have proven to be a powerful combination for producing new nuclei where precision measurements can be made to further define the Ft value results and to test the theoretical corrections that must be applied to the nuclear beta decay measurements.

Head-on collisions of heavy nuclei at high energies will create highly excited, very dense, neutron-rich nuclear matter in the laboratory. Understanding the properties of nuclear matter under such extreme conditions is intrinsically interesting and will add greatly to our ability to understand astro-physical phenomena such as supernovae explosions, neutron-star formation, and the evolution of the early universe. Using models based on quantum chromodynamics and effective hadronic theories, nuclear theorists at TAMU (Ko) are studying the properties of hadrons in such a nuclear medium. Work is underway to describe the evolution of heavy-ion collision dynamics with a multiphase transport model that includes effects of a quark-gluon plasma. Also, lattice models and Monte Carlo methods (Chin) are used to study a variety of strongly interacting quantum systems such as pionic collective states, finite nuclei, atomic clusters and helium droplets.

External Nuclear Physics Research: Nuclear physics research is carried out at a variety of accelerators around the world. The group at TAMU is now involved in experiments at Fermilab, Argonne National Laboratory (ANL), TRIUMF and RHIC. At Fermilab, Drell-Yan, and J/Psi production have been studied using the 800 GeV beam in fixed target mode. Some results of these measurements, including the work by former student Eric Hawker (see image), have already been published. Work at ANL and TRIUMF is aimed at precision studies of the Standard Model. An improved technique for making very precise mass measurements of radioactive nuclei was pioneered at Chalk River in Canada; one of our faculty members was part of that collaboration. The apparatus, the Canadian Penning Trap or CPT (see image), has now been relocated to ANL using the ATLAS accelerator. Mass measurements using this device will supplement lifetime and branching ratio measurements being made at TAMU to provide new tests of the unitarity of the CKM matrix. The CPT will likely move to TRIUMF within the next two to three years when low-energy radioactive beams become available there.

Another experiment now underway at TRIUMF, E614, is a precision measurement of the Michel parameters in the normal decay of the muon. The Standard Model gives exact predictions for the four Michel parameters which determine the muon decay. Since the decay involves only leptons, the Standard Model predictions are not subject to the usual uncertainties associated with the strong interaction. A new spectrometer is being constructed at TRIUMF specifically for this project. It consists of state-of-the-art wire chambers that are used for tracking positrons from muon decay in a 2.2 T magnetic field. The frames for the wire chambers are made with optical quality glass and are positioned in the spectrometer with glass spacers that are polished to obtain an alignment precision of a few microns.