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Nuclear Physics Research

Experimental research is carried out at the on-campus Cyclotron Institute, as well as at other laboratories and accelerators around the world. Theoretical research encompasses low and high energy nuclear physics. A description of the Cyclotron Institute is included in a separate overview describing its research programs. The Institute is nearing completion of an upgrade project which is re-commisioning the original K150 cyclotron and coupling it to the present workhorse of the facility, the K500 superconducting cyclotron. This upgrade will extend the Institute's production capability for radioactive ion beams, as well as provide high-quality re-accelerated secondary beams in an energy range unique to the world.

Ion beams currently available at the Institute span a wide range of nuclei and energies, from 8–70 MeV protons up to uranium ions with energies between 500 MeV (2MeV per nucleon) and 3.5 GeV (15 MeV per nucleon). Facilities of the Institute that purify and take advantage of the rare ion beams provided by the cyclotrons include: MARS, the momentum achromat recoil separator; the MDM spectrometer; NIMROD, a 4π neutron and charged particle detector; a precision on-line decay station; light and heavy ion guides; and a radiation effects facility used by a wide variety of commercial, governmental and educational groups.

Experimental Faculty

Prof. Carl Gagliardi's group is part of the spin physics working group of the STAR collaboration and are investigating the origin of the proton spin via polarized p+p collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Deep-inelastic scattering (DIS) experiments with polarized leptons and polarized nucleons have found that the spins of quarks and antiquarks account for only about 25% of the nucleon spin. By carefully measuring the asymmetry of p + p vs p + p spin-polarized collisions, the gluon and sea quark polarization distributions — which presently represent the poorly constrained part of the proton's spin function — can be determined. Results so far have decisively excluded models that proposed large positive gluon polarization to explain the small quark spin contribution to the nucleon spin, and agree with next-to-leading order perturbative QCD evaluations. Present research is focused on di-jet measurements to provide direct information about the gluon polarization distribution.

Prof. John Hardy uses nuclei and nuclear techniques to probe the fundamental properties of the weak force. He and his group specialize in precision measurements of beta-decay half-lives, branching ratios and Q-values. By measuring these quantities to a precision of 0.1% or better, they have used a series of "superallowed" beta decays to test the constancy of the weak interaction in nuclei to ±0.01%. This result has also led to a wider test of the weak-interaction universality, via the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, one of the pillars of the Standard Model. Currently the sum of squares of the experimentally determined top-row elements of the CKM matrix is 0.9999(6), in excellent agreement with unitarity. This important result limits the scope for physics beyond the Standard Model. The group is currently striving to improve the precision of their measurements and thus to improve the precision of these tests. The group has also been active recently in other tests requiring high precision: 1) a search for any possible dependence of radioactive half-lives on external physical properties; and 2) measurements of internal conversion probabilities for high-multipolarity electromagnetic transitions, to test the validity of the theory used to calculate internal conversion coefficients.

Prof. Dan Melconian and his group use the atomic nucleus as a laboratory for probing the fundamental symmetries of the electroweak interaction and searching for physics beyond the Standard Model (SM) of particle physics. To be sensitive to new physics and complement the direct searches being made at colliders, their measurements of the correlation parameters from β decay must be extremely precise: 0.1% or better. To attain that sensitivity, the group employs trapping techniques on short-lived atoms and ions to provide them with a clean, cold, point-like source of β-decaying nuclei. Prof. Melconian is spokeperson for an experiment at TRIUMF which uses a magneto-optical trap to cool and confine neutral isotopes of Potassium and polarizes the sample to >99% via optical pumping techniques; the goal is to measure the β and ν asymmetries (and other correlations) of this decay to check whether Nature is purely left-handed — as the SM predicts — or if a right-handed sector exists but is only difficult to see at our low energies. The group is also in the process of constructing a Penning trap to confine ions of any species within a 210mm-diameter bore, 7T magnet. This novel trap and completion of the upgrade to the Cyclotron Institute will allow the group to study β-delayed particle decays of the proton-rich nuclei 20Mg, 24Si, 28S, 32Ar, and 36Ca. These studies will search for scalar contributions to the weak interaction and help test shell models of isospin mixing in pure Fermi decays.

Prof. Saskia Mioduszewski is experimentally investigating properties of the medium created in ultra-relativistic heavy-ion collisions, as a member of the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC). At high enough temperature and density, the matter created in these collisions has been predicted to undergo a phase transition between ordinary hadronic matter and one in which quarks and gluons interact freely (deconfined matter). The heavy-ion collisions at RHIC recreate the hot and dense conditions of the universe just milliseconds after the Big Bang, where quarks and gluons (partons) are believed to have combined to form hadrons. Prof. Mioduszewski's group focuses on two probes of the matter created at RHIC. One probe addresses the density of the matter through the study of energy loss of hard-scattered partons as they traverse the medium. The other probe is heavy quarkonium, which is a signature of deconfinement in heavy-ion collisions, and its production mechanism in p+p collisions.

Prof. Robert Tribble and his research group are carrying out experiments to determine rates for important reactions that occur in stellar evolution. With the deployment of new ground-based and satellite-based observatories, astronomers have much better information about stellar compositions from a wide range of stars including those that formed at the earliest times in our universe. Nuclear reactions that occur in the stars and also occur during stellar explosions lead to the synthesis of the elements that we exist in our world today. These same reactions provide the energy that powers the stars. Many of the reactions that we need to determine in order to model stellar evolution take place on short-lived radioactive nuclei. We have developed new indirect techniques to determine these reaction rates at energies appropriate to stars. The indirect approach uses beams of radioactive nuclei that are produced at the Cyclotron Institute from interactions of stable beams that are accelerated in the K500 superconducting cyclotron and a recoil spectrometer to form a secondary beam. Reactions using the radioactive beams provide the information that is needed to determine the rates of many proton-capture reactions that are important in stellar evolution. Beginning near the end of 2011, we will produce radioactive beams that are accelerated directly by the K500 cyclotron using the upgraded facilities at the Cyclotron Institute.

Prof. Dave Youngblood and his group investigate giant resonances in nuclei to ascertain general properties of nuclei and of nuclear matter. The isoscalar giant monopole resonance (ISGMR) is of particular importance, because the energy of this resonance is directly related to the compressibility of the nucleus. From this, the compressibility of nuclear matter, one of the constituents of the nuclear equation of state, can be determined. Dr. Youngblood's group pioneered the techniques necessary to study this resonance and was the first to identify it. Texas A&M is one of only two laboratories in the world where these studies can be carried out. Recent studies have focused on the symmetry energy [proportional to (NZ)/A] in nuclei, and a new detector will allow studying these resonances in unstable nuclei. The compressibility and the symmetry energy are important for studies of nuclear reactions and for understanding the behavior of nuclear matter in stars. Recent data from Youngblood's group have suggested that the details of nuclear structure may be altering the energy of the ISGMR, which could have a significant effect on compressibility and symmetry energy studies and this is being investigated.

Theoretical Faculty

Prof. Rainer Fries is interested in collisions of hadrons and nuclei at high energies in order to investigate the properties of the underlying Strong Interaction (QCD). One direction of his research focuses on the dynamics of relativistic heavy ion collisions, beginning from the Color Glass Condensate in nuclei, to the formation of a Quark-Gluon Plasma (QGP) phase in the nuclear collisions. Quark-Gluon Plasma is a primordial form of matter which existed a few microseconds after the big bang at temperatures larger than 1,000,000,000,000 Kelvin. He is also conducting research to effectively use QCD jets and weakly interacting particles (photons, leptons) as tomographic probes for both cold nuclei and Quark-Gluon Plasma. A related research goal is a consistent treatment of multiple scattering in perturbative QCD computations. His recent research accomplishments include a phenomenologically very successful recombination model describing how quarks in a cooling Quark-Gluon Plasma form hadrons, and fundamental work on properties of the proton wave function. Prof. Fries's theoretical work is connected to the large experimental programs at the Relativistic Heavy Ion Collider (RHIC), the upcoming Large Hadron Collider (LHC) at CERN and a future Electron Ion Collider (EIC) in the US.

Prof. Che Ming Ko's recent research includes theoretical work in both isospin physics and RHIC physics. Using an isospin-dependent transport model that includes different proton and neutron mean-field potentials and scattering cross sections, he and his collaborators have found many interesting phenomena in collisions of radioactive nuclei that are sensitive to the properties of nuclear symmetry energy. In particular, they have extracted from the experimental isospin diffusion data valuable information on the density dependence of the nuclear symmetry energy at subnormal densities, which has further allowed them to put stringent constraints on the parameters in nuclear effective interactions. For relativistic heavy ion collisions, they have developed a multi-phase transport model that includes interactions in both initial partonic and final hadronic matters as well as the transition between these two phases of matters. Using this model, they have obtained useful information on the properties of the quark-gluon plasma produced at RHIC from studying the abundance, collective dynamics, and correlations of produced hadrons of both light and heavy flavors. They are currently extending these studies to make predictions for heavy ion collisions at future Facilities for Rare Isotope Beams and Large Hadron Collider.

Prof. Ralf Rapp and his group conduct systematic theoretical studies of spectral properties of hadrons in hadronic matter and in the Quark-Gluon Plasma (QGP), their relations to QCD phase transitions and observable signatures in heavy-ion collisions. Most of the visible mass in the universe is believed to be generated by the spontaneous breaking of chiral symmetry in QCD, which, however, will be restored in the QGP. Employing hadronic many-body theory, Rapp and his co-workers are evaluating spectral functions of the rho meson (and its chiral partner, the a1 in hot and dense matter. The calculations predict a strong broadening (even "melting") of the rho resonance in hot and dense matter. Recent measurements of dilepton invariant-mass spectra in heavy-ion collisions at the CERN-SPS have confirmed these calculations. Employing heavy-quark potentials from finite-temperature lattice QCD computations, Rapp's group is evaluating the properties of heavy-quark bound states in the QGP. The resulting quarkonium spectral functions are checked against Euclidean correlation functions computed in lattice-QCD and applied to charmonium and bottomonium observables at RHIC and LHC. Lattice-QCD potentials are furthermore implemented to compute heavy-quark transport properties of the QGP. Large friction and small diffusion coefficients support the notion of a strongly coupled QGP as suggested by experimental results from RHIC. Rapp and co-workers are also interested in color-superconducting cold dense quark matter and associated signatures in observables from compact ("neutron") stars, including mechanisms for gamma ray bursters.