Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session X3: Topics In Nuclear Physics |
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Sponsoring Units: DNP Chair: Richard Casten, Yale University Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), St. Louis E |
Tuesday, April 15, 2008 1:30PM - 2:06PM |
X3.00001: Correlations in ground-state nuclei and their dependence on the neutron-proton ratio Invited Speaker: Although a mean-field description enables an understanding of many aspects of nuclear structure, a full description of nuclei and nuclear matter requires the consideration of the correlations between the nucleons. These include short-range correlations associated with hard-core and tensor interactions and the longer-range correlations due to couplings of the nucleons to low-lying collective states and giant resonances. These correlations and their dependence on the neutron-proton ratio are investigated in Ca isotopes with the dispersive optical model developed by Mahaux. In addition to the standard optical model potential, a dispersive correction is included which then gives the correct relationship between the real and imaginary potentials due to causality. This dispersion correction allows the optical model to be used for bound single-particle states. By fitting proton elastic-scattering data, reaction cross sections, and level properties of valence hole states deduced from (e,e$'$p) reactions, it is shown that proton long-range correlations increase with neutron richness. Additional data is needed to determine the dependence for neutrons. The dispersive optical model is shown to allow for data-driven extrapolations to the drip-lines, but more data is presently needed to make useful predictions. [Preview Abstract] |
Tuesday, April 15, 2008 2:06PM - 2:42PM |
X3.00002: The strange electromagnetic form factors of the nucleon at low $Q^2$ Invited Speaker: The strange electric and magnetic form factors of the nucleon, $G_{E}^s$ and $G_M^s$, give the contributions of strange quarks to the charge and magnetization distributions of the nucleon, which solely arise from the sea of $s\bar s$. $G_E^s$ and $G_M^s$ can be determined by combining the well-known electromagnetic form factors of the proton and the neutron, $G_{E,M}^{\gamma,p}$ and $G_{E,M}^{\gamma,n}$, with the neutral weak form factors of the proton, $G_{E,M}^{Z,p}$, which can be measured via parity-violating (PV) elastic electron scattering. In the past 10 years, a series of definitive PV electron scattering experiments along with several theoretical studies now provide a basis for extracting precision information on these strange quark contributions. In this talk, I will briefly review the experimental technique, and give a summary of the PV elastic electron scattering measurements (at low momentum transfer in particular). A global analysis to extract $G_E^s$ and $G_M^s$ from the data and the physics implications to the ``strange sea'' will then be presented. [Preview Abstract] |
Tuesday, April 15, 2008 2:42PM - 3:18PM |
X3.00003: Test QCD symmetries via precision measurement of the neutral pion lifetime Invited Speaker: Symmetries and their dynamical breaking effects play fundamental roles in the nature. In particular, the three light neutral pseudoscalar mesons $\pi ^{0}$, $\eta $ and $\eta \prime $ contain fundamental information about QCD symmetries. While $\pi ^{0}$ and $\eta $ are Goldstone bosons due to spontaneous chiral symmetry breaking, the $\eta \prime $ is not due to the U(1)$_{A}$ anomaly. There is a second type of anomaly driving the two-photon decays of these mesons. Since $\pi ^{0}$ is the lightest meson in the hadron spectrum, the chiral anomaly prediction for the $\pi ^{0}$\textit{$\to \gamma \gamma $ }decay width is more accurate and is exact in the limit of massless quarks. In the real world, the SU(3) and isospin breaking by the light quark masses lead to important mixing effects among these mesons. Theoretical activities in this field over the past several years have resulted in high precision (1{\%} level) predictions for the decay amplitude of the $\pi ^{0}$ into two photons. As a result, the experimental measurement on this quantity with a comparable precision will provide an important test on the fundamental QCD predictions. The present experimental uncertainty of the $\pi ^{0}$ decay amplitude, according to the PDG average, is an order of magnitude greater than the theoretical uncertainties. The PrimEx collaboration at Jefferson Lab has recently developed and performed a new experiment to measure the neutral pion life time with high precision using the small angle coherent photoproduction of $\pi ^{0}$'s in the Coulomb field of nucleus, $i.e.$, the Primakoff effect. A new level of experimental precision ($\sim $2.9{\%} total error) has been achieved by implementing the new high intensity and high resolution photon tagging facility in Hall B at Jefferson Lab and by developing a novel high resolution electromagnetic hybrid calorimeter (HYCAL). The final result of this experiment will be presented. The advent of a 12 GeV electron beam at Jlab will make it possible to extend the program to $\eta $ and $\eta \prime $. A plan for future $\eta $ and $\eta \prime $ program will be discussed. [Preview Abstract] |
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