Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session JA: From Nucleons to the Origins of Nuclei |
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Chair: Witek Nazarewicz, University of Tennessee Room: Newport News Marriott at City Center Grand Salon I |
Saturday, October 13, 2007 2:00PM - 2:36PM |
JA.00001: The Various Proposed Mechanisms of Core-collapse Supernova Explosions: A Status Report Invited Speaker: The explosion mechanism for core-collapse supernova explosions has exercised theorists for more than four decades. During that time, much progress was made in understanding the basic physics and hydrodynamics, but no robust, definitive, and satisfactory solution emerged. Although, the neutrino-driven heating mechanism is still the favorite of most researchers, it has not been demonstrated to work generically, particularly in 1D and 2D simulations. Recently, an acoustic mechanism and magnetohydrodynamic jets have been added to the mix and it has been shown that the majority of gamma-ray bursts must be associated with a small subset of core collapses. Moreover, a new class of energetic supernovae (``hypernovae'') have been discovered. As a result, the study of the supernova mechanism has assumed a far wider portfolio and a greater richness than ever in the past. In this talk, I will discuss the menu of explosion mechanisms now available, and the status of multi-dimensional numerical simulations of the death of massive stars, the birth of neutron stars and black holes, and the origin of the elements. A theme of this talk will be the synergistic roles played by both sophisticated numerical simulation and nuclear physics, in all its particulars, in one of nature's most dramatic and important phenomena. [Preview Abstract] |
Saturday, October 13, 2007 2:36PM - 3:12PM |
JA.00002: Electric Form Factor of the Neutron Invited Speaker: Recent polarization-based precision measurements of the nucleons' elastic electric form factors have led to surprising results. The measurement of the ratio of the proton's electromagnetic form factors, $\mu_p G_E^p/G_M^p$, was found to drop nearly linearly with $Q^2$ out to at least $5 \mathrm{GeV}^2$, inconsistent with the older Rosenbluth-type experiments. A recent measurement of $G_E^n$, the neutron's electric form-factor saw $G_E^n$ does not fall off as quickly as commonly expected up to $Q^2 \approx 1.5 \mathrm{GeV}^2$. Extending this study, a precision measurement of $G_E^n$ up to $Q^2=3.5 \mathrm{GeV}^2$ was completed in Hall A at Jefferson Lab. The ratio $G_E^n/G_M^n$ was measured through the beam-target asymmetry $A_\perp$ of electrons quasi-elastically scattered off polarized neutrons in the reaction ${}^{3}\overrightarrow{He}(\overrightarrow{e},e' n)$. The experiment took full advantage of the electron beam, recent target developments, as well as two detectors new to Jefferson Lab. The measurement used the accelerator's 100\% duty-cycle high-polarization (typically 84\%) electron beam and a new, hybrid optically-pumped polarized ${}^{3}\overrightarrow{He}$ target which achieved in-beam polarizations in excess of 50\%. A medium acceptance (80msr) open-geometry magnetic spectrometer (BigBite) detected the scattered electron, while a geometrically matched neutron detector observed the struck neutron. Preliminary results from this measurement will be discussed and compared to modern calculations of $G_E^n$. [Preview Abstract] |
Saturday, October 13, 2007 3:12PM - 3:48PM |
JA.00003: Two-proton decay of 45Fe Invited Speaker: The decay of the extremely neutron deficient $^{45}$Fe is the the best candidate to study two proton radioactivity. This process was first predicted in 1960 by V.I. Goldansky to occur for only very few nuclei near the proton drip-line, for which the emission of the single proton is energetically forbidden, due to the pairing interaction. It was hypothesized, that the two protons will be emitted as a strongly correlated pair, a di-proton. Since then other scenarios have been predicted. The measurement of angular correlation between protons should provide an experimental signature for the nature of this process. In the recent experiment, performed at the NSCL at the Michigan State University, decay of $^{45}$Fe has been studied in detail using a new type of gaseous detector developed at the Warsaw University. Two-proton radioactivity channel was clearly identified. The production rate of the $^{45}$Fe nuclei was sufficient to measure enough events to determine the distribution of the angular correlations between the protons indicating a three-body character of this decay. Details of the experiment and the results will be presented. [Preview Abstract] |
Saturday, October 13, 2007 3:48PM - 4:24PM |
JA.00004: Shell Model Approach to Many-Body Open Quantum Systems Invited Speaker: The theoretical description of weakly-bound/unbound nuclei requires the treatment of the many-body correlations, the continuum of positive-energy states and decay channels. Solution of this problem has been advanced recently in the open quantum system formulation of the nuclear shell model (SM), the so-called Gamow Shell Model (GSM). GSM is the SM with a single-particle (s.p.) basis given by the Berggren ensemble consisting of Gamow states and the non-resonant continuum of scattering states. The principal limitation of GSM applications is the explosive growth in the number of configurations with both the number of particles and the size of the s.p. space. To ensure completeness of the basis, one should include a large set of non-resonant continuum states. Because of their presence, the dimension of the matrix representing the Hamiltonian $H_{\rm GSM}$ grows extremely fast and this matrix is also significantly denser than that of a conventional SM. To overcome these difficulties, we propose a method based on the density matrix renormalization group (DMRG) approach for finding the eigenstates of $H_{\rm GSM}$. During my presentation I will show results we obtained for the description of weakly-bound/unbound states for Helium and Lithium isotopes. [Preview Abstract] |
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