Bulletin of the American Physical Society
2005 APS April Meeting
Saturday–Tuesday, April 16–19, 2005; Tampa, FL
Session C13: Nuclear Theory I |
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Sponsoring Units: DNP Chair: James Vary, Iowa State University Room: Marriott Tampa Waterside Room 12 |
Saturday, April 16, 2005 1:30PM - 1:42PM |
C13.00001: Neutron rich nuclei in heaven and earth Bonnie Rutel, Jorge Piekarewicz An accurately calibrated relativistic parametrization is introduced to compute the ground state properties of finite nuclei, their linear response, and the structure of neutron stars. While similar in spirit to the successful NL3 parameter set, it produces an equation of state that is considerably softer---both for symmetric nuclear matter and for the symmetry energy. This softening appears to be required for an accurate description of various giant resonances of nuclei with different neutron-to-proton ratios. Among the central predictions of this model are a compression modulus for symmetric nuclear matter of $K\!=\!230$~MeV and a neutron skin of ${}^{208}Pb$ of $R_{n}-R_{p}\!=\!0.207$~fm. Further, the impact of such a softening of the equation of state on the properties of neutron stars is as follows: we obtain a limiting neutron star mass of $M_{\rm max}\!=\!1.722~M_{\odot}$ and a radius for a ``canonical'' $M_{\star}\!=\!1.4~M_{\odot}$ neutron star of $R_{\star}\!=\!12.655$~km. [Preview Abstract] |
Saturday, April 16, 2005 1:42PM - 1:54PM |
C13.00002: Tunneling of a system with internal degrees of freedom Vladimir Zelevinsky, Alexander Sakharuk The tunneling reactions play crucial role in nuclear physics, astrophysics and chemical physics. However, the old question of how the internal structure of a tunneling composite object can ifluence the tunneling process remains poorly understood. We consider a model of a bound two-particle system tunneling through a potential barrier that acts only on one particle. In adiabatic motion, the intrinsic structure is adjusted as a function of the distance from the barrier (polarization effects). The effective potential for the tunneling of the center-of-mass of the two-body system is constructed and the penetrability coefficient is calculated. Comparison with standard point-like tunneling is done. Being simple enough to allow an accurate numerical solution, the problem is important for different physical and astrophysical applications. In addition, the problem allows straightforward generalizations to more realistic processes. [Preview Abstract] |
Saturday, April 16, 2005 1:54PM - 2:06PM |
C13.00003: Renormalization of general one- and two-body operators in the no-core shell model Ionel Stetcu, Bruce R. Barrett, Petr Navratil, James P. Vary We implement an effective operator formalism for general one- and two-body operators, obtaining results consistent with the no-core shell model (NCSM) wave functions. The Argonne V8' nucleon-nucleon potential was used in order to obtain realistic wave functions for $^4$He, $^6$Li and $^{12}$C. In the NCSM formalism, we compute electromagnetic properties using the two-body cluster approximation for the effective operators and obtain results which are sensitive to the range of the bare operator. To illuminate the dependence on the range, we employ a Gaussian two-body operator of variable range, finding weak renormalization of long range operators (e.g., quadrupole) in a fixed model space. This is understood in terms of the two-body cluster approximation which accounts mainly for short-range correlations. Consequently, short range operators, such as the relative kinetic energy, will be well renormalized in the two-body cluster approximation. In particular, we show that the expectation values of operators involving large momentum transfer become independent of the model space and harmonic oscillator frequency used in calculation, even in the two-body cluster approximation. I.S. and B.R.B acknowledge partial support by NFS grants PHY0070858 and PHY0244389. P.N. received support from LDRD contract 04-ERD-058 and USDOE contract No. W-7405-Eng-48. J.P.V. acknowledges partial support by USDOE grant No DE-FG-02-87ER-40371. [Preview Abstract] |
Saturday, April 16, 2005 2:06PM - 2:18PM |
C13.00004: The Electromagnetic Current in Nuclear Many-Body Physics Carrie E. Halkyard, Brian D. Serot The electromagnetic current is studied in a recently proposed effective field theory of the nuclear many-body problem.\footnote{% R.~J.~Furnstahl, B.~D.~Serot, H.-B.~Tang, Nucl.\ Phys.\ {\bf A615}, 441 (1997).} The Lorentz-invariant lagrangian contains nucleons, pions, isoscalar scalar $(\sigma )$ and vector $(\omega )$ fields, and isovector vector $(\rho )$ fields. The theory exhibits nonlinear $SU(2)_L \times SU(2)_R$ chiral symmetry and has three desirable features: it uses the same degrees of freedom to describe the nuclear currents and strong-interaction dynamics, it satisfies the symmetries of the underlying theory of QCD, and its parameters can be calibrated using strong-interaction phenomena. The low-energy structure of the nucleon is described using vector-meson dominance, so that external form factors are not needed. For normal nuclear systems, it is possible to expand systematically the effective lagrangian in powers of the fields (and their derivatives) and to truncate the expansion reliably after the first few orders. The fully U(1)-gauge-invariant lagrangian is derived for the first few orders of the field expansion, including terms that are both linear and quadratic in the electromagnetic charge, and the electromagnetic current is exhibited to all orders in the pion field. Explicit expressions are derived for the Lorentz-covariant amplitudes of the one- and two-nucleon electromagnetic currents. [Preview Abstract] |
Saturday, April 16, 2005 2:18PM - 2:30PM |
C13.00005: Correlated nucleons in nuclei Athanasios Petridis, Drew Fustin, Aaron Frazier, James Vary Jefferson Laboratory data (K. Egiyan et. al., talk given at the April 2004 meeting of the American Physical Society) indicate that there exist short-range correlations in nucleons bound in nuclei leading to a step-like behavior of the heavy-to-light nuclei cross section ratios versus the Bjorken-x in deeply inelastic scattering (DIS). A possible theoretical model to interprete this effect is the formation of multi-quark clusters in the nucleus. These are due to the overlap of nucleon wavefucntions leading to six, nine, or more valence quark color singlets. A semiclassical algorithm has been developed to calculate the multi-quark-cluster probabilities for arbitrarily large nuclei. Two-body correlations are introduced and the results are compared to those obtained with the independent particle model. The calculated probabilities are close to those needed to fit the DIS data as well as data on Drell-Yan production and J/$\Psi$ suppression in nucleon-nucleus collisions. [Preview Abstract] |
Saturday, April 16, 2005 2:30PM - 2:42PM |
C13.00006: Modelling Nucleon-Nucleon Scattering Above 1 GeV Ruprecht Machleidt, Oleg Eyser, Wolfgang Scobel Motivated by the recent measurement of proton-proton spin-correlation parameters up to 2.5 GeV laboratory energy, we investigate models for nucleon-nucleon (NN) scattering above 1 GeV. Signatures for a gradual failure of the traditional meson model with increasing energy can be clearly identified. Since spin effects are large up to tens of GeV, perturbative QCD cannot be invoked to fix the problems. We discuss various theoretical scenarios and come to the conclusion that we do not have a clear phenomenological understanding of the spin-dependence of the NN interaction above 1 GeV. [Preview Abstract] |
Saturday, April 16, 2005 2:42PM - 2:54PM |
C13.00007: Sensitivity of the Derived B(E2) Transition Strength to Nuclear Structure Models Stephen Weppner Identifying the structure of exotic nuclei via scattering observables is an interesting theoretical challenge. Starting with a microscopic optical model, which has a clear delineation between structure and the interaction introduced at the onset, we have a natural tool to examine the role of the structure in the calculation. In the past this model was used to ascertain the sensitivities of the elastic scattering observables to the structure of the nucleus, with the differences between models being minimal at intermediate scattering energies. We now examine the role of the nucleur structure model in the calculation of an inelastic proton-nucleus scattering observable: the differential cross-section for the first $0+\rightarrow2+$ transition of the nucleus. We will study how this observable and the related $B(E2)$ matrix element and deformation parameter are affected by the use of a variety of different structure inputs used in the calculation. [Preview Abstract] |
Saturday, April 16, 2005 2:54PM - 3:06PM |
C13.00008: Exact removal of the center-of-mass spurious states from level density Mihai Horoi We present recursive formulae to exactly remove the contribution of the ceter-of-mass spurious states from the fixed-J and parity shell model nuclear level density, if one knows the level density for restricted classes of shell model configurations. The method is valid for a large class of problems that use a harmonic oscillator shell model basis and translational invariant shell model Hamiltonians. Using our methods for calculating nuclear level densities based on fixed-J configuration centroids and widths for restricted classes of shell model configuration (see e.g. M. Horoi et al, Phys. Rev. C {\bf 69}, 041307(R) (2004)), such as $N\hbar \omega$ excitations, one can calculate very accurately nuclear level densities free of center-of-mass spurious states. [Preview Abstract] |
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