Bulletin of the American Physical Society
Spring 2013 Meeting of the APS Ohio-Region Section
Volume 58, Number 2
Friday–Saturday, March 29–30, 2013; Athens, Ohio
Session D1: Nuclear Theory I |
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Chair: Charotte Elster, Ohio University Room: Grover Hall E205 |
Saturday, March 30, 2013 8:24AM - 8:36AM |
D1.00001: Shear Viscosities of Hadrons with K-Matrix Cross Sections Anton Wiranata, Volker Koch, XinNian Wang, Madappa Prakash In this work we show how interactions play a key role in determining the magnitudes of shear viscosity. In order to include all resonances that are produced near the phase transition temperature, a better and consistent formalism for transport cross sections in a multi-component mixture is used. Thus far, two avenues have been pursued: (i) Parametrize phase shifts (where experimentally available) to obtain the differential cross sections, and (ii) Employ the Briet-Wigner parametrization of total cross sections as available in the Particle Data book. The first method fails to include all resonances of relevance (as data are not available), whereas the second method fails to preserve unitarity except in the case of well separated resonances. The K-Matrix parametrization of differential cross sections (necessary for both transport and equation of state calculations) overcomes both of these drawbacks, while preserving unitarity. The K-Matrix formalism also treats close-lying resonances in a consistent way by properly considering interference terms in the differential cross section. The interference terms are important in determining the magnitude of the cross section, because these terms could be negative; hence they will either reduce or increase the magnitudes of both viscosities. [Preview Abstract] |
Saturday, March 30, 2013 8:36AM - 8:48AM |
D1.00002: Effect of varying charge and matter radii on observables in $^6$He and $^8$He Azamat Orazbayev, Charlotte Elster, Stephen Weppner The helium isotopes $^6$He and $^8$He have total spin zero, but they are open-shell nuclei. Conventional microscopic optical potentials are derived for closed shell nuclei. When the ground state is assumed to have neutrons occupying the $p_{3/2}$-shell, two additional terms need to be added to the microscopic optical potential. These terms modify the differential cross section and the analyzing power. Here we study the influence of charge and matter radii on the differential cross-section and on the analyzing power $A_y$. It is found that the analyzing power $A_y$ is more sensitive to the variations of charge radii than the differential cross-section. The difference between the conventional and the modified microscopic optical potentials decreases at higher energies. This phenomenon is explained by the fact that the additional central and spin-orbit terms of the optical potential have the opposite effects on the observables. At higher energies, the magnitudes of these effects become approximately equal and the overall effect reduces. The study of varying matter radii shows the smaller sensitivity of the observables to this parameter. The effect of the valence neutrons on the observables is larger in $^8$He than in $^6$He. [Preview Abstract] |
Saturday, March 30, 2013 8:48AM - 9:00AM |
D1.00003: The Supernova Equation of State: Finite-Range, Momentum-Dependent Potential Approach Constantinos Constantinou, Brian Muccioli, Madappa Prakash Experimental evidence on nucleon-nucleus scattering suggests that the real part of the optical potential has a strong momentum dependence which causes it to be attractive for low energies while it becomes repulsive and saturates at high energies. Results of microscopic nuclear matter calculations are consistent with this picture. However, Skyrme-like single-particle potentials grow quadratically with momentum in contrast with experimental data. This behavior results from approximating non-local exchange forces by local effective ones. Field-theoretical models at the mean-field level are also inconsistent with experimental data. To explore the effect momentum-dependent interactions have on the thermal properties of dense, isospin-symmetric nucleonic matter, we study the schematic model constructed by Welke et al. in which the correct momentum dependence that fits optical potential data is built through finite-range exchange forces of the Yukawa type. The exact numerical results are compared to analytical ones in the quantum regime where we rely on Landau's Fermi-Liquid Theory, and in the classical regime where the state variables are obtained through a steepest descent calculation. Detailed comparisons with similarly calibrated Skyrme models are also performed. [Preview Abstract] |
Saturday, March 30, 2013 9:00AM - 9:12AM |
D1.00004: Universality in SRG Evolved Potential Matrix Elements and K Matrix Equivalence Brian Dainton Utilizing Similarity Renormalization Group (SRG) transformations, we examine universality in potential matrix elements of various realistic potentials. Using several modern potentials and an inverse scattering separable potential (ISSP), we observe that the diagonal matrix elements evolve to a universal form only in regions of phase-shift equivalence. The universality of off-diagonal potential matrix elements seems to require equivalence of half-on-shell K matrix elements. We discuss differences in modern realistic potentials and the ISSP for the two-nucleon problem, specifically explicit pion physics and off-shell behavior, and gain insight into the consequences for the 3- and many-body problem. [Preview Abstract] |
Saturday, March 30, 2013 9:12AM - 9:24AM |
D1.00005: Neutron matter based on consistently evolved chiral three-nucleon interactions R.J. Furnstahl, K. Hebeler We present the first results for the neutron matter equation of state (EOS) using nucleon-nucleon and three-nucleon chiral effective field theory interactions that are consistently evolved in the framework of the Similarity Renormalization Group (SRG). The dependence of the EOS on the SRG resolution scale is greatly reduced when induced three-nucleon forces (3NF) are included and the residual variation, which in part is from missing induced four-body interactions, is comparable to estimated many-body perturbation theory truncation errors. The relative growth with decreasing resolution of the 3NF contributions to the energy per neutron is of natural size, but it accelerates at the lowest resolutions where strong renormalization of the long-range 3NF matrix elements is also observed. [Preview Abstract] |
Saturday, March 30, 2013 9:24AM - 9:36AM |
D1.00006: Effects of Random Interactions in a Finite Fermi System Jennifer Glick, Vladimir Zelevinsky The unexpected observation in [C. Johnson, G. Bertsch, D. Dean, Phys. Rev. Lett. 80 (1998) 2749] that a randomly chosen rotationally invariant interaction in many-body fermi systems leads to the statistical predominance of a ground state of zero spin, suggested that pairing correlations alone are not sufficient to explain the zero spin ground state of even-even nuclei. Despite numerous efforts and proposed explanations, this problem still lacks a satisfactory solution. We suggested in [V. Zelevinsky, A. Volya, Physics Reports 391 (2004) 311-352] that a statistical approach, using a simple single-j fermion model, shows how conservation laws and the geometry of the single-particle space can be used to determine the distribution of ground state spins. [Preview Abstract] |
Saturday, March 30, 2013 9:36AM - 9:48AM |
D1.00007: Fluctuating flow angles and anisotropic flow measurements in heavy-ion collisions Ulrich Heinz, Zhi Qiu, Chun Shen Event-by-event fluctuations in the initial density distributions of the fireballs created in relativistic heavy-ion collisions lead to event-by-event fluctuations of the final anisotropic flow angles, and density inhomogeneities in the initial state cause these flow angles to vary with the transverse momentum of the emitted particles. It is shown that these effects lead to characteristically different transverse momentum dependencies for anisotropic flow coefficients extracted from different experimental methods. These differences can be used to experimentally constrain flow angle fluctuations in the final state of heavy-ion collisions which, in turn, are sensitive to the initial state density fluctuations and the shear viscosity of the expanding fireball medium. [Preview Abstract] |
Saturday, March 30, 2013 9:48AM - 10:00AM |
D1.00008: Separabilization of Optical Potentials in Momentum Space Linda Hlophe, Charlotte Elster Separable representations of optical potentials have important applications in Faddeev calculations for (d,p) reactions~[1]. A way to construct separable representations of local potentials was suggested by Ernst, Shakin, and Thaler (EST)~[2]. In order to employ the EST scheme, we obtained a semi-analytic Fourier transform of the Woods-Saxon potential as input to the momentum space Lippmann-Schwinger equation. The resulting half-shell t-matrices at given support points are the form factors of the separable expansion. Starting from the Chapel-Hill 89 (CH89) optical potential partial wave S-matrix elements in the range from 0 to 50 MeV are constructed for three closed shell nuclei, $^{48}$Ca, $^{132}$Sn, and $^{208}$Pb. The quality of the separable representation of the S-matrix elements depends on (a) the choice of support points, (b) the partial wave of interest, and (c) the rank of the separable optical potential.\\[4pt] [1] C.~Elster and L.~Hlophe, Journal of Physics: Conference Series (\textbf{403}), 012025 (2012).\\[0pt] [2] D.~J.~Ernst, C.~M.~Shakin and R.~M.~Thaler, Phys.\ Rev.\ C {\bf 8}, 46 (1973). [Preview Abstract] |
Saturday, March 30, 2013 10:00AM - 10:12AM |
D1.00009: Universal properties of infrared oscillator basis extrapolations Sushant More It has been shown recently that a finite harmonic oscillator basis in nuclear many-body calculations effectively imposes a hard-wall boundary condition in coordinate space, motivating infrared extrapolation formulas for the energy and other observables. We present further refinement of these formulas by studying two-body models and the deuteron (More et al., arXiv:1302.3815). We accurately determine the box size as a function of the model space parameters, and compute scattering phase shifts in the harmonic oscillator basis. We show that the energy shift can be well approximated in terms of the asymptotic normalization coefficient and the bound-state momentum, discuss higher-order corrections for weakly bound systems, and illustrate this universal property using unitarily equivalent calculations of the deuteron. [Preview Abstract] |
Saturday, March 30, 2013 10:12AM - 10:24AM |
D1.00010: Universality and the matter radius of Carbon-22 Danel Phillips, Bijaya Acharya, Chen Ji Recently, Tanaka et al. measured the matter radius of $^{22}$C to be $\langle {\rm r}_m^2 \rangle^{1/2}$=5.4 $\pm$ 0.9 fm. This suggests that ${}^{22}$C is an s-wave two-neutron halo, with the two neutrons orbiting a ${}^{20}$C core. We address this finding using an effective field theory (EFT) that employs core and neutron degrees of freedom and is designed for systems with a large two-body scattering length. This EFT enables the derivation of universal predictions for three-body systems which are built on such two-body interactions and have a large matter radius. We show that, at leading order in the EFT, the matter radius of any s-wave two-neutron halo is given by a function of the neutron-core scattering length and the halo nucleus' two-neutron separation energy. We display this function and discuss its general properties. Specializing to the case of ${}^{22}$C, we use our general function, together with the datum of Tanaka et al., to set limits on the binding energy of $ {}^{22}$C for different values of the ${}^{21}$C resonance energy. Our analysis includes a consideration of the higher-order corrections in the EFT, allowing us to set an upper bound on the ${}^{22}$C binding energy which includes both these uncertainties and those in the original measurement. [Preview Abstract] |
Saturday, March 30, 2013 10:24AM - 10:36AM |
D1.00011: Hanbury Brown-Twiss (HBT) interferometry relative to the triangular flow plane in heavy-ion collisions Christopher Plumberg, Chun Shen, Ulrich Heinz The PHENIX Collaboration reported third-order harmonic oscillations of the source radius parameters when measuring the Hanbury Brown-Twiss correlation function for charged hadrons relative to the triangular flow angle. They interpreted their result geometrically as a measurement of the final triangular deformation at freeze-out. Such a purely geometric interpretation is not tenable: we show that a triangular source deformation can not lead to third-order oscillations unless the source also features radial flow.-- Using a simple Gaussian source featuring both geometric deformations and anisotropic flow, we analyze possible origins of the observed third-order oscillations. It is known that second-order oscillations of the HBT radii relative to the elliptic flow plane can arise from an elliptic spatial deformation of the source at freeze-out, both with and without radial flow, as well as from elliptic flow; in general the first (geometric) effect is strongest. We show that this is different for third-order oscillations relative to the triangular flow plane. We present the dependence of the third-order oscillation amplitudes of all HBT radii on the model parameters. In particular we explore the origin of the observed much larger third-order oscillation amplitude for Rout than Rside. [Preview Abstract] |
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