Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session B11: Nuclear Theory |
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Sponsoring Units: DNP Chair: James Vary, Iowa State University Room: Embassy F |
Saturday, March 31, 2012 10:45AM - 10:57AM |
B11.00001: Contribution of isovector mesons to the symmetry energy in a microscopic model Francesca Sammarruca The equation of state of isospin asymmetric nuclear matter (IANM) has broad applications, ranging from the structure of rare isotopes to the properties of neutron stars. Important quantities that emerge from IANM studies are the symmetry energy and the symmetry potential. The latter arises from the difference between the neutron and proton single-particle potentials in IANM, along with the closely related proton and neutron effective masses. In this paper, we concentrate on the role of isovector mesons in the symmetry energy. The impact of isovector mesons on the symmetry energy has been discussed in the literature, particularly in the context of mean-field approaches, both relativistic and non-relativistic. Because our framework is microscopic, our findings are easily understood in terms of contributions of each meson to the appropriate component of the nuclear force and the isospin dependence naturally generated by isovector mesons. We demonstrate the outstanding role of the pion in building the symmetry energy. We comment on fundamental differences between our approach and the one of mean-field models, particularly pionless QHD theories. [Preview Abstract] |
Saturday, March 31, 2012 10:57AM - 11:09AM |
B11.00002: Charge form factors of light nuclei in chiral effective field theory Maria Piarulli We use potentials and charge operators derived in chiral effective field theory ($\chi$EFT) to calculate charge form factors of $A=2-4$ nuclei up to next-to-next-to-next-to-next-to leading order (N4LO). The sensitivity of the results is studied as function of the cutoff $\Lambda$ in the chiral potentials and charge operators. We also present results obtained in a hybrid approach using the conventional AV18 potential and the $\chi$EFT charge operators. The theoretical results are compared to the experimental data for momentum transfers $Q$ up to $(2-3)\,m_{\pi}$. Reasonable agreement is obtained between theory and experiment. [Preview Abstract] |
Saturday, March 31, 2012 11:09AM - 11:21AM |
B11.00003: Spin-Orbit interactions in auxiliary field diffusion Monte Carlo Jie Zhang, Kevin Schmidt Nuclear Matter and light nuclei have been successfully calculated using the Auxiliary field diffusion Monte Carlo method and a truncated two-body potential without spin-orbit interactions, the Argonne v6' potential. In order to have realistic calculations where the remaining parts of the interaction can be treated perturbatively, the Argonne v8' potential is commonly sampled in Monte Carlo methods. Here we will show that by using a pair-wise break up of the Hamiltonian, along with additional auxiliary fields, that the additional spin-orbit and isospin dependent spin-orbit terms can be sampled. We will discuss calculations of the nuclear matter equation of state using this method. This work was support by NSF grant PHY-1067777. [Preview Abstract] |
Saturday, March 31, 2012 11:21AM - 11:33AM |
B11.00004: Building Real-Space, Imaginary-Time Propagators for Non-Local Nucleon-Nucleon Potentials Joel Lynn Monte Carlo methods often used in nuclear physics, such as auxiliary field diffusion Monte Carlo and Green's function Monte Carlo, have used phenomenological local real-space potentials containing as few derivatives as possible, such as the Argonne-Urbana family of interactions, to make sampling simple and efficient. Basis set methods such as no-core shell model or coupled-cluster techniques typically use softer non-local potentials because of their more rapid convergence with basis set size. These non-local potentials are typically defined in momentum space and are often based on effective field theory. Comparisons of the results of the two types of methods can be difficult when different potentials are used. I will show methods for evaluating the real-space, imaginary-time propagators needed to perform quantum Monte Carlo calculations using such non-local potentials. I will compare the consistency of the large imaginary-time propagators for different potentials and discuss their use in quantum Monte Carlo calculations. [Preview Abstract] |
Saturday, March 31, 2012 11:33AM - 11:45AM |
B11.00005: Evidence of nonlocality due to a gradient term in the optical model Mahmoud Jaghoub, George Rawitscher We demonstrate that the presence of a velocity-dependent term in the phenomenological optical potential simulates a source of nonlocality. This is achieved by showing that, in the interior of the nucleus, the nonlocal wave functions are different from the corresponding local ones obtained in the absence of the velocity-dependent term in accordance with the Perey effect. It is also shown that the enhancement or suppression of the nonlocal wave function is energy as well as angular momentum dependent. The latter is in line with the results of previous works that introduced parity dependent terms in the conventional optical potential. [Preview Abstract] |
Saturday, March 31, 2012 11:45AM - 11:57AM |
B11.00006: An Exactly Solvable Many-Body Model Nouredine Zettili, Abdelkrim Boukahil We deal here with the construction of a simple many-body model that can be solved exactly. This model serves as a tool for testing the validity and accuracy of many-body approximation methods, most notably those encountered in nuclear theory. The model consists of a system of two distinguishable, one-dimensional sets fermions interacting via a schematic two-body force. We construct the Hamiltonian of the model by means of vector operators that satisfy a Lie algebra and which are the generators of an SO(2,1) group. The Hamiltonian depends on an adjustable parameter which regulates the strength of the two-body interaction. The size of the Hamiltonian's matrix is rendered finite by means of a built-in symmetry: the Hamiltonian is represented by a five-diagonal square matrix of finite size. The energy spectrum of the model is obtained by diagonalizing this matrix. The energy eigenvalues obtained from this diagonalization are exact, for we don't need to resort to any approximation in the diagonalization. This model offers a rich and flexible platform for testing quantitatively the various many-body approximation methods especially those that deal with nuclear collective motion. [Preview Abstract] |
Saturday, March 31, 2012 11:57AM - 12:09PM |
B11.00007: Single j-shell studies of cross-conjugate nuclei and isomerism Larry Zamick, Alberto Escuderos In the single j-shell with the same interaction cross -conjugate pairs should have identical spectra. There are differences.For the lighter members of cross-conjugate pairs of four nucleons with T=1 The ground state spin is two for the heavier members it is (2j-1). This result can be obtained by using the spectrum of 2 particles as an effective interaction for the lighter member and the spectrum of 2 holes for the heavier member. But what is most new in this work is the observation in many single j-shell calculations and some experiments of criss-cross behavior. The J=2 state in the heavier member, if not the ground state is still low lying and hence isomeric. Likewise the (2j-1) state in the lighter member is also low lying and is isomeric. An exception to the isomers comes when J=(2j-1) differs from J=2 by two units or less. A key point in the difference of the particle-particle and the hole-hole interaction is that in the latter the state with J$^{max}$=2j is much lower than in the former. In $^{44}$Sc the J=2 state is calculated to be the ground state and in $^{52}$Mn J=6. But J=6 is at 0.381 MeV in the former and J=2 at 0.202 MeV in the latter.In ${96}$Ag J=(2j-1)=8 is the ground state and J=2 is at 0.097 MeV. J=15 is also isomeric. We thus a (2j-1) rule. [Preview Abstract] |
Saturday, March 31, 2012 12:09PM - 12:21PM |
B11.00008: Violation of time reversal invariance in ensembles with random interactions. Volha Abramkina, Alexander Volya, Vladimir Zelevinsky Studies of low-energy spectrum in fermionic systems driven by random Hamiltonians allow to shift a focus of attention from specifics of interactions to the role of conservation laws and geometry of a system. The two-body random ensemble (TBRE) with rotational symmetry is based on the shell model approach and is relevant to such a discussion in context of nuclear structure. It has been shown that the probability for the ground-state spin to be zero is enhanced in the rotationally invariant TBRE. A zero ground-state spin in all realistic even-even nuclei is attributed to a superconducting state, this paired state is not what determines the ground state properties when interactions are random. It is plausible that one of the reasons of this enhancement is the time-reversal (T) invariance. To elucidate the effect of the T-invariance we embedded the randomly interacting system in an external magnetic field and thus broke the symmetry. The effect of this Coriolis force is compared with the statistical theory. The violation of the T-invariance by different types of scalar one-body Hamiltonian terms is also considered. [Preview Abstract] |
Saturday, March 31, 2012 12:21PM - 12:33PM |
B11.00009: Electron in Strong Time-Dependent Laser Field Xingbo Zhao, Anton Ilderton, Pieter Maris, James Vary The basis light front quantization (BLFQ) approach has recently been developed as a nonperturbative method for addressing forefront problems in QED and QCD [1,2]. This approach represents the quantum field in an optimal basis exploiting the symmetries of the underlying dynamics and is therefore numerically efficient. In the present work we extend BLFQ to the time-dependent regime. As an example we study the interaction of a single electron with a strong time-dependent laser background field, which necessitates a nonperturbative treatment. We study a process called ``nonlinear Compton scattering,'' in which the electron is excited by the laser field and emits a photon. We present numerical results for the time-evolution of the average invariant mass of the one-electron-one-photon system and show explicitly the invariant mass distribution of the system at intermediate times and at pulse termination. Hopefully, these observables can be tested in experiments at planned strong laser facilities [3]. Finally we compare our results in the weak laser field limit with those obtained in perturbation theory.\\[4pt] [1] J. P. Vary, et al, Phys. Rev. C 81, 035205 (2010)\\[0pt] [2] X. Zhao, et al, LC2011 Proceedings, Few-Body Systems (accepted)\\[0pt] [3] T. Heinzl, A. Ilderton, Eur.Phys.J. D55 (2009) 359. [Preview Abstract] |
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