Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session CE: Mini-Symposium on Toward a Predictive Model of Nuclei I |
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Chair: Filomena Nunes, Michigan State University Room: Sweeney Ballroom D |
Thursday, October 29, 2015 8:30AM - 9:06AM |
CE.00001: Predictive Nuclear Many-Body Theory with Ab Initio Methods: A Brief Survey and A Look Ahead Invited Speaker: Heiko Hergert The reach of \emph{ab initio} many-body techniques has increased tremendously in recent years, owing to new developments in many-body theory as well as advances in their numerical implementation. Coupled Cluster, Self-Consistent Green's Function, and In-Medium Similarity Renormalization Group (IM-SRG) calculations are routinely performed for isotopes in the $A\sim100$ region. Moreover, these techniques have been extended to tackle open-shell nuclei, either directly or through the auxiliary step of deriving valence-space interactions for use with existing Shell Model technology. One of the most powerful aspects of \emph{ab initio} methods is their capability to provide results for energies and other observables with systematic uncertainties. Together with new accurate nuclear forces (and operators) derived from Chiral Effective Field Theory, they provide a consistent framework --- and a road map --- for a predictive description of nuclei. This will have a critical impact on the search for the limits of nuclear existence, tests of fundamental symmetries (e.g., the search for neutrinoless double beta decay), our understanding of quenching and effective charges in phenomenological Shell Model calculations etc. Using the Multi-Reference IM-SRG as a representative example, I will survey the current capabilities of \emph{ab initio} methods with an emphasis on uncertainty quantification, highlight successes in the description of ground-state properties and spectra, and preview upcoming developments like the construction of consistent transition operators. [Preview Abstract] |
Thursday, October 29, 2015 9:06AM - 9:18AM |
CE.00002: A variational Monte Carlo approach for the study of medium-mass nuclei Diego Lonardoni, Steven C. Pieper, Robert B. Wiringa, Alessandro Lovato We report on an accurate variational many-body technique (cluster variational Monte Carlo) suitable for the study of medium-mass nuclei. The employed many-body nuclear Hamiltonian contains realistic two- and three-nucleon interactions and the trial wave function is constructed from pair- and triplet-correlation operators acting on a product of single-particle determinants. As opposed to traditional variational Monte Carlo calculations, that are limited to $A=12$ nuclei, expectation values are evaluated with a cluster expansion for the non-central correlations. The cluster expansion drastically reduces the computational effort necessary for the study of an $A$-body system, allowing us to extend the calculations in the medium-mass region, currently up to $40$ nucleons. We present results for the closed-shell nuclei $^{16}$O and $^{40}$Ca and prospects for open-shell nuclei like $^{40}$Ar. Of particular interest is the derivation of the momentum distributions that can be used to constrain the spectral functions of these nuclei. This has a crucial interplay with electron-nucleon and neutrino-nucleon scattering experiments, where Argon is among the typical targets and the scattering data at high momentum transfer can be analyzed by means of the spectral function formalism. [Preview Abstract] |
Thursday, October 29, 2015 9:18AM - 9:30AM |
CE.00003: Light nuclei with improved order-by-order chiral interactions Pieter Maris, James Vary We present recent results for light nuclei~[1] obtained with improved NN interactions derived from chiral effective field theory up to N$^4$LO~[2]. The many-body calculations are performed order-by-order in the chiral expansion. We show results for the ground state energies and the low-lying spectrum; in addition we discuss other observables such as magnetic and quadrupole moments. We discuss both the theoretical uncertainties due to the truncation of the chiral expansion, as well as the numerical uncertainties associated with the many-body method. Depending on the value chiral order, additional renormalization using the Similarity Renormalization Group is needed in order to improve numerical convergence of the many-body calculations.\\[4pt] [1] S.~Binder {\it et al.}, arXiv:1505.07218 [nucl-th].\\[0pt] [2] E.~Epelbaum, H.~Krebs, and U.-G.~Mei\ss ner, Eur.~Phys.~J.~A51 (2015) 5, 53; {\it ibid} arXiv:1412.4623 [nucl-th]. [Preview Abstract] |
Thursday, October 29, 2015 9:30AM - 9:42AM |
CE.00004: Shell model effective operators from IM-SRG Steven Stroberg, Heiko Hergert, Scott Bogner, Angelo Calci, Jason Holt, Titus Morris, Petr Navratil, Nathan Parzuchowski, Achim Schwenk, Johannes Simonis The past decade has witnessed the development of a number of ab-initio many-body methods which can reach to medium-mass nuclei and beyond. Recently, these techniques have been extended beyond doubly-magic systems by producing effective interactions for shell-model valence spaces, extending the reach of ab-initio theory to a large swath of the nuclear chart. This talk will discuss the use of the In-Medium Similarity Renormalization Group (IM-SRG) to produce consistent effective operators for use with these shell model interactions, yielding ab-initio radii, multipole moments, and transition rates for open-shell nuclei. [Preview Abstract] |
Thursday, October 29, 2015 9:42AM - 9:54AM |
CE.00005: Two- and Three-Nucleon Chiral Interactions in Quantum Monte Carlo Calculations for Nuclear Physics Joel Lynn, Joseph Carlson, Stefano Gandolfi, Alexandros Gezerlis, Kevin Schmidt, Achim Schwenk, Ingo Tews I present our recent work on Green's function Monte Carlo (GFMC) calculations of light nuclei using local two- and three-nucleon interactions derived from chiral effective field theory (EFT) up to next-to-next-to-leading order (N2LO). GFMC provides important benchmarking capabilities for other methods which rely on techniques to soften the nuclear interaction and also allows for nonperturbative studies of the convergence of the chiral EFT expansion. I discuss the choice of observables we make to fit the two low-energy constants which enter in the three-nucleon sector at N2LO: the $^4$He binding energy and $n$--$\alpha$ elastic scattering $P$-wave phase shifts. I then show some results for light nuclei. I also show our results for the energy per neutron in pure neutron matter using the auxiliary-field diffusion Monte Carlo method and discuss regulator choices. Finally I discuss some exciting future projects which are now possible. [Preview Abstract] |
Thursday, October 29, 2015 9:54AM - 10:06AM |
CE.00006: Electromagnetic structure of light nuclei Saori Pastore I present a number of {\it ab initio} Quantum Monte Carlo calculations of electromagnetic observables for $A\le 10$ nuclei, which account for two-body effects due to the coupling of external photons with pairs of interacting nucleons. Nuclear wave functions are generated from a nuclear Hamiltonian with the Argonne v18 two-nucleon and Illinois-7 three-nucleon potentials, while chiral effective field theory is utilized to construct the two-body electromagnetic current operators. Emphasis is on recent calculations of magnetic radii and Zemach moments of light nuclei. [Preview Abstract] |
Thursday, October 29, 2015 10:06AM - 10:18AM |
CE.00007: Separable coupled-channels momentum space potentials for nuclear reactions Linda Hlophe, Vasily Eremenko, Charlotte Elster, Filomena Nunes, Arbanas Deltuva, Jutta Escher, Ian Thompson Many nuclei are deformed and their properties may be described using a rotational model. This involves defining a deformed surface of the nucleus and constructing the nuclear interaction as a function of distance to the surface. The resulting potential has non-zero matrix elements between different rotational states which are characterized by the nuclear spin-parity $I^\pi$, leading to channel couplings. Our goal is to utilize these coupled-channels potentials in momentum space Faddeev calculations which take into account core excitations. For this purpose their separable representation in momentum space is necessary. We accomplish this by employing the separable representation scheme developed by Ernst, Shakin, and Thaler (EST). Since the potentials are complex, the multichannel EST scheme is generalized to non-Hermitian potentials. In the case of proton-nucleus interactions the EST scheme is further extended to include charged particles. The multichannel EST scheme is applied to scattering off $^{10}$Be and $^{12}$C . For $^{10}$Be only couplings to the first excited state ($I^\pi=2^+$) were included while for $^{12}$C the first two excited states ($I^\pi=2^+,4^+$) were taken into account. [Preview Abstract] |
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