Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session EM: Nuclear Theory II |
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Chair: Saori Pastore, Washington U. in St. Louis Room: White Hill |
Tuesday, October 12, 2021 11:45AM - 11:57AM |
EM.00001: Ab initio theory for physics beyond the standard model Jason D Holt Breakthroughs in our treatment of the many-body problem and nuclear forces are rapidly transforming modern nuclear theory into a true first-principles discipline. This allows us to address some of the most exciting questions at the frontiers of nuclear structure and physics beyond the standard model, such as the nature of dark matter and neutrino masses, as well as searches for violations of fundamental symmetries in nature. |
Tuesday, October 12, 2021 11:57AM - 12:09PM |
EM.00002: Large-Nc constraints for one- and two-nucleon currents in Pionless Effective Field Theory for dark matter direct detection Thomas R Richardson, Xincheng Lin, Son Nguyen The detection of dark matter is a high priority in searches for Beyond the Standard Model physics. One candidate for dark matter is a weakly interacting massive particle (WIMP) with a mass above the GeV scale. Proposals for the use of light nuclei for dark matter direct detection require a strong theoretical understanding of the nuclear matrix elements involved. The momentum transfer in direct detection experiments for light nuclei is at most a few MeV, so these systems are amenable to the techniques of pionless effective field theory (EFT). However, the effective theory contains undetermined low energy coefficients that must be determined either from data or from lattice calculations. Fortunately, theoretical constraints can be obtained from other means. We use the spin-flavor symmetry of nucleons in the large-Nc limit of quantum chromodynamics, where Nc is the number of colors, to constrain the coefficients of one- and two-nucleon currents that are not presently known from data. We also examine the impact of these constraints on the cross sections of WIMP-nucleon and WIMP-deuteron elastic scattering. Lastly, these constraints can be used to organize the currents required in many-body calculations. |
Tuesday, October 12, 2021 12:09PM - 12:21PM |
EM.00003: Ab initio calculations of 10C → 10B super-allowed Fermi transition Michael Gennari, Petr Navratil, Mack C Atkinson
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Tuesday, October 12, 2021 12:21PM - 12:33PM |
EM.00004: Calculations of radiative capture and pair production for the 8Be composite system Peter H Gysbers, Petr Navratil, Sofia Quaglioni, Konstantinos Kravvaris, Guillaume Hupin Through first-principles calculations of 7Li(p, γ)8Be and 7Li(p, e+e-)8Be capture, we examine the nuclear processes related to the ATOMKI anomaly (which posits the existence of a new boson with a mass of 17 MeV). We calculate the energy and decay of the relevant 1+ resonances in 8Be. We apply the no-core shell model with continuum technique to investigate reactions involving both p+7Li and n+7Be with 8Be as the composite system. This method enables accurate description of both bound states and the continuum using chiral nucleon-nucleon and three-nucleon forces as input. We report phase-shifts and cross-sections for a suite of scattering and reaction processes. |
Tuesday, October 12, 2021 12:33PM - 12:45PM |
EM.00005: Pionless EFT Calculations of Nuclear Response Functions Andrew Andis, Sebastian Koenig Nuclear response functions encode the interaction of atomic nuclei with electromagnetic (or electroweak) probes. These observables can be used to understand the details of nuclear dynamics and in particular to relate theoretical predictions to experiments. Pionless effective field theory (EFT), and in particular the expansion of light nuclei around the unitarity limit, have been shown to successfully describe the binding energies and radii of light nuclei. To further assess the predictive power of this framework, we study in this work the longitudinal response function of few-nucleon states in Pionless EFT. We implement the Lorentz Integral Transform (LIT), an established method that makes it possible to calculate continuum observables like response functions with effective bound-state methods, trading the complexity of an explicit break-up calculation for a delicate numerical inversion. We present our implementation of the LIT in momentum space, using interactions provided by Pionless EFT, as well as first results derived within this framework. |
Tuesday, October 12, 2021 12:45PM - 12:57PM |
EM.00006: Chiral EFT corrections to ab initio M1 observables in p-shell nuclei Patrick J Fasano, Mark A Caprio, Pieter Maris, James P Vary, Shiplu Sarker, Soham Pal, Robert Basili In ab initio nuclear theory we strive to make quantitative predictions of nuclear observables, starting with the internucleon interaction. Modern realistic interactions, such as the LENPIC interactions, are derived systematically from chiral effective field theory (χEFT). The same χEFT treatment used for deriving the interaction can be used to derive consistent effective operators for electroweak properties. Using the effective M1 operator derived consistently with the semi-local coordinate-space LENPIC interaction up to N2LO, we use the no-core configuration interaction (NCCI), or no-core shell model (NCSM), approach to calculate electromagnetic properties of a variety of nuclei in the p-shell. We present results for magnetic moments and M1 transition matrix elements, and explore convergence behavior of the χEFT corrections. |
Tuesday, October 12, 2021 12:57PM - 1:09PM |
EM.00007: Ab initio analysis of $\beta$-delayed proton emission in $^{11}$Be Mack C Atkinson, Petr Navratil The exotic $\beta$-delayed proton emission is calculated in $^{11}$Be from first principles using chiral two- and three-nucleon forces. To investigate the unexpectedly large branching ratio measured in [PRL 123, 082501 (2019)] we calculate the proposed $1/2^+$ proton resonance in $^{11}$B using the no-core shell model with continuum (NCSMC). This calculation helps to resolve whether this large branching ratio is caused by unknown dark decay modes or an unobserved proton resonance. |
Tuesday, October 12, 2021 1:09PM - 1:21PM |
EM.00008: \textit{Ab initio} calculated $M1$ observables: The role of symmetry, structure and selection rules Zhou Zhou, Patrick J Fasano, Mark A Caprio, Anna E McCoy, Pieter Maris, James P Vary Electromagnetic observables provide probes for nuclear structure. In particular, $M1$ observables provide information on angular momentum structure. We explore $M1$ observables in \textit{ab initio} no-core shell model (NCSM) predictions for light nuclei ($p$-shell). In order to understand how the predicted $M1$ moments and transition strengths relate to underlying structure of the nucleus, we study the contributions of the different components of the $M1$ operator (orbital and spin, broken into isoscalar and isovector parts). Each of these components is subject to different selection rules on angular momentum (orbital and spin), isospin and $\mathrm{SU}(3)$ quantum numbers. We use the Lanczos decomposition method to determine the dominant $LS$ and $\mathrm{SU}(3)$ contributions to the calculated wave functions and thus understand the relevant selection rules for each transition. We also study the convergence properties of NCSM calculations for $M1$ observables, including the role of symmetry, structure and selection rules. |
Tuesday, October 12, 2021 1:21PM - 1:33PM |
EM.00009: Quadrupole moments and proton-neutron structure in p-shell mirror nuclei Mark A Caprio, Patrick J Fasano, Pieter Maris, Anna E McCoy Electric quadrupole (E2) matrix elements provide a measure of nuclear deformation and related collective structure. While the experimental electric quadrupole moment only measures the proton distribution, both proton and neutron quadrupole moments are needed to probe proton-neutron asymmetry in the nuclear deformation. In ab initio calculations, converged no-core configuration interaction (NCCI) results for quadrupole moments are particularly challenging to obtain, due to sensitivity to long-range behavior of the wave functions. However, we find that ratios of quadrupole moments (either across a mirror pair, or the proton/neutron moment ratio) are more robustly converged. We compare with measured mirror ratios, explore how well mirror symmetry holds for calculated quadrupole moments, and interpret the results in terms of cluster and Elliott SU(3) descriptions. A similar approach may be applied to other long-range observables, such as proton and neutron root-mean-square radii as indicators of halo structure. |
Tuesday, October 12, 2021 1:33PM - 1:45PM |
EM.00010: Feature selection in machine learning algorithms for nuclear DFT Rodrigo Navarro Perez, Nicolas Schunck We present a new process to identify features for machine learning algorithms that will be used to make predictions in nuclear structure calculations. We focus on the particular task of quantifying the model bias in nuclear binding energies calculated with Density Functional Theory (DFT). While the model bias can be directly calculated when experimental data is available, only an estimate can be made in the absence of measured data. Although machine learning algorithms have found applications in many areas of nuclear science, some of these applications require to extrapolate the input variables to regions outside of the domain that was used to train the algorithm. Current approaches quickly lose predictive power when the input variables are extrapolated. Our process of feature selection avoids this situation by incorporating the distribution of different features in all regions of the nuclear chart. This allows for reliable predictions of the model bias and a direct improvement of DFT calculations. |
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