Session R06: Nuclear Theory I

Lattice simulations of nuclear structure, thermodynamics, and superfluidity

This talk presents recent results obtained by the Nuclear Lattice Effective Field Theory Collaboration for the binding energies, radii, shapes, and transitions of light and medium-mass nuclei; thermodynamics of nuclear and neutron matter; and superfluid correlations at zero temperature.   

Connecting Lattice QCD Nucleon-Pion Scattering to Nuclear Ab Initio Calculations

Quantum chromodynamics (QCD) is the driving mechanism behind the binding of quarks and gluons into nucleons and nuclei. Though rich in physics, the nonperturbative nature of QCD stymies the direct formulation of nuclear physics using quark and gluon degrees of freedom. Instead, perturbative approaches such as chiral effective theory that use pions and nucleons as degrees of freedom have become a mainstay of nuclear physics. State of the art ab initio calculations provide a systematic approach for obtaining properties of nuclei with uncertainty quantification. These calculations are based on chiral effective theories with low energy constants (LECs) that are calibrated against experimental data. We present work towards further grounding ab initio calculations in QCD by providing additional constraints on nucleon-pion LECs entering chiral EFT nucleon-nucleon and three-nucleon forces using Lattice QCD (LQCD) simulations of nucleon-pion scattering. We will show that a combined fit employing a recent LQCD computation of nucleon-pion scattering at unphysical pion mass is consistent with the baseline fit to LECs using experimental data only. Even a modest set of LQCD spectra is sufficiently uncorrelated with experimental data to significantly improve nucleon-pion LEC constraints.   

Ab initio calculations for dark matter detection and CEvNS

Over the past decades, ab initio nuclear calculation has made dramatic progress, especially reaching the heavy mass region as ²⁰⁸Pb [1]. This means that it becomes possible to obtain first-principles computation (with quantified uncertainties) of quantities which even reside in the heavy-mass region. The quantities include these relevant to astrophysics and searches for physics beyond the Standard Model. In this talk, I will present a conceptual introduction to modern ab initio theory. Then, I will focus on recent advances in ab initio calculations of nuclear responses for dark matter (DM) direct detection [2] and coherent elastic neutrino-nucleus scattering (CEvNS), including nuclei ¹⁹F, ²³Na, ²⁷Al, ^28-30Si, ^70,72-74,76Ge, ¹²⁷I, ¹³³Cs, and ^{128-132,134,136}Xe. 

[1]. Ab initio predictions link the neutron skin of ²⁰⁸Pb to nuclear forces. B.S. Hu, W.G. Jiang, T. Miyagi, Z.H, Sun, et al. Nat. Phys. 118, 1196 (2022) arXiv:2112.01125v1.

[2]. Ab initio structure factors for spin-dependent dark matter direct detection. B.S. Hu, et al. Phys. Rev. Lett. 128, 072502 (2022). arXiv:2109.00193.   

Ab initio computation of the magnetic dipole transition in 48Ca

We perform ab initio computation of the magnetic dipole transition in ⁴⁸Ca in coupled-cluster theory using interactions and currents from chiral effective field theory. Our computation includes the effects of the continuum as well as the contributions of many-body correlations in the nuclear wave functions and electromagnetic currents. We find that the transition strength B(M1) lies in the range from 7.0 to 10.2 μ_N², which agrees with a (γ,n) experiment but is larger than the results from inelastic electron- and proton-scattering experiments. We validate our prediction by computing magnetic moments of several odd-mass isotopes of Calcium and by performing benchmarks with the no-core shell model calculations on Oxygen isotopes.    

Collectivity in neutron-rich neon isotopes from ab-initio methods

We compute the low-lying collective structure of ^20-34Ne using ab-initio coupled-cluster methods and chiral nucleon-nucleon and three-nucleon interactions. For ^20-30Ne we accurately describe the first 2⁺ and 4⁺ energies and the quadrupole transitions from the first 2⁺ to the ground-state. For ^32,34Ne less is known and we predict that they are strongly deformed and collective. To understand how individual terms of the nuclear Hamiltonian impacts deformation we use emulators of the E(4+)/E(2+) ratio and perform a global sensitivity analysis in ^20,32Ne and ³⁴Mg. We find that more than 50% of deformation is driven by S-wave contacts and that adding short-range repulsion (reducing pairing) increases deformation. These are first steps towards answering the question: What drives deformation in nuclei?   

Bounds on the equation of state from QCD inequalities and lattice QCD

Determining the equation of state (EoS) of QCD matter at high baryon density from first principles is a vital challenge in modern nuclear theory because of its non-perturbative nature. The most promising method in the non-perturbative regime is lattice QCD, but it does not work at high baryon density due to the sign problem. However, there is a well-known exception to avoid the sign problem by taking opposite up and down quark chemicals; it is referred to as QCD at finite isospin chemical potential.

In this talk, I describe the uses of QCD inequalities and input from the recent lattice QCD calculations at finite isospin chemical potential to derive robust bounds on the EoS at finite baryon density. We find that the lattice data puts an upper bound on the baryon density of the symmetric nuclear matter at a given baryon chemical potential and a lower bound on the pressure as a function of the energy density. I also discuss additional constraints from perturbative calculations of the QCD EoS at high density derived in earlier work and causality to delineate robust bounds on the EoS of isospin symmetric matter at densities relevant to heavy-ion collisions.   

Tunable-fidelity wave functions for ab initio scattering and reactions

The no-core shell model (NCSM) is a first-principle method that computes static properties of light nuclei employing an expansion of the many-body wave function onto a basis of Slater determinants. Extensions of the NCSM (such as the NCSM with continuum or NCSMC) have further allowed for the description of light-ion reactions relevant to astrophysics and nuclear technology applications. Nevertheless, the reach of the NCSMC is hindered by an explosion in the problem dimensionality as nuclei beyond oxygen are targeted. From the large number of basis states that are needed to compute nuclear properties in the NCSM, however, only a smaller subset is relevant in the description of reactions. In this talk, we discuss an approach that aims to circumvent the dimensionality explosion problem within the NCSM (and NCSMC) by allowing for wave functions of near-continuous fidelity to be generated. We present first findings on the convergence properties of the approach as well as demonstrate its efficacy when describing heavier nuclei.   

QED nuclear medium effects in neutrino-nucleus and electron-nucleus scattering

The exchange of photons with nuclear medium modifies (anti)neutrino and electron scattering cross sections. We study the distortion of (anti)neutrino-nucleus and charged lepton-nucleus cross sections and estimate the QED-medium effects on the final-state kinematics, scattering cross sections, and bremsstrahlung. We find new permille-to-percent level effects, which were never accounted for in either (anti)neutrino-nucleus or electron-nucleus scattering. We quantitatively compute the effects of Glauber photon-mediated multiple re-scattering within the nuclear medium and find that the relativistic charged lepton acquires a momentum of order 10 MeV transverse to its direction of propagation inside the nucleus. This broadening sizably deflects expected electron tracks and suppresses scattering cross sections. Precise extraction of the nucleon and nuclear structure by electron and muon probes should, thus, take the QED nuclear medium angular redistribution of particles into account.