Session GG02: V: Nuclear Theory

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Proton and neutron electric and magnetic form factors, G_E and G_M, are the primary characteristics of their spatial structure. Their behavior at large values of the momentum transfer Q² may enlighten us on the QCD dynamics in transition from nonperturbative to perturbative regime and effects of quark orbital angular momenta and diquark correlations in particular. Several experiments at JLab and elsewhere are focussing on the momentum region up to Q²=18 GeV² for the proton and up to 14 GeV² for the neutron. I will report our study of nucleon electromagnetic form factors with momenta up to Q²=12 GeV² on a lattice with a range of parameters approaching physical values. The G_E/G_M form factor ratios and flavor contributions are in reasonable agreement with experiment. Comparison to new JLab results will be an important test of lattice QCD methods in the near-perturbative regime.   

Power Accuracy in Lattice Calculations of Parton Distributions

In lattice-QCD calculations of the parton distribution functions (PDFs) via large-momentum effective theory (LaMET), the leading power correction appears at ${cal O}(Lambda_{ m QCD}/P^z)$ due to the linearly divergent self-energy of the Wilson line in quasi-PDF operators. 

For lattice data with hadron momentum $P^z$ of a few GeV, power accuracy is important both for an estimation of theoretical error and a wider-$x$ range application of LaMET. We show how to attain this by resumming the relevant infrared renormalon effect in the matching coefficients in the large $eta_0$-function approximation and, at the same time, fixing phenomenologically the non-perturbative mass parameter in the renormalization of the linear divergence through the zero-momentum lattice matrix element. A demonstrative example

is shown on the pion PDF data at $P^z = 1.9$ GeV.   

The Big-Bang Nucleosynthesis within the Scale-Invariant Vacuum (SIV) paradigm

We will present our results on the study of the Li7 problem for testing the Scale-Invariant Vacuum (SIV) paradigm with high-precision Primordial Nucleosynthesis simulations using the PRIMAT'21 code that utilizes partially decoupled neutrinos during the Big-Bang Nucleosynthesis. Possible connections of the SIV cosmology to the Dark Matter and Early Dark Energy phenomena will be highlighted.   

Predictions of microscopic and conventional CRMs for light nuclei

The conventional cranking model (CCRM3) uses a constant angular velocity. To study the effect of a microscopic angular velocity on the model-predicted results, a quantal, microscopic cranking model Hamiltonian for triaxial rotation (MSCRM3) is derived from the action of a microscopic rotation operator on a deformed nuclear state using Hartree-Fock method. This derivation is exact. MSCRM3 and CCRM3 Hamiltonians have identical forms (hence MSCRM3 and CCRM3 are equally easy to use), but MSCRM3 includes residual correction terms, and uses a dynamic angular velocity, hence it is time-reversal and D₂ invariant. For a self-consistent deformed harmonic oscillator potential, MSCRM3 and CCRM3 equations are determined in closed algebraic forms using Feynman’s theorem and solved iteratively. The two cranking models are used to investigate the stability of nuclear rotational states, nuclear shapes, rotation modes and their transitions, and the differences between these predictions in some light nuclei. The impact of spin-orbit and residuals of the square of the angular momentum and a two-body interaction is studied. It is shown that MSCRM3 predicts rotational relaxation of the intrinsic system, triaxial rotation, inherent instability of the cranked rotational states, rotational-band termination on spherical symmetry, the observed reduced rotational-energy-level spacing in ²⁰Ne, whereas CCRM3 does not. In some cases, MSCRM3 predicts rotations and band terminations whereas CCRM3 does not, and vice versa.

    

Charged pion electric polarizability in lattice QCD

We explore a four-point-function-based method in lattice QCD that mimics low-energy Compton scattering and offers a transparent picture on how polarizabilities arise from quark and gluon interactions. We carry out a proof-of-concept simulation on the electric polarizability of a charged pion, using quenched Wilson action on a $24^3 imes 48$ lattice at beta=6.0 with pion mass from 1100 to 370 MeV. We report results on pion mass, rho mass, form factor and charge radius, and electric polarizability.   

Energy and pressure in neutron matter: current status of ab initio predictions vs. empirical constraints

Modern theoretical predictions of the neutron matter equation of state and the symmetry energy are timely. Recent and planned experiments aim at extracting constraints on the density dependence of these quantities, which is of paramount importance for a number of nuclear and astrophysical systems. We discuss ab initio predictions in relation to recent empirical information. We emphasize and demonstrate that free-space NN data pose stringent constraints on the density dependence of the neutron matter equation of state and thus the slope of the symmetry energy at saturation, which we find to be extremely sensitive to the isovector component of the NN interaction through the ¹S₀ partial wave.

  

Ab Initio nuclear physics: proceeding from the "bottom up"

The Holy Grail of nuclear theory is the ab initio description of all (finite) atomic nuclei and (infinite) nuclear matter. The past 20 years have seen great progress towards this goal. However, around 2015, the program hit a stumbling block, when it turned out to be difficult to explain the properties of intermediate-mass nuclei and nuclear matter simultaneously. Solutions to this problem have been advanced, and we will analyze to what extent these solutions can be perceived as genuine ab initio.