Operator evolution for ab initio electric dipole transitions of 4He

TL;DR

This paper investigates how evolving the electric dipole operator using the similarity renormalization group affects ab initio calculations of 4He, revealing significant impacts on observables like photoabsorption cross sections.

Contribution

It introduces a consistent method for evolving external operators alongside the Hamiltonian in ab initio nuclear calculations, focusing on electric dipole transitions.

Findings

Evolved operators significantly affect photoabsorption cross sections.

Operator evolution effects are comparable to three-nucleon force corrections.

The method improves the accuracy of ab initio predictions for 4He.

Abstract

A goal of nuclear theory is to make quantitative predictions of low-energy nuclear observables starting from accurate microscopic internucleon forces. A major element of such an effort is applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence of ab initio calculations as a function of the model space size. The consistent simultaneous transformation of external operators, however, has been overlooked in applications of the theory, particularly for nonscalar transitions. We study the evolution of the electric dipole operator in the framework of the similarity renormalization group method and apply the renormalized matrix elements to the calculation of the 4He total photoabsorption cross section and electric dipole polarizability. All observables are calculated within the ab initio no-core shell model. We find that, although seemingly small, the effects of evolved operators on the photoabsorption cross section are comparable in magnitude to the correction produced by including the chiral three-nucleon force and cannot be neglected.