Quantum Monte Carlo calculations of electromagnetic transitions in 8Be with meson-exchange currents derived from chiral effective field theory

TL;DR

This paper uses quantum Monte Carlo methods with chiral effective field theory to calculate electromagnetic transitions in 8Be, achieving good agreement with experimental energies and providing insights into meson-exchange current contributions.

Contribution

It introduces detailed quantum Monte Carlo calculations of electromagnetic transitions in 8Be including meson-exchange currents from chiral EFT, improving understanding of nuclear electromagnetic properties.

Findings

Energies of states match experimental data closely.

Two-body meson-exchange currents contribute 20-30% to M1 transitions.

Transitions between same symmetry states agree with experiment, others are underpredicted.

Abstract

We report quantum Monte Carlo calculations of electromagnetic transitions in 8Be. The realistic Argonne v18 two-nucleon and Illinois-7 three-nucleon potentials are used to generate the ground state and nine excited states, with energies that are in excellent agreement with experiment. A dozen M1 and eight E2 transition matrix elements between these states are then evaluated. The E2 matrix elements are computed only in impulse approximation, with those transitions from broad resonant states requiring special treatment. The M1 matrix elements include two-body meson-exchange currents derived from chiral effective field theory, which typically contribute 20--30% of the total expectation value. Many of the transitions are between isospin-mixed states; the calculations are performed for isospin-pure states and then combined with empirical mixing coefficients to compare to experiment. Alternate mixings are also explored. In general, we find that transitions between states that have the same dominant spatial symmetry are in reasonable agreement with experiment, but those transitions between different spatial symmetries are often underpredicted.