Ab initio prediction of the $^4{\rm He}(d,\gamma)\,^6\rm Li$ big bang radiative capture

TL;DR

This paper presents ab initio calculations of the helium-deuterium fusion rate producing lithium-6, resolving uncertainties and revealing the importance of magnetic dipole transitions in the primordial nucleosynthesis process.

Contribution

The study provides the first ab initio predictions of the $^4$He$(d, extgamma)^6$Li astrophysical S-factor using chiral effective field theory and no-core shell model with continuum, improving accuracy and reducing uncertainties.

Findings

Enhanced capture probability below 100 keV due to M1 transitions.

Reduced uncertainty of the thermonuclear capture rate by a factor of 7.

Provides a more accurate prediction of primordial lithium-6 production.

Abstract

The rate at which helium ($^4$He) and deuterium ($d$) fuse together to produce lithium-6 ($^6$Li) and a $\gamma$ ray, $^4$He$(d,\gamma)^6$Li, is a critical puzzle piece in resolving the roughly three orders of magnitude discrepancy between big bang predictions and astronomical observations for the primordial abundance of $^6$Li. The accurate determination of this radiative capture rate requires the quantitative and predictive description of the fusion probability across the big bang energy window ($30$ keV $\lesssim E\lesssim 400$ keV), where measurements are hindered by low counting rates. We present first-principles (or, ab initio) predictions of the $^4$He$(d,\gamma)^6$Li astrophysical S-factor using validated nucleon-nucleon and three-nucleon interactions derived within the framework of chiral effective field theory. By employing the ab initio no-core shell model with continuum to describe $^4{\rm He}$-$d$ scattering dynamics and bound $^6\rm Li$ product on an equal footing, we accurately and consistently determine the contributions of the main electromagnetic transitions driving the radiative capture process. Our results reveal an enhancement of the capture probability below 100 keV owing to previously neglected magnetic dipole (M1) transitions and reduce by an average factor of 7 the uncertainty of the thermonuclear capture rate between $0.002$ and $2$ GK.