Ab initio calculation of the $np \to d \gamma$ radiative capture process

TL;DR

This paper uses lattice QCD to directly calculate the electromagnetic contributions to the neutron-proton radiative capture process, providing results consistent with experimental data and advancing understanding of nuclear interactions from fundamental theory.

Contribution

It introduces a first-principles lattice QCD approach to determine two-body electromagnetic effects in nuclear reactions, bypassing phenomenological models.

Findings

Calculated cross section at physical pion mass matches experimental value.

Performed calculations at two different quark masses to enable extrapolation.

Demonstrated the feasibility of ab initio QCD calculations for nuclear processes.

Abstract

Lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process $np \to d\gamma$, and the photo-disintegration processes $\gamma^{(\ast)} d \to np$. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of $m_\pi \sim 450$ and 806 MeV, and are combined with pionless nuclear effective field theory to determine these low-energy inelastic processes. Extrapolating to the physical pion mass, a cross section of $\sigma^{lqcd}(np\to d\gamma)=332.4({\tiny \begin{array}{l}+5.4 \\ - 4.7\end{array}})\ mb$ is obtained at an incident neutron speed of $v=2,200\ m/s$, consistent with the experimental value of $\sigma^{expt}(np \to d\gamma) = 334.2(0.5)\ mb$.