Proton-proton fusion and tritium $\beta$-decay from lattice quantum chromodynamics

TL;DR

This paper uses lattice QCD to calculate nuclear matrix elements relevant for proton-proton fusion and tritium beta decay, providing first-principles insights into weak nuclear processes.

Contribution

It presents the first lattice QCD calculations of key nuclear matrix elements at SU(3)-flavor-symmetric quark masses, connecting fundamental quark interactions to nuclear physics.

Findings

Gamow-Teller matrix element close to experimental value

Fusion cross section consistent with accepted value

Determined two-nucleon axial counterterm L_{1,A}

Abstract

The nuclear matrix element determining the $pp\to d e^+ \nu$ fusion cross section and the Gamow-Teller matrix element contributing to tritium $\beta$-decay are calculated with lattice Quantum Chromodynamics (QCD) for the first time. Using a new implementation of the background field method, these quantities are calculated at the SU(3)-flavor-symmetric value of the quark masses, corresponding to a pion mass of $m_\pi$ ~ 806 MeV. The Gamow-Teller matrix element in tritium is found to be 0.979(03)(10) at these quark masses, which is within $2\sigma$ of the experimental value. Assuming that the short-distance correlated two-nucleon contributions to the matrix element (meson-exchange currents) depend only mildly on the quark masses, as seen for the analogous magnetic interactions, the calculated $pp\to d e^+ \nu$ transition matrix element leads to a fusion cross section at the physical quark masses that is consistent with its currently accepted value. Moreover, the leading two-nucleon axial counterterm of pionless effective field theory is determined to be $L_{1,A}=3.9(0.1)(1.0)(0.3)(0.9)\ {\rm fm}^3$ at a renormalization scale set by the physical pion mass, also in agreement with the accepted phenomenological range. This work concretely demonstrates that weak transition amplitudes in few-nucleon systems can be studied directly from the fundamental quark and gluon degrees of freedom and opens the way for subsequent investigations of many important quantities in nuclear physics.