Proton-proton fusion and tritium beta-decay from lattice QCD

I will discuss a recent study of axial-current matrix elements in two and three-nucleon systems directly from the underlying theory of the strong interactions using lattice QCD. Key results are determinations of both the nuclear matrix element determining the proton-proton fusion cross section and the Gamow-Teller matrix element contributing to tritium beta-decay. I will discuss the calculation, details of systematic uncertainties, and the implications of the results, and show that this work leads to a QCD expectation for the proton-proton fusion cross section at the physical quark masses that is consistent with its currently accepted value from experiment. 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.