Towards a precise calculation of the nucleon axial charge with lattice QCD

Abstract:  The nucleon axial charge, gA, is one of the fundamental properties of the nucleon, measuring the strength with which the weak axial current couples to the nucleon. The axial charge governs many fundamental nuclear processes, such as nuclear beta decay and the strength of the nuclear force through long-range pion exchange. Given the prominence of gA, this quantity was originally thought to be one of the first important benchmark calculations for demonstrating that uncertainties associated with lattice QCD calculations relevant to nuclear physics can be appropriately quantified and controlled. However, this quantity has been notoriously difficult to compute with lattice QCD, largely due to systematics that were not fully appreciated, and its theoretical determination remains an outstanding challenge. I will describe a recent lattice QCD calculation, using a new computational strategy motivated by the Feynman-Hellmann Theorem, in which we demonstrate control over all sources of systematic uncertainties using relatively few stochastic samplings. For the first time, a precise and accurate determination of gA has been achieved with lattice QCD. I will also discuss the remaining sources of the largest systematic uncertainty and what is required to reduce the overall uncertainty below the 2% level.