The MuCap Experiment

Muon capture on the proton is sensitive to the weak pseudoscalar form factor, gp, one of the basic parameters characterizing the nucleon electroweak charged‐current interaction. There is a precise theoretical prediction for gp based on chiral symmetry of QCD, yet the results of past experiments are controversial: they are dependent on a poorly‐known, mu‐molecular kinetic parameter, and there is no point of mutual agreement between theory and all experiments. The goal of the MuCap experiment is to clarify the situation by measuring the rate of muon capture on the free proton to 1%, which determines gp to 6%, higher precision than achieved in previous experiments and without ambiguities in the interpretation. This talk will cover the motivation for and design of the MuCap experiment, first physics results from the new measurement, and improvements toward the final precision goal.