Muon Capture and the Axial Structure of the Nucleon

The weak charged-current reaction of muon capture has much in common with several aspects of neutrino physics. Its basic process, μp->nν is the inverse of quasi-elastic νn scattering, while muon capture on deuterium is governed by the same physics as solar pp fusion and νd scattering at SNO. In heavier nuclei, recent muon capture experiments explore virtual intermediate states of 0ν2β candidates. The presentation will focus on muon capture in hydrogen and the axial radius of the nucleon. We [R. Hill et al., 2018, Rep. Prog. Phys. in press] used the recent model independent reevaluation of of the axial radius r_A² (z exp.) = 0.46(22) fm² from νd scattering and the hydrogen capture rate from the MuCap experiment, to update the value of the induced pseudoscalar form factor g_P and, alternatively, to determine r_A² (MuCap) = 0.46(24) fm² from muon capture. The impact of this result and the motivation for new, 3-fold improved muon capture experiment regarding neutrino quasi-elastic neutrino scattering in the GeV region is discussed.