Radiative Muon Capture on Hydrogen and the Induced Pseudoscalar Coupling

The first measurement of the elementary process μ^-p-->ν_μnγ is reported. A photon pair spectrometer was used to measure the partial branching ratio \(2.10+/-0.22\)×10^-8 for photons of k>60 MeV. The value of the weak pseudoscalar coupling constant determined from the partial branching ratio is g_p\(q² = -0.88m²_μ\) = \(9.8+/-0.7+/-0.3\) g_a\(0\), where the first error is the quadrature sum of statistical and systematic uncertainties and the second error is due to the uncertainty in λ_op, the decay rate of the ortho to para pμp molecule. This value of g_p is ~1.5 times the prediction of partial conservation of axial current and pion-pole dominance.