Measurement of the induced pseudoscalar coupling using radiative muon capture on hydrogen

The photon energy spectrum from the elementary process μ^-→pν_μnγ was measured using a photon pair-spectrometer at the TRIUMF cyclotron. Various sources of backgrounds are discussed in detail and evaluated. From the final spectrum the partial branching ratio R_γ and the pseudoscalar coupling gₚ were extracted. The values obtained are R_γ(k>60 MeV)=(2.10±0.21)×10^-8 and gₚ(q²=-0.88m_μ²)=(9.8±0.7±0.3)gₐ(0), respectively. The first error is the quadrature sum of statistical and systematic errors, while the second error is due to the uncertainty in λₒₚ, the transition rate of the pμp molecule from ortho to para states. This measurement of gₚ, the most precise to date, agrees with all but one of the previous measurements. However it is 1.46 times the value predicted by the partially conserved axial current hypothesis and pion pole dominance.