The photon energy spectrum from the elementary process ${\mu }^{-}\overrightarrow{p}{\nu }_{\mu }n\gamma$ 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 $R_{\gamma }$ and the pseudoscalar coupling $g_p$ were extracted. The values obtained are $R_{\gamma }(k>\mathrm{}60\mathrm{}$ MeV$)=(2.10\mathrm{}\pm 0.21)\times {10}^{-8}$ and $g_p{(q}^2=-{0.88m}_{\mu }^2)=(9.8\mathrm{}\pm 0.7\pm {0.3)g}_a\mathrm{}\left(0\right)\mathrm{}$, respectively. The first error is the quadrature sum of statistical and systematic errors, while the second error is due to the uncertainty in ${\lambda }_{\mathrm{op}},$ the transition rate of the $p\mu p$ molecule from ortho to para states. This measurement of $g_p,$ 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.

• Received 8 July 1997

DOI:https://doi.org/10.1103/PhysRevC.57.373