We report a measurement of the ortho-para transition rate in the $p\mu p$ molecule. The experiment was conducted at TRIUMF via the measurement of the time dependence of the 5.2 MeV neutrons from muon capture in liquid hydrogen. The measurement yielded an ortho-para rate ${\Lambda }_{\mathrm{op}}=(11.1\pm 1.7\pm \genfrac{}{}{0ex}{}{0.9}{0.6})\times {10}^4{\mathrm{s}}^{-1}$, which is substantially larger than the earlier result of Bardin et al. The result has striking implications for the proton’s induced pseudoscalar coupling $g_p$, changing the value of $g_p$ obtained from the most precise ordinary muon capture measurement from $10.6\pm 2.7$ to $0.8\pm 2.8$, and from the sole radiative muon capture measurement from $12.2\pm 1.1$ to $10.6\pm 1.2$, bringing the latter result closer to theoretical predictions.

• Received 19 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.073401