Ortho-para transition rate in $mu$-molecular hydrogen and the protons induced pseudoscalar coupling $g_p$

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 $Lambda_{op} = (11.1 pm 1.7 pm^{0.9}_{0.6}) times 10^4$ s$^{-1}$ that is substantially larger than the earlier result of Bardin {it et al.} We discuss the striking implications for the protons induced pseudoscalar coupling $g_p$.