Ortho-para transition rate in $\mu$-molecular hydrogen and the proton's   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 proton's induced pseudoscalar coupling $g_p$. 
J.H.D. Clark, D.S. Armstrong, T.P. Gorringe, M.D. Hasinoff, P.M. King,   T.J. Stocki, S. Tripathi, D.H. Wright and P.A. Zoln...