Photon asymmetry measurement in radiative muon capture on 40Ca -: art. no. 034605

A measurement of the photon asymmetry (alpha(gamma)) in radiative muon capture RMC on Ca-40 has been carried out at TRIUMF. Negative muons were stopped in a Ca-40 target and the resulting RMC photons were then converted by a 5-cm thick NaI detector placed immediately in front of our main NaI detector, a cylindrical crystal of diameter 46 cm and depth 51 cm. The data sample consisted of 5200 high-energy (57 MeV<E-gamma<95 MeV) RMC photon candidates. Our measured value for the energy-averaged photon asymmetry is alpha(gamma)=1.00+/-0.23. The extracted values for the induced pseudoscalar coupling constant (g(P)) based on this photon asymmetry measurement are g(P)/g(A)<8.0, utilizing the impulse approximation IA model, and g(P)/g(A)<14.5 in terms of the modified impulse approximation (MIA) model. The two extracted values, although both consistent with the Goldberger-Treiman value of g(P)/g(A)approximate to7, are considerably different, indicating a significant theoretical model dependency.