Photon asymmetry measurement in radiative muon capture on ⁴⁰Ca

A measurement of the photon asymmetry (α_γ) in radiative muon capture RMC on ⁴⁰Ca has been carried out at TRIUMF. Negative muons were stopped in a ⁴⁰Ca 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 _γ <95 MeV) RMC photon candidates. Our measured value for the energy-averaged photon asymmetry is α_γ = 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 ≈ 7, are considerably different, indicating a significant theoretical model dependency.