The spin-neutrino correlation revisited in 28Si muon capture: a new determination of the induced pseudoscalar coupling gP/gA Original Research Article

We describe an improved follow-up of our previous spin-neutrino correlation experiment, where the 1229 and 2171 keV gamma-rays emitted after the 28Si(μ,ν)28Al(1+, 2202 keV) reaction were observed by high-resolution HPGe detectors at different angles with respect to the muon spin. In the experiment described here, a magnetic field was used both to select events according to the spin-gamma angle and to measure the residual muon polarization by the muSR method. We further increase the precision of our result making use of new experimental and theoretical information. We obtain for the parameter x≡M(2)/M(−1) the value of x=0.239±0.029, which can be compared to our previous result (Brudanin et al., Nucl. Phys. A 587 (1995) 577) x=0.254±0.034 and to the value x=0.315±0.080 obtained recently at TRIUMF with a different technique. The coupling-constant ratio gP/gA was deduced from the mean value x=0.247±0.027. Using renormalized transition operators calculated recently, we obtained for gP/gA the solution range of −1.0 to +1.3 to be compared to the value of 7 predicted by PCAC for the free nucleon. This result confirms the strong quenching of gP/gA reported earlier.