Vector leptoquark pair production in e+e− annihilation

Negative muon capture in 28Si was used to determine the angular correlation between the neutrino and the 1229 keV γ-ray from the de-excitation of the 2201 keV 1+ level in 28Al. To suppress backgrounds in the HPGe detectors, a coincidence was required with the 942 keV γ-ray in the de-excitation cascade detected in a NaI array. The extracted angular correlation coefficient α is 0.360 ± 0.059, consistent with previous measurements. Using a recent shell model calculation employing a full 1s-od model space and the USD empirical interaction, this value of α implies a value for the induced pseudoscalar coupling of gpga = 0.0 ± 3.2, substantially smaller than the prediction of PCAC and pion-pole dominance.