New limit of the G-parity irregular weak nucleon current detected in β decays of spin aligned [sup 12]B and [sup 12

Abstract

The G-parity irregular induced tensor form factor in the weak nucleon axial vector current has been precisely determined by measuring the alignment correlation terms in the β-ray angular distributions of the purely spin aligned mirror pair 12B(Iπ=1+,T1/2=20.2 ms) and 12N(Iπ=1+,T1/2=11.0 ms) in order to place a new limit on the applicability of the G-parity conservation law. The coefficient of the induced tensor term was determined to be 2MfT/fA=−0.15±0.12±0.05(theor.) in 90% confidence level. The obtained 2MfT/fA is vanishingly small and is consistent with the theoretical prediction based on QCD in the framework of which 2MfT/fA is proportional to the mass difference between up and down quarks. Also we set constraints on the KDR parameters from the present result together with the results of correlation type experiments in the mass A=8 and 20 systems as ζ=−(0.12±0.15)×10−3 MeV−1 and λ=+(0.30±0.89)×10−3 in 1σ level.