Search for the G-parity irregular term in weak nucleon currents extracted from mirror beta decays in the mass 8 system

The alignment correlation terms in the β-ray angular distributions from purely spin aligned 8Li and 8B have been measured to search for the G-parity violating induced tensor term gII in the weak nucleon currents. The gII was extracted from the present alignment correlation terms, combined with the known β-α angular correlation terms and weak magnetism. This analysis permits an experimental determination of all the matrix elements necessary to extract gII. As a result, the induced tensor term was extracted as gII/gA=-0.28±0.28(stat.)±0.15(syst.) at a 1σ (68%) level. The present data and the data in the mass A=12 and 20 systems were analyzed under the KDR model in which medium effects including the off-shell effect and meson exchange current were taken into account. We determined the 1-body contribution to be ζ=-(0.13±0.13)x10-3MeV-1 and the 2-body contribution to be λ=+(0.27±0.97)x10-3 at a 1σ level