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 gIIgII in the weak nucleon currents. The gIIgII was extracted from the present alignment correlation terms, combined with the known β–αβ–α angular correlation terms and weak magnetism. This analysis permits an experimental determination of all the matrix elements necessary to extract gIIgII. 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=12A=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)×10−3MeV−1 and the 2-body contribution to be λ=+(0.27±0.97)×10−3λ=+(0.27±0.97)×10−3 at a 1σ level.