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

Abstract The alignment correlation terms in the β -ray angular distributions from purely spin aligned 8 Li and 8 B have been measured to search for the G -parity violating induced tensor term g II in the weak nucleon currents. The g II 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 g II . As a result, the induced tensor term was extracted as g II / g A = − 0.28 ± 0.28 (stat.) ± 0.15 (syst.) at a 1 σ (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 ) × 10 −3 MeV −1 and the 2-body contribution to be λ = + ( 0.27 ± 0.97 ) × 10 −3 at a 1 σ level.