Beta decays in mass 8 system and test of conserved vector current hypothesis

We observed the alignment correlation terms in the $\beta$-ray angular distribution for $^8$Li and $^8$B. The alignment correlation terms are useful probes to test the conserved vector current (CVC) hypothesis or the $G$-parity conservation in the $\beta$-decay process. The experiment was performed using the Van de Graaff accelerator at Osaka University. The pure nuclear-spin alignments were successfully produced by applying the $\beta$-NMR techniques and the alignment correlation terms were extracted from the $\beta$-ray angular distribution as a function of the $\beta$-ray energy. Under the assumption of the $G$-parity conservation, not only the weak magnetism (WM) but also the second-forbidden matrix element (M$_2$) of the vector current was obtained both from the present alignment correlation terms and the previous $\beta$-$\alpha$ angular correlation terms. The WM and M$_2$ are related by the CVC hypothesis to the M1 and E2 analog $\gamma$-decays from the isobaric analog state of $^8$Be. It is found that the M$_2$ is inconsistent with the previously observed E2 $\gamma$-decay.