G-parity violation of weak nucleon current and in-medium mass renormalization of nucleons detected through the beta decays of spin aligned B-12 and N-12

The beta-ray angular distributions from purely spin aligned B-12 and N-12 w ere precisely measured to determine a new limit of the G-parity irregular i nduced tensor form factor in weak nucleon axial vector currents and to stud y the in-medium mass renormalization of nucleons through the axial charge. Since the major systematic error in the previous result which originated fr om the intensity fluctuation of the incident beam used for the production o f the nuclei was removed in the present measurement, the more reliable resu lt was obtained: 0.01 less than or equal to 2M f(T)/f(A) less than or equal to 0.34 (90 % CL). The result is consistent with the theoretical predictio n in the framework of which induced tensor form factor is proportional to t he mass difference between the up and down quarks. We also determined the a xial charge of the weak nucleon current to be y = 4.66 +/- 0.12, which may disclose an in-medium mass reduction of the decaying nucleon of 11 +/- 4 %.