G-Parity Violation of Weak Nucleon Current and In-Medium Mass Renormalization of Nucleons Detected Through the β Decays of Spin Aligned 12B and 12N

The β-ray angular distributions from purely spin aligned ¹²B and ¹²N were precisely remeasured to determine a new limit of the G-parity irregular induced tensor form factor in weak nucleon axial vector currents and to study the in-medium mass renormalization of nucleons through the axial charge. Since the major systematic error in the previous result which originated from the intensity fluctuation of the incident beam used for the production of the nuclei was removed in the present measurement, the more reliable result was obtained: 0.01 ≤ 2M f_T/f_A ≤ 0.34 (90% CL). The result is consistent with the theoretical prediction in the framework of which induced tensor form factor is proportional to the mass difference between the up and down quarks. We also determined the axial 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%.