Low-energy test of second-class current in β decays of spin-aligned 20 F and 20 Na

The G-parity-violating, second-class current-(SCC) induced tensor term in the weak-nucleon axial-vector current was determined from beta-decay correlation measurements of the mass A = 20 mirror pair. The alignment correlation terms in the beta-decay angular distribution from the purely spin aligned mirror pair, F-20 and Na-20, were measured in the present study. Combining the present results with existing results of beta and delayed gamma-ray angular correlation measurements for the A = 20 mirror pair, the SCC-induced tensor term was extracted to be d(II)/Ac = 0.18 +/- 0.48. In the extraction, the weak-magnetism term, b/Ac = 8.58 +/- 0.28, evaluated from the M1 analog gamma-ray decay strength, was used in a framework of the conserved vector current hypothesis. This is the first unambiguous extraction of the (b-d(II))/Ac term free from the contribution of the second-forbidden form factor, j(2), in the axial-vector current, which was a limiting factor for accurate determination of nonexistence of the SSC in the beta decay of the A = 20 system.