Low-energy test of second-class current inβdecays of spin-aligned20F and20Na

Abstract

The $G$-parity-violating, second-class current- (SCC) induced tensor term in the weak-nucleon axial-vector current was determined from $\ensuremath{\beta}$-decay correlation measurements of the mass $A$ = 20 mirror pair. The alignment correlation terms in the $\ensuremath{\beta}$-decay angular distribution from the purely spin aligned mirror pair, ${}^{20}$F and ${}^{20}$Na, were measured in the present study. Combining the present results with existing results of $\ensuremath{\beta}$ and delayed $\ensuremath{\gamma}$-ray angular correlation measurements for the $A$ = 20 mirror pair, the SCC-induced tensor term was extracted to be ${d}_{\mathit{II}}/Ac$ = 0.18 $\ifmmode\pm\else\textpm\fi{}$ 0.48. In the extraction, the weak-magnetism term, $b/Ac$ = 8.58 $\ifmmode\pm\else\textpm\fi{}$ 0.28, evaluated from the $M$1 analog $\ensuremath{\gamma}$-ray decay strength, was used in a framework of the conserved vector current hypothesis. This is the first unambiguous extraction of the $(b\ensuremath{-}{d}_{\mathit{II}})/Ac$ 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 $\ensuremath{\beta}$ decay of the $A$ = 20 system.