New limit of the G-parity irregular weak nucleon current detected in β decays of spin aligned 12B and 12N

Abstract

The G-parity irregular induced tensor form factor in the weak nucleon axial vector current has been precisely determined by measuring the alignment correlation terms in the $\ensuremath{\beta}$-ray angular distributions of the purely spin aligned mirror pair ${}^{12}\mathrm{B}{(I}^{\ensuremath{\pi}}{=1}^{+},$ ${T}_{1/2}=20.2$ ms) and ${}^{12}\mathrm{N}{(I}^{\ensuremath{\pi}}{=1}^{+},$ ${T}_{1/2}=11.0$ ms) in order to place a new limit on the applicability of the G-parity conservation law. The coefficient of the induced tensor term was determined to be ${2Mf}_{T}{/f}_{A}=\ensuremath{-}0.21\ifmmode\pm\else\textpm\fi{}0.09$ (stat.) \ifmmode\pm\else\textpm\fi{}0.07 (syst.)\ifmmode\pm\else\textpm\fi{}0.05 (theory) at a 90% confidence level. The previously obtained data in the year 1996 was reanalyzed to be added to the present result. The combined result is ${2Mf}_{T}{/f}_{A}=\ensuremath{-}0.15\ifmmode\pm\else\textpm\fi{}0.12\ifmmode\pm\else\textpm\fi{}0.05$ (theory) at a 90% confidence level. The obtained induced tensor coefficient is vanishingly small and is consistent with the theoretical prediction based on QCD in the framework of which the induced tensor form factor is proportional to the mass difference between up and down quarks. Also we set constraints on the Kubodera-Delorme-Rho parameters from the present result together with the results of correlation-type experiments in the mass $A=8$ and 20 systems as $\ensuremath{\zeta}=\ensuremath{-}(0.12\ifmmode\pm\else\textpm\fi{}0.14)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{MeV}}^{\ensuremath{-}1}$ and $\ensuremath{\lambda}=+(0.30\ifmmode\pm\else\textpm\fi{}0.88)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in the $1\ensuremath{\sigma}$ level.