Ft values of the mirror β transitions and the weak-magnetism-induced current in allowed nuclear β decay

Abstract

The precision of correlation measurements in neutron and nuclear β decay has now reached the level of about 1% and better. At this level of precision, higher-order corrections such as recoil-order corrections induced by the strong interaction and radiative corrections cannot necessarily be neglected anymore. We provide here an update of the Ft values of the isospin T=1/2 mirror β decays including the neutron, of interest to determine the Vud quark-mixing matrix element. We also provide an overview of current experimental and theoretical knowledge of the most important recoil term, weak magnetism, for both these mirror β decays and a large set of β decays in higher isospin multiplets. The matrix elements determining weak magnetism were calculated in the nuclear shell model and cross-checked against experimental data, showing overall good agreement. We show that the neutron and the mirror nuclei now effectively contribute to the value of Vud, but we also stress the need for further work on the radiative correction ΔRV. Our results provide new insight into the size of weak magnetism, extending the available information to nuclei with masses up to A= 75. This provides important guidance for planning and improved sensitivity for interpreting correlation measurements in searches for new physics or to extract Vud in mirror β decays. It can also be of interest for further theoretical work related to the reactor neutrino problem.