High-precision half-life measurement for the isospin T = 1/2 mirror $\beta^+$ decay of $^{21}$Na

The half-life of the isospin T = 1/2 mirror $\beta^+$ decay of 21Na was measured at the GANIL- SPIRAL radioactive ion beam facility to be T1=2 = 22.422 (10) s, a result that is more than a factor of 5 times more precise than the previous world average, with a resulting ft value that is now two times more precise. The precision of this new result implies that the half-life is no longer the dominant source of uncertainty in the Ft value used in the calculation of Vud from mirror decays to test the conserved vector current hypothesis of the Standard Model. The value of Vud deduced from mirror decays using the new half-life result is now in better agreement with the Vud derived from T = 1 superallowed 0+ to 0+ transitions.