High-precision half-life measurement for the isospin T =1 /2 mirror β+ decay of 21Na

The half-life of the isospin T =1 /2 mirror β⁺ decay of ²¹Na was measured at the GANIL-SPIRAL radioactive ion beam facility to be T_{1 /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 f t 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 F t value used in the calculation of V_ud from mirror decays to test the conserved vector current hypothesis of the standard model. The value of V_ud deduced from mirror decays using the new half-life result is now in better agreement with the V_ud derived from T =1 superallowed 0⁺ to 0⁺ transitions.