Second unique forbidden β decay of 115In and neutrino mass

The measurement of the electron spectrum in β decays close to the end point provides a robust direct determination of the values of neutrino masses. The most sensitive experiments use tritium and rhenium β decays because these transitions have low Q value. Recent measurement with Penning traps established that the β decay of 115In(9/2+) to the first excited state of 115Sn(3/2+) is a transition with the smallest Q value among β decays. The decay is associated with a change of spin and parity ΔJπ = 3+ (ΔL = 2, ΔS = 1) of nucleus, i.e., classified as unique second forbidden β decay. Our investigation shows that in this transition electrons are predominantly emitted in d5/2 partial waves. In addition, it is found that the Kurie function associated with this transition near the end point within a good accuracy reflects a behavior the Kurie function of superallowed β transitions.