Search

We present a precision analysis of the ^{136}Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ββ NMEs.

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.

Already long time ago the shape of the electron spectrum in β‐decays of 3H and 187Re has been recognized as an important tool for understanding of neutrino masses. The sensitivity of KATRIN (in preparation, tritium β‐decay) and the MARE (under consideration, 187Re β‐decay) experiments to neutrino mass will reach the sub eV domain. In view of this experimental progress there is a request for a highly accurate theoretical description of the electron endpoint spectra. By taking the advantage of the elementary particle treatment of 3H and 3He the relativistic form for β‐decay endpoint spectrum of tritium is obtained by taking into account also the effect of nuclear recoil. Further, the currently unknown shape of the electron spectrum for the β‐decay of 187Re is presented. It is found that the first forbidden 187Re(5/2+)→187Os(1/2−)β‐decay transition is accompanied with emission of mostly p3/2‐state electrons.