Press Releases
Jul. 14, 2009

Is the current upper limit of the neutrino mass reliable?

study of nuclear matrix element of double-beta decay
  • Hideyuki Sakai (Professor, Department of Physics, The University of Tokyo)
  • Kentaro Yako (Research Associate, Department of Physics, The University of Tokyo)

In double-beta decay of nucleus, two neutrons in the nucleus are converted to protons, and two electrons are emitted. If double-beta decay without neutrino emission is observed, it may be concluded that (1) the neutrino is a Majorana particle and that (2) its mass can be determined from its lifetime. To derive the mass from the lifetime, one needs the nuclear matrix element (nme), taking into account of the fact that the double-beta decay is occurring in the nucleus. Since there is no direct experimental means to obtain the nme, the current upper limit of neutrino mass (~1eV) is obtained from nme’s derived from nuclear-structure theories. To test the theories, we performed nuclear-scattering measurements to study the Gamow-Teller states in the intermediate nucleus 48Sc, which, as virtual intermediate states, constitute a major contribution to the nme of the 48Ca → 48Ti double-beta decay. It was found that the Gamow-Teller strengths are distributed more broadly in excitation energy spectra than the predictions by the up-to-date nuclear-structure theories. Therefore nuclear-structure theories need to be improved to reproduce the present results before we evaluate the upper limit of neutrino mass.

Paper information

Physical Review Letters (advanced online publication: July 1, 2009)
"Gamow-Teller Strength Distributions in 48Sc by the 48Ca(p,n) and 48Ti(n,p) Reactions and Two-Neutrino Double-beta Decay Nuclear Matrix Elements"
K. Yako, M. Sasano, K. Miki, H. Sakai, S. Noji et al.