Scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have reported evidence of neutrinoless double beta decay. This result comes from members of the long-running Heidelberg-Moscow double beta decay experiment at Italy’s Gran Sasso underground laboratory. Neutrinoless double beta decay violates lepton number conservation, a fundamental tenet of particle physics. If confirmed, the consequences for particle physics will be profound.
In the conventional picture of particle physics, neutrinos and antineutrinos are distinct. Beta decay proceeds via the transformation of a neutron into a proton with the emission of a neutrino and an electron. However, if the neutrino is its own antiparticle, a rather more exotic process becomes possible. In this case, two successive neutron decays could occur, with the neutrino emitted by the first being absorbed by the second. Two electrons would emerge from the nucleus with no neutrinos, leaving a nucleus that contains two more protons than the original.
In the latest paper from the Heidelberg-Moscow collaboration (2001 Modern Physics Letters A 16 (37) 2409-2420), the authors claim evidence for neutrinoless double beta decay with a half-life of around 1.5 x 1025 years from 10 years of running with an enriched sample of germanium-76. This would be compatible with a neutrino mass of around 0.4 eV, which is difficult to reconcile with neutrino mass results from other experiments and would require a special neutrino mass scenario. A previous limit from this experiment was reported in an earlier issue of CERN Courier.
