neutrinoless double beta decay
/ no-neutrino double BAY-ta decay /
In ordinary beta decay, a neutron in a nucleus turns into a proton and spits out an electron plus an antineutrino. In a few special nuclei this can happen twice at once, with two neutrons converting together and emitting two electrons and two antineutrinos — a rare but established process called two-neutrino double beta decay. The hunt is for an even rarer version that has never been seen: the same double conversion but with no neutrinos coming out at all, only the two electrons.
How could the neutrinos simply vanish? Only if the neutrino is its own antiparticle. Then the antineutrino that would normally fly out from one conversion can instead be swallowed as a neutrino by the second conversion, the two cancelling internally and leaving just two electrons carrying off all the energy. Such an event would change the number of leptons by two units, violating lepton-number conservation — something never observed in any reaction. Its signature would be unmistakable: the combined energy of the two electrons would land at one sharp, predictable value, instead of the smeared spread seen in ordinary double beta decay.
This is one of the most coveted measurements in physics. Finding neutrinoless double beta decay would prove neutrinos are Majorana particles, demonstrate that lepton number is not conserved, and offer a handle on the absolute scale of neutrino mass. It would even bolster theories that the early universe's matter-antimatter imbalance arose in the neutrino sector. Experiments using hundreds of kilograms of special isotopes, buried deep underground to hide from cosmic rays, are pushing the limits ever lower, but no confirmed signal has yet appeared.
Experiments watch isotopes like germanium-76 or xenon-136. In ordinary double beta decay the two electrons share energy randomly with the escaping neutrinos, giving a broad spectrum; a neutrinoless event would instead pile up as a single sharp spike right at the maximum energy.
A single sharp energy spike, not a broad smear, would be the telltale sign.
Past claims of a discovery have not held up, and the predicted rate also depends on hard-to-calculate nuclear physics; a non-detection limits but does not strictly rule out a Majorana neutrino.