I know this sounds esoteric, but bear with me. Physicists have measured the slowest radioactive decay process yet – (2-neutrino) double-beta decay in xenon-136. The half-life is figured to be 2.11×1021 years. With the age of the universe around 1.37×1010 years, this half-life is more than 100 billion times as long. Don’t wait up for it.
Howwever, as impressive a feat as this is, the experiment is aiming at something even more interesting: a decay process that’s not even predicted by the Standard Model, namely neutrinoless double beta decay. If this actually occurs, it would imply neutrinos are their own antiparticles, which is very non-SM.
What the EXO 200 team wants to find is another decay process – one that is not only even more fantastically rare than 2nubb, but that no one is certain even exists. It’s called zero-neutrino double-beta decay, or 0nubb, and it is decidedly not a Standard Model process.
In 0nubb, two neutrons once again decay into two protons and two electrons, but the antineutrinos are nowhere to be found. They must have been there; the IRS has nothing on Nature for keeping the books balanced. The two antineutrinos must have annihilated each other, like positrons and electrons can annihilate each other, or protons and anti-protons, or any particle and its antiparticle.
This means in order for 0nubb decay to happen, neutrinos must be their own antiparticles.
Odd as this sounds, the possibility of a particle that could be both itself and its anti-self was hypothesized by an Italian theoretical particle physicist named Ettore Majorana in 1937. Such particles are called Majorana particles, and if they exist physicists would need to get busy revising the Standard Model.