Several major experiments around the world are designed to catch the elusive neutrino in the act of not showing up. In a radioactive metamorphosis called single beta decay, a neutron (a neutral particle) in the nucleus of an unstable atom spontaneously turns into a proton (a positive particle) and emits an electron and an antineutrino - the antimatter twin of a neutrino. In double beta decay, the interaction is doubled: Two neutrons simultaneously decay into two protons. However, instead of producing two electrons and two antineutrinos, as one might expect, physicists such as Giorgio Gratta of Stanford University suspect that in some instances, no antineutrinos are emitted. That can happen only if neutrinos are their own antiparticle, in which case an antineutrino would be emitted by a neutron and then - presto! - absorbed as a neutrino by a neutron.
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