Neutrinos are the second most abundant particles in the cosmos. Because they are so ubiquitous, their properties have an important influence on the structure of the universe.
An open question about neutrinos is whether they are their own anti-particles. An experiment in Japan recently reported that it failed to find “strong evidence” that this is so, ruling out a few theories trying to explain neutrinos’ many mysterious properties.
Every elementary particle has an anti-particle. If the two meet, they will destroy each other in a flash of energy. The electron’s anti-particle is the positron. They can be distinguished because they have opposite charges. Similarly, neutrinos have anti-neutrinos.
However, neither is electrically charged, nor possesses any other properties to really differentiate between them.
But physicists working with the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) in Japan recently reported that after analysing two years’ data, they could not find signs that neutrinos could be their own anti-particles.
KamLAND looks for an event called neutrinoless double beta-decay. In normal double beta-decay, two neutrons in an atom turn into two protons by emitting two electrons and two anti-neutrinos. In neutrinoless double beta-decay, the anti-neutrinos aren’t emitted, which can happen only if anti-neutrinos are just different kinds of neutrinos.
A KamLAND team looked for signs of neutrinoless double beta-decay in 750 kg of xenon-136. It reported on January 30, in Physical Review Letters, that if a xenon-136 nucleus does undergo neutrinoless double beta-decay, it happens at most once every 230 million billion billion years. Even the universe is only 13.8 billion years old.
The result rejects theories that predict more frequent occurrences of neutrinoless double beta-decay. The physicists plan to upgrade KamLAND and test theories that predict lower frequencies in future.
The frequency can be used to estimate the mass of neutrinos, explained Itaru Shimizu from the Research Centre for Neutrino Science, Tohoku University, and a KamLAND team member.
So, 2.3×1026 years implies “an effective neutrino mass of 36-156 meV,” he said in an email. This is at least 5,000-times lighter than an electron.
Another unknown about neutrinos is their mass. So as KamLAND continues its quest for neutrinoless double beta-decay, it may be able to help crack this mystery as well.
