First T2K measurement of antineutrino identity-shifting behaviour announced

Antineutrinos have been observed changing their identities in the same way as their normal neutrino counterparts by the T2K experiment.

The latest results from the T2K experiment in Japan were announced this week by a team of researchers including physicists from Imperial College London.

Differences between the identity-shifting behaviour of neutrinos and antineutrinos could explain why the universe is made up of normal matter, and was not obliterated by antimatter shortly after the Big Bang.

Every type of particle that makes up the universe has an antimatter counterpart – an identical particle with the opposite charge. Physicists predict that during the Big Bang, the creation of the universe, equal amounts of matter and antimatter should have been created.

However, matter and antimatter annihilate each other, so the persistence of matter making up our universe is a mystery. Small differences in the way matter and antimatter behave could explain why one survived at the expense of the other.

Differences found so far between matter and antimatter particles have been too small to account for the makeup of the universe as we know it, but the strange behaviour of neutrinos may hold the answer.

Different flavours
Neutrinos have the smallest mass of any known particle, and are created in several ways, including during radioactive decay, nuclear reactions and when cosmic rays from the Sun hit the Earth's atmosphere.

Through interactions with other matter, neutrinos are known to come in three types, or 'flavours,' – one paired with the electron (called the electron neutrino), and two more paired with the electron's heavier cousins, the muon and tau leptons (called the muon and tau neutrinos).

The fact that neutrino masses and flavours do not exactly overlap each other means that the three different flavours of neutrinos can spontaneously change into each other as they travel, a phenomenon called neutrino oscillation. Scientists have previously observed all three flavours changing into each other, and measured the degree of change in each type of identity shift.

To explore the neutrinos' oscillations, the T2K experiment fired a beam of neutrinos from the J-PARC laboratory at Tokai Village on the eastern coast of Japan, and detected them at the Super-Kamiokande neutrino detector, 295 km away in the mountains of the north-western part of the country. Here, the scientists looked to see if the neutrinos at the end of the beam matched those emitted at the start.

Now, they have measured the degree of change for the first of the antineutrino identity shifts: muon antineutrinos oscillating into tau antineutrinos. When comparing these to their results for the muon to tau neutrino shift, there appears to be no difference in their behaviour.

Weirdness wanted
The Standard Model of Physics predicts the consistency in behaviour, but deviations from the expected answers are what the team wants in order to try and explain the difference between matter and antimatter. ''We want the weird stuff,'' says Dr Asher Kaboth, a Post-Doctoral researcher from the Department of Physics who announced the results at a meeting earlier this week.

The new measurements are not as precise as those for normal neutrinos, and the team will collect more data, but they are reasonably confident in their result. ''Even this small amount of data is a promising result,'' said T2K scientist Dr Yoshi Uchida from the Department of Physics at Imperial.

''It's a small step but we've already achieved world-best sensitivity,'' adds International Co-Spokesperson of T2K and Imperial physicist Dr Morgan Wascko. In 2011 the team saw the first hints of the as-yet unobserved shift between muon neutrinos and electron neutrinos, which they later confirmed in 2013 with more data.