Researchers at the T2K (Tokai to Kamioka – which involves sending neutrinos 295 km through the earth across Japan) experiment have discovered that the symmetry between matter and antimatter may be violated for neutrino oscillations. This could help us to understand why there is so much matter in the universe, but very little antimatter.

Neutrinos are one of the fundamental particles that make up our Universe and are among the least understood. Yet every second around 50 trillion neutrinos from the Sun pass through your body. These tiny particles, produced copiously by processes within the sun and other stars, come in three varieties or flavours, and may spontaneously change from one flavour to another.

Each flavour of neutrino has an associated antineutrino. If flavour-changing, or oscillations, are different for neutrinos than for antineutrinos, it could help to explain the observed dominance of matter over antimatter in our universe, a question that has puzzled scientists for a century.

The latest results from the T2K experiment show that the appearance of electron neutrinos in a muon-neutrino beam is higher than expected if the probability of neutrino and antineutrino oscillations are the same. In addition, results show the appearance of electron antineutrinos in a muon-antineutrino beam are lower than expected.

T2K is the first experiment to find such a strong indication of matter/anti-matter asymmetry in the behaviour of neutrinos. Although researchers do not have enough data yet to make a conclusive statement, the results are of significance for current and future neutrino experiments. The T2K data exclude the hypothesis of matter/antimatter symmetry in neutrino oscillations at the 95% confidence level.

To explore the oscillations in neutrinos the T2K experiment fires a beam, which can switch from neutrinos to antineutrinos, from the J-PARC laboratory on the eastern coast of Japan. When the beam reaches the Super-Kamiokande detector, 295 km away in Western Japan, scientists then look for a difference in the oscillations of neutrinos and anti-neutrinos.

Dr Laura Kormos, Senior Lecturer in Physics at Lancaster University, head of Lancaster’s neutrino physics group and researcher at T2K, said: “I'm excited by our new results and proud of my collaboration. When Lancaster built part of the near detector for T2K, no one seriously thought T2K would be likely to say anything significant about the matter/antimatter asymmetry question.

“Now we have this surprisingly strong indication that perhaps there is matter/antimatter asymmetry in neutrino oscillations, and in fact our data suggest that nature prefers the maximal value of asymmetry for this process. So the trend has been good and let's hope that nature continues co-operating.”

This international science experiment is part-funded by the Science and Technology Facilities Council (STFC).

Dr Morgan Wascko, international co-spokesperson for the T2K experiment from the Department of Physics at Imperial, said: “The current T2K result shows a fascinating hint that there's an asymmetry between the behaviour of neutrinos and antineutrinos, in other words an asymmetry between the behaviour of matter and antimatter.

“We now need to collect more data to enhance the significance of our observed asymmetry.”

Although this work is promising, there are still systematic uncertainties, so the T2K team is designing an upgrade to the detector to enhance its sensitivities.

Professor David Wark, Director of Particle Physics at the STFC Rutherford Appleton Laboratory, Professor in Experimental Particle Physics at Oxford University and former spokesperson for the experiment said: “These results confirm the high rate of electron neutrino appearance seen in the earlier data, which is exciting.

“More running and further experiments will be needed to confirm if this has the exciting explanation that neutrinos and anti-neutrinos don’t oscillate the same way, which could be a clue to why there is so much matter in the universe.

“Or the explanation could be a more mundane difference, we don’t know yet, but this certainly gives us a strong incentive to continue the search.”

Scientists at the STFC’s Rutherford Appleton and Daresbury Laboratories were heavily involved in collaborating with UK university scientists on designing, building and operating key parts of the T2K detectors and the neutrino beam.