A step closer to solving the mystery of matter

15 April 2020

Scientists from Oxford University’s Department of Physics are part of an international collaboration that has taken a major step forward in understanding the difference between matter and antimatter. The findings of the T2K collaboration, published today in Nature, start to reveal a basic property of neutrinos that has not been measured until now.

The T2K collaboration has published new results that hint at differences between the oscillation of neutrinos and anti-neutrinos, which may be a clue to the answer to the great mystery of why there is more matter than anti-matter in the universe. Using beams of muon neutrinos and muon antineutrinos, T2K has studied how these particles and antiparticles transition into electron neutrinos and electron antineutrinos, respectively. In their findings, the group have managed to disfavour almost half of the possible values thus gaining a deeper insight into the phenomena.

Oxford and T2K

Physicists from Oxford have played a key role in the T2K experiment from its inception; Professor of Particle Physics Dave Wark has been the project’s principal investigator over the project’s 15-year lifetime and is T2K’s international co-spokesperson while fellow academics Professor Alfons Weber and Professor Giles Barr have developed the electronics and data acquisition systems. Many postdocs and more than a dozen graduate students from Oxford’s Department of Physics have been involved in T2K over the years.

Professor Weber, who studied the details how neutrinos interact with the detector, comments: ‘The universe is now almost entirely made from matter and contains almost no anti-matter – despite the fact that equal amounts of each were produced during the big bang. So there has to be some difference between matter and anti-matter. Physicists have found some differences studying quarks, but these are much too small to explain the matter dominance. Our research shows that this difference may be much bigger for neutrinos and will help us to understand why we live in a matter-dominated universe.”

How the experiment works

The T2K experiment uses a beam consisting primarily of muon neutrinos or muon antineutrinos created using the proton beam from the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai village on the east coast of Japan. A small fraction of the neutrinos (or antineutrinos) are detected 295 km away at the Super-Kamiokande detector, located under a mountain in Kamioka, near the west coast of Japan. As the muon neutrinos and muon antineutrinos traverse from Tokai to Kamioka (hence the name T2K), a fraction will oscillate or change flavour into electron neutrinos or electron antineutrinos respectively – which can then be identified in the Super-Kamiokande detector by the rings of Cherenkov light that they produce. While Super-Kamiokande cannot identify each event as a neutrino or antineutrino interaction, T2K is able to study the neutrino and antineutrino oscillations separately by operating the beam in neutrino mode or antineutrino mode.

Professor Wark concludes: ‘These hints of difference between matter and anti-matter are fascinating, but they are only hints. We need far more precise measurements before we can really know if we have found something, which will require new and more powerful experiments like Hyper Kamiokande – a vastly larger version of Super Kamoikande now being built in Japan – and DUNE, a planned experiment in America with a huge detector of liquid argon. The Oxford neutrino group is working on them both, but it will probably be almost ten years before we will surpass the sensitivity of T2K with either one.’

Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations’ is published in Nature, 580, 339-344 (2020)

Physics in focus
New to physics? Look for the 'physics in focus' explanations to help you along the way...
What is a neutrino? As far as we know, a neutrino is an elementary particle – it is not composed of anything else. An electron neutrino is very similar to the more familiar electron – except that neutrinos have no electric charge, and almost no mass. The lack of electric charge means that neutrinos interact only weakly with ordinary matter, and hence pass almost without attenuation through vast thicknesses of material (which is why T2K can aim its neutrino beam through 295 km of rock without losing it). The mass is so small that no conventional 'mass' measurement (where you push one and see how fast it goes) has ever succeeded in observing it. We only see the effects of neutrino mass by observing neutrino oscillations in experiments like T2K.

Image: © Kamioka Observatory ICRR