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UK scientists have contributed to new research that puts further support behind the theory that neutrinos are the reason the universe is dominated by matter.

Understanding whether neutrinos and antineutrinos behave differently is important, because if all types of matter and anti-matter behave the same way, they should have completely wiped each other out shortly after the Big Bang.

Instead matter dominates and these new experimental results show a difference in the way neutrinos and antineutrinos behave - which could help us understand why there is so much matter in the universe, but very little anti-matter.

The new results published by the T2K Collaboration show the strongest evidence yet that neutrinos and antineutrinos behave differently, and therefore may not wipe each other out. Using beams of muon neutrinos and muon antineutrinos, the T2K project has studied how these particles and antiparticles transition into electron neutrinos and electron antineutrinos, respectively. 

Professor Christos Touramanis from the University of Liverpool said “This new result exceeds our original hopes for the discovery potential of T2K. It also fills us with confidence that major discoveries lie ahead of us in the next generation of neutrino experiments.”

Neutrinos and antineutrinos can come in three ‘flavours’, known as muon, electron and tau. As they travel, they can ‘oscillate’ – changing into a different flavour. To explore these changes in neutrinos the T2K experiment fires a beam, which can be switched from neutrinos to antineutrinos, from the J-PARC laboratory on the eastern coast of Japan to the Super-Kamiokande detector 295km away. At the detector scientists look for differences in how the neutrinos or antineutrinos changed flavour and are now finding that neutrinos appear to be much more likely to change than antineutrinos.

Dr Patrick Dunne, from the Department of Physics at Imperial College London, said “This result brings us closer than ever before to answering the fundamental question of why the matter in our universe exists. If confirmed – at the moment we’re over 95 per cent sure – it will have profound implications for physics and should point the way to a better understanding of how our universe evolved.”

While this result shows a strong preference for enhancement of the neutrino rate in T2K, it is not yet clear if CP symmetry is violated or not. To improve the experimental sensitivity to a potential CP symmetry violating effect, the T2K Collaboration will now upgrade the near detector suite to reduce systematic uncertainties and accumulate more data, and J-PARC will increase the beam intensity by upgrading the accelerator and beamline.

Professor David Wark, former 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 are providing very strong hints that the oscillations of neutrinos and anti-neutrinos differ in a way we did not expect.  Whether that is a clue as to why there is more matter than anti-matter in the universe, or just a more trivial difference due to some statistical fluke or some unexpected asymmetry in the experiment, we will need more data and to undertake more sensitive experiments before we can tell. 

“Over the next few years we hope to keep running T2K with upgrades to the near detectors and to Super Kamiokande, which would allow us to test various sources of uncertainty.  However to really probe this potential new physics in detail we will need entirely new, larger, and more sensitive experiments, like the Hyper Kamiokande experiment recently approved in Japan and the DUNE experiment now under construction in the United States”.  

Reviewing these results and looking ahead to what comes next Professor Morgan Wascko, international co-spokesperson for the T2K experiment from the Department of Physics at Imperial College London said “From the earliest days of T2K, the UK universities and national laboratories have played major roles in the near detectors and the neutrino beam, and of course in data analysis. In the past five years, we have also now expanded into the far detector, Super Kamiokande, and are looking to expand our work into the accelerator that feeds the neutrino beam as well."

These results, using data collected through 2018, have been published in Nature on April 16.

More information about the T2K experiment can be found on the T2K public website (http://t2k- experiment.org).

T2K (Tokai-to-Kamioka) – is an international experiment led by Japan and part funded by the UK’s Science and Technology Facilities Council (STFC). It probes the strange properties of the enigmatic neutrino to unprecedented precision, by firing the most intense neutrino beam ever designed from the east coast of Japan, all the way under the country, to a detector near Japan’s west coast.

Scientists and engineers 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. 

In particular, engineers from the STFC Technology Department, working with physicists from STFC’s Particle Physics Department, designed and built major parts of the electronics for the T2K near detectors, the data acquisition software and hardware for the near detector, the engineering design and supports for the Electromagnetic Calorimeter (one of the major near detectors), the supports for the entire near detector (which showed the quality of their design by surviving the 2011 Great East Japan Earthquake unscathed).  Technology Department engineers also made major contributions to one of the most challenging pieces of the experiment – the target system where a powerful proton beam hits a target to produce pions, which are then focussed so that when they decay a well-collimated neutrino beam is produced. 

The T2K experiment was constructed, and is operated, by an international collaboration that currently consists of nearly 500 scientists from 68 institutions in 12 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, UK, USA and Vietnam]. The results are made possible by the efforts of J-PARC to deliver high-quality beam to T2K.

The following UK institutes participate in the T2K experiment:

• STFC Daresbury Laboratory (UK)
• Imperial College London (UK)
• Lancaster University (UK)
• Oxford University (UK)
• Queen Mary, University of London (UK)
• Royal Holloway University of London (UK)
• STFC Rutherford Appleton Laboratory (UK)
• University of Glasgow(UK)
• University of Liverpool (UK)
• University of Sheffield (UK)
• University of Warwick (UK)