New results from T2K conclusively show muon neutrinos transform to electron neutrinos

19 July 2013

Today at the European Physical Society meeting in Stockholm, the international T2K collaboration announced definitive observation of muon neutrino to electron neutrino transformation. In 2011, the collaboration announced the first indication of this process, a new type of neutrino oscillation, then; now with 3.5 times more data this transformation is firmly established. The probability that random statistical fluctuations alone would produce the observed excess of electron neutrinos is less than one in a trillion. Equivalently the new results exclude such possibility at 7.5 sigma level of significance. This T2K observation is the first of its kind in that an explicit appearance of a unique flavour of neutrino at a detection point is unequivocally observed from a different flavour of neutrino at its production point.

In the T2K experiment in Japan, a muon neutrino beam is produced in the Japan Proton Accelerator Research Complex, called J-PARC, located in Tokai village, Ibaraki prefecture, on the east coast of Japan. The neutrino beam is monitored by a detector complex in Tokai and aimed at the gigantic Super-Kamiokande underground detector in Kamioka, near the west coast of Japan, 295 km (185 miles) away from Tokai. An analysis of the data from the Super-Kamiokande detector associated with the neutrino beam time from J-PARC reveals that there are more electron neutrinos (a total of 28 events) than would be expected (4.6 events) without this new process.

Neutrino oscillation is a manifestation of a long range quantum mechanical interference. Observation of this new type of neutrino oscillation leads the way to new studies of charge-parity (CP) violation which provides a distinction in physical processes involving matter and antimatter. This phenomenon has only been observed in quarks (for which Nobel prizes were awarded in 1980 and 2008). CP violation in neutrinos in the very early universe may be the reason that the observable universe today is dominated by matter and no significant antimatter, which is one of the most profound mysteries in science. Now with T2K firmly establishing this form of neutrino oscillation that is sensitive to CP violation, a search for CP violation in neutrinos becomes a major scientific quest in the coming years, and T2K will continue to play a leading role.
Prof. Weber said: "This is a first time that a definite measurement has been made that electron neutrinos (one of 3 different types of neutrinos) have appeared in a beam of muon neutrinos. The UK particle physics community was one of the driving forces behind this experiment. We not only provided part of the detector that characterises the beam, but also designed the target that produced the neutrinos in the first place. The long years of hard work have now come to fruition. Our findings now open the possibility to study this process for neutrinos and their anti-matter partners the anti-neutrinos. A difference in the rate of electron or anti-electron neutrino being produced may lead us to understand why there is so much more matter than anti-matter in the universe. The neutrino may be the very reason why we are here."

The T2K experiment expects to collect 10 times more data in the near future, including data with anti-neutrino beam for studies of CP violation in neutrinos.
Prof. Dave Wark, the UK spokesperson explained: "It is a joy to see T2K deliver the science we designed it for. I have been working on this for more than a decade, and what these results tell us is that we have more than another decade of work ahead of us. We have seen a new way for neutrinos to change, and now we have to find out if neutrinos and anti-neutrinos do it the same way. If they don't, it may be a clue to help solve the mystery of where the matter in the universe came from in the first place. Surely answering that is worth a couple of decades of work!"

The T2K experiment was constructed and is operated by an international collaboration. The current T2K collaboration consists of over 400 physicists from 59 institutions in 11 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Switzerland, Spain, UK and US]. The experiment is primarily supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). Additional support is provided by the following funding agencies from participating countries: NSERC, NRC and CFI, Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MICINN and CPAN, Spain; SNSF and SER, Switzerland; STFC, U.K.; DOE, U.S.A. This discovery was made possible with the unyielding and tireless effort by the J-PARC staff members and the management to deliver high quality beam to T2K after the devastating March 2011 earthquake in eastern Japan which caused severe damage to the accelerator complex at J-PARC, and abruptly discontinued the data-taking run of the T2K experiment.

More detailed information on this announcement including images, T2K experiment and T2K collaboration can be found from the T2K public webpage: and in the STFC press release