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A new type of neutrino oscillation was discovered in the T2K experiment

19.07.2013
Today at the European Physical Society meeting in Stockholm (EPS HEP 2013), the international T2K collaboration announced a definitive observation of muon neutrino to electron neutrino transformation which is a new type of neutrino oscillation (a process when neutrinos convert into other type of neutrinos predicted by Academician Bruno Pontecorvo, whose 100th anniversary is celebrated this year). In 2011, the collaboration announced the first indication of this process, which was then a new type of neutrino oscillation; 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 a possibility at the 7.5 sigma level of significance. This T2K observation is the first of its kind in that an explicit appearance of a unique flavor of neutrino at a detection point is unequivocally observed from a different flavor of neutrino at its production point.

In the T2K experiment in Japan, a muon-neutrino beam is produced at the Japan Proton Accelerator Research Complex (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 Cerenkov detector in Kamioka, near the west coast of Japan, 295 km (185 miles) away from Tokai.



Image 1: Layout of T2K


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.


The muon-neutrino beam near the target (i.e before possible oscillations) is studied and controlled by the complex of neutrino detectors situated on the J-PARC territory. A significant part of this detector has been designed and created by INR scientists. Physicists from JINR DLNP participated in this work in the frames of the agreement on JINR-IN2P3 (France) cooperation.

In this 3D image (Image 2) of the cylindrically-shaped Super Kamiokande, each coloured dot represents a photomultiplier that detected light. An electron neutrino interacts with a neutron in a nucleus of a water molecule to produce an electron and a proton. The electron often travels faster than the speed of light in water, and causes Cerenkov light to be emitted from the water atoms. This light is seen as a ring by the photomultipliers of Super Kamiokande. The image shows the first electron-neutrino candidate observed after the recovery from the earthquake on the east coast of Japan in 2011.


Image 2: Electron-neutrino candidate in Super Kamiokande


The distribution of reconstructed energies of the electron-neutrino candidates in Super Kamiokande is shown in the below figure, with black points representing the data, the pink histogram representing the expected energy distribution of electron-neutrino candidates that have oscillated from muon neutrinos, and the green histogram representing the expected energy distribution of non-oscillated electron neutrinos. These non-oscillated electron neutrinos are expected to be seen in Super Kamiokande, and are due in part to a small fraction (1%) of electron neutrinos in the muon-neutrino beam produced at J-PARC.


Image 2: Reconstructed energy distribution of electron-neutrino candidates


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 between 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. The T2K experiment expects to collect 10 times more data in the near future, including data with an antineutrino beam for studies of CP violation in neutrinos.

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.

The RAS Institute for Nuclear Research (INR) is a participant of the T2K experiment from Russia.

Physicists from JINR DLNP made the decisive contribution to precision measurements of the production of hadrons in proton-carbon interactions at energy of 30 GeV in the NA61/SHINE experiment at the SPS accelerator (CERN). These results is used for exact computation of spectrum and flow of neutrino in the T2K experiment.

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 some images can be found at http://t2k-experiment.org/supporting-material-for-t2k-press-release-19-7-2013.
More detailed information about the experiment  and the collaboration can be found at http://t2k-experiment.org.