The T2K international collaboration has announced the definitive observation of a new phenomenon in physics: the transformation of muon neutrinos into electron neutrinos. The observation of this new form of neutrino oscillation will allow the study of the differences between matter and antimatter, which are key to solving one of the biggest mysteries in science: why is the universe made of matter instead of antimatter? The T2K experiment involves over 400 physicists from 11 countries, including Spain, which is represented by the High Energy Physics Institute (IFAE, Generalitat-UAB consortium) and the Corpuscular Physics Institute (IFIC), of the University of Valencia.
In 2011 T2K presented the first indications of this new form of neutrino oscillation, and now, with 3.5 times more data, the indications have become scientific certainty: the probability that the phenomenon observed is fortuitous (a statistical fluctuation) is less than one in a trillion. Thus, T2K is the first experiment to reveal explicitly the appearance of neutrinos of a different type to those produced originally.
The T2K experiment involves sending an intense beam of muon neutrinos to be picked up by a complex system of detectors, placed at varying distances, and capable of measuring the transformation of the initial neutrinos as they travel. The neutrino beam is produced at the J-PARC laboratory (Japan Proton Accelerator Complex) in Tokai, on the east coast of Honshu, Japan’s largest island. The initial properties of the beam are measured by various detectors at the point of production.
After travelling 295 kilometres, the neutrinos reach the west coast of the island and are detected by Super-Kamiokande, a gigantic 50-kiloton detector buried one kilometre underground, in an old zinc mine. Analysis of the data compiled by Super-Kamiokande shows a number of electron neutrinos (28 altogether) that is far higher than would be expected without the new oscillation phenomenon (4.6).
Neutrino oscillation is a manifestation of a long-range quantum mechanical interference caused by the difference between the mass of the different types of neutrinos. Observation of this new type of oscillation can lead to the study of another phenomenon, of even greater importance: charge-parity (CP) violation, which is responsible for the mechanisms that distinguish between the behaviours of matter and antimatter.
CP violation, only previously observed in quarks (for which Nobel prizes were awarded in 1980 and 2008), is today accepted as the most probable reason why the observable universe today is dominated by matter and contains no significant amounts of antimatter, It is one of the most exciting mysteries in science, given that the Big Bang ought to have produced equal amounts of matter and antimatter.
Now that T2K has firmly established this new form of neutrino oscillation, sensitive to CP violation, research into this phenomenon has become one of the major scientific quests of the decade, and T2K is in a position to lead this research. T2K expects to collect 10 times more data in the coming years, including data obtained with an antineutrino beam (“neutrino antimatter”), to look for differences in the oscillations of matter and antimatter that will allow CP violation to be measured.
This discovery was made possible by the efforts of the J-PARC staff members and management to deliver an intense high-quality beam to T2K even after the devastating March 2011 earthquake, which caused severe damage to the accelerator complex at J-PARC, and interrupted data-taking in the T2K experiment.
The T2K experiment was constructed and is operated by an international collaboration, currently made up of over 400 physicists from 59 institutions in 11 countries (Germany, Canada, USA, Spain, France, UK, Italy, Japan, Poland, Russia, and Switzerland). The experiment is primarily funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Spain contributes two research groups: The High Energy Physics Institute (IFAE, Generalitat-UAB consortium) and the Corpuscular Physics Institute (IFIC), of the University of Valencia, which have been taking part in the design, construction and operation of the experiment for over ten years. Both groups have made highly significant contributions to the study of neutrino oscillation, working with the Tokai detector, the nearest to the beam source, to measure its initial properties. Funding for the study from within Spain has come from the Spanish Ministry of the Economy and Competitiveness, the Government of Catalonia (Generalitat), and the Spanish National Centre for Particle, Astroparticle and Nuclear Physics (CPAN).