Indication of electron neutrino appearance caught in T2K experiment
2011. 10. 31

In our Universe, the basic elements of matter are the Fermions: 6 quarks and 6 leptons. Among the 6 kinds of leptons, the electron is well-known and easily detected one while three kinds of neutrinos are also distributed within our space but extremely hard to detect and recognize. The three neutrinos are distinguished by their flavors, and be recognized as electron neutrino, muon neutrino, and tau neutrino. Something hiding in these neutrino properties may lead us to find “new physics” beyond the Standard Model, which is our current explanation of particle behavior.

Among the marvelous properties of neutrinos, neutrino oscillation is regarded as the most notable one. This oscillation means that traveling neutrinos will change their flavor, due to neutrinos have a mix of mass states in each flavor. However, these mixed mass states contradict what the Standard Model predicts: that neutrinos should be massless particles. In 1998, the oscillation evidence was first presented by Super-Kamiokande from atmospheric neutrinos (neutrinos from cosmic rays interacting with the atmosphere). Since then, there have been various neutrino oscillation experiments attempting to measure the oscillation parameters―mass differences and mixing angles―precisely. Three mixing angles (theta12, theta23, and theta13) are used to connect between the three flavor states (electron, muon, tau) and the three mass states (labeled 1, 2, 3). Based on current results, numerical values of theta12 and theta23 are known (theta12 ~ 35 deg, theta23 ~ 45 deg), but we only have an upper limit on theta13 of 0.15, as theta13 value is still concealed in unobserved oscillation modes. Physicists are eager to measure this final number, which is also important in another topic: CP violation. As long as theta13 has a non-zero value, this non-zero number leads a chance to observe CP violation, which could explain why antimatter disappeared in our Universe.

Kyoto University High Energy Particle Physics Group (KUHEP) are participating in the T2K (Tokai to Kamioka) neutrino oscillation experiment, which is trying to observe an unfound mode: electron neutrinos coming from a muon neutrino beam. The J-PARC facility in Tokai, Ibaraki can generate the most intense proton beam in the world, which can be used to produce a pure muon neutrino beam, directed to a (297km-distant) far neutrino detector: Super-Kamiokande in Kamioka, Gifu. In this giant water Cherenkov detector, electron neutrinos appearing in the JPARC beam are looked for, in order to measure the last angle theta13. The T2K experiment is a very large scale experiment and T2K has 500 members, from 12 countries. Among these international groups, KUHEP play a significant role in T2K. KUHEP members take a central role in the following J-PARC detectors: Muon Monitor, INGRID, Proton Module, MIZUCHE, and ND280, which measure neutrino flux, cross-section, beam direction and intensity. On the Kamiokande side, we are also doing studies to estimate systematic errors. Our members have done a lot of indispensable work relating to the core problems of T2K experiment. However, on March 11th, when T2K just got 2% data of our intended quantity of data, East Japan suffered a major earthquake. Although T2K has had to stop operating for a while, the members still devoted themselves into analysis work with these limited data. Fortunately, we found a fruitful result, of 6-electron neutrino events appearing in Super-Kamiokande. With a probability, in the absence of oscillations, of just 0.7%, this is the first indication of this electron-flavor appearing mode, which suggests that theta13 must have a nonzero value.

These analysis results were submitted to PRL (Physical Review Letters, one of the world’s most influential journals of Physics) on June 13th, we found that theta13 is about 10 degrees, and this helps physicists to complete the puzzle of neutrino mass. After submitting, these results have had a big impact on the field of physics. Presently, T2K are still aiming at more precise measurement of theta13, and hope it will lead to research into CP violation with neutrinos. The breaking of this symmetry has already been seen in hadronic systems and is described by Kobayashi-Maskawa theory. If breaking of this symmetry can be seen with neutrinos, it will be a great help to solve the puzzle of missing anti-matter.

In the meantime, KUHEP are helping recovery work of J-PARC and improving treatment of systematic errors. We believe that T2K will produce more exciting results from our efforts in the future.