14 June 2011
Neutrinos exist in three different forms, or 'flavors': electron neutrinos, muon neutrinos and tau neutrinos. The T2K experiment, based in Japan, is studying the oscillation mechanism of these particles, in other words their ability to change flavor while traveling through space. The experiment is designed to observe neutrino oscillations over a distance of 295 km, between the Tokai site, where the muon neutrinos are produced using the J-PARC(2) particle accelerator on Japan's east coast, and the Super-Kamiokande detector, a cylindrical tank of water 40 meters in diameter and 40 meters in height, located 1 000 meters underground, near the west coast (hence its name T2K, which stands for "Tokai to Kamiokande").
Analysis of the data collected between the beginning of the experiment in January 2010 and March 2011 (the experiment was suspended as a result of the 11 March earthquake) shows that, over the period, the Super-Kamiokande detector recorded a total of 88 neutrinos, of which six were electron neutrinos that probably result from a change of muon neutrinos into electron neutrinos. The remaining 82 neutrinos are thought to be mainly muon neutrinos that underwent no transformation between their production to their detection. Measurements by GPS confirm that the neutrinos identified by the Super-Kamiokande detector were indeed produced on the east coast of Japan. The physicists therefore estimate that the results obtained point to a 99.3% probability that electron neutrino appearance was detected.
The T2K experiment will resume at the end of the year. Although located in a seismic zone near the epicenter of the earthquake of 11 March 2011, the J-PARC laboratory and T2K's near detectors were fortunately only slightly damaged. T2K's next goal is to confirm the appearance of electron neutrinos with further data and, even more importantly, to measure the 'mixing angle', a parameter of the Standard Model which would open the way to studying the asymmetry between matter and antimatter in our Universe.
The T2K collaboration brings together over 500 physicists from 62 institutions across 12 countries (Japan, a number of European countries and the US). The teams from CNRS and CEA/IRFU developed some of the measuring instruments used in the near detectors (located 280 meters from the neutrino production target, and used to monitor the experiment). The researchers also took part in the calibration of the Super-Kamiokande detector and helped to analyze the data.
© Kamioka Observatory, ICRR, University of Tokyo.
View of the huge Super-Kamiokande detector, which had already been used to study 'natural' neutrinos coming from the Sun as well as those produced by cosmic rays in the upper atmosphere. At the bottom of the image, a team can be seen inspecting the detector while it is still empty.
© T2K collaboration.
View of a T2K electron neutrino appearance event at the Super-Kamiokande far detector. The dots show where Cherenkov radiation is detected by photomultipliers located on the walls of the huge tank. The circle shows where an electron was produced due to the interaction of the incident electron neutrino with the water in the tank. This event was perfectly synchronous with the production of the muon neutrino at JPARC.
(1)Laboratoire de physique nucléaire et de hautes énergies (CNRS / Université Pierre et Marie Curie / Université Paris Diderot-Paris 7), Institut de physique nucléaire de Lyon (CNRS / Université Lyon 1), Laboratoire Leprince-Ringuet (CNRS / École Polytechnique).
(2)Japan Proton Accelerator Research Complex
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