T2K (Tokai to Kamioka) is the world leading experiment dedicated to the study of neutrino oscillations over a long distance using µ-neutrino and µ-anti-neutrino beams.
Neutrinos exist in three different types (called 'flavours'): νe, νμ and ντ. In the last fifteen to twenty years, several experiments have proven that neutrinos undergo a quantum mechanical phenomenon called 'oscillation', where neutrinos oscillate from one type to another. Neutrino oscillations demonstrate that neutrinos have mass, contrary to what was predicted in the Standard Model. The oscillation probability is a function of L/E, where E is the neutrino energy and L is the length travelled by the neutrino between its production and its detection points.
Depending on the neutrino type (νe, νμ or ντ) at the time of its detection, an electron (e), a muon (μ) or a tau (τ) can be produced through weak interaction with a nucleon in matter. So starting from a μ-neutrino beam, due to oscillation, electron neutrinos can be detected far from the source. Tau neutrinos are not detected since the energy of the neutrinos in the T2K beam is not sufficient to create a tau particle through the weak interaction. Comparing the rate of muon neutrinos produced at the source and detected far from the source, it looks like some muon neutrinos are ‘disappearing’.
The beam originates from the J-PARC accelerator in Tokai and is aimed at Super Kamiokande, a detector located 295km away in Kamioka. The near detector called ND280 (280m) measures the beam properties before the oscillation and the cross sections of the neutrino’s interactions with nuclei. These measurements are crucial for extracting accurate neutrino oscillation parameters using the Super Kamiokande data.
T2K has been taking data since 2009. The initial goal of the experiment was to measure the angle θ13 of the neutrino PMNS mixing matrix. In 2011, T2K was the first experiment to show indications (at 2.5 σ) that θ13 is different from zero. In 2013 T2K presented the first observation of µ-neutrinos oscillating into e-neutrinos ("e-neutrino appearance"). This was the first direct observation of appearance of a new flavor in neutrino oscillation.
T2K has now started to explore CP violation in the leptonic sector using the combined analysis of the data collected with a neutrino beam and an anti-neutrino beam. The latest results were presented at the ICHEP 2016 summer conference [arXiv:1701.00432]. The data includes runs from Jan 2010 to May 2016 which yielded in Super Kamiokande 32 e-like and 135 μ-like events in neutrino mode, and 4 e-like and 66 μ-like events in antineutrino mode. Using reactor measurements of sin2(2θ13) as an additional constraint, the CP conservation hypothesis (PMNS matrix parameter δCP=0, π) is excluded at 90% C.L. This first hint of non zero CP violation in the leptonic sector, if confirmed, could help in understanding the asymmetry between matter and antimatter in the universe and open a window on new physics.
IRFU has designed and built the front-end electronics and the "bulk" Micromegas detectors for the near detector TPCs (a crucial element of ND280). The TPCs allow the momenta of muons and electrons produced by neutrino interactions to be measured with an accuracy better than 10% up to 1GeV/c. Information from the TPCs is also used for electron and muon identification.
DPhP physicists are currently working on the analysis of the near detector data. Most of the work aims at measuring the neutrino interaction cross-sections with nuclei. These measurements are crucial to reduce the systematical uncertainties that limit the accuracy of oscillation analyses, since despite the recent progress by nuclear physicists to take into account nuclear effects in the models describing neutrino interactions, large theoretical uncertainties remain. An additional analysis of the near detector data is devoted to the search for hypothetical heavy sterile right-handed neutrinos at the GeV scale.
The team is also involved in detector development for future long-baseline experiments, like the WA105 project at CERN, which is based on a Liquid Argon double phase TPC.
Contact: Marco Zito