T2K released its first measurement of muon-antineutrino disappearance in a seminar at the KEK High Energy Accelerator Research Organization on 18th May 2015. This result has been obtained from data taken from May 2014 – March 2015.
During this time, T2K has been running with a beam composed mostly of muon antineutrinos, and 59.8 events would have been expected in T2K’s far detector Super-Kamiokande if there were no antineutrino oscillations. However only 17 events were observed, which matches the expected “disappearance” due to muon antineutrinos oscillating into tau antineutrinos.
T2K also compared these new results with its earlier measurement of muon-neutrino disappearance, and found no difference between neutrinos and antineutrinos in these oscillations.
The graph below shows the T2K best-fit values and 68% and 90% credible intervals in the antineutrino oscillation parameters sin2θ23 and Δm232. The first of these parameters is a measure of the fraction of antineutrinos that oscillates, while the second is a measure of the difference between the squares of the masses of two types of antineutrino. T2K’s measurement of the antineutrino oscillation probability parameter sin2θ23 is the world’s best.