| Summary: | T2K is a long-baseline neutrino oscillation experiment, which studies the changing avour composition of a beam over a 295 km baseline from an accelerator at J-PARC to Super-Kamiokande, a 50 kt water Cerenkov detector. The T2K neutrino beam has an energy peak at 0.6 GeV which gives strong sensitivity to oscillations at the atmospheric mass squared splitting. The beam can be run in two modes, producing a beam either dominated by neutrinos or by antineutrinos. Collecting data in antineutrino-mode allows the measurement of the neutrino mixing parameters on antineutrinos only. In the first analysis of T2K antineutrino-mode data, we use beam data collected up to June 2015 to measure sin2⊖23 and j m2 32j. The 90% CL allowed values for mixing angle are 0.327 < sin2⊖23 < 0.692 (normal hierarchy) and 0.332 < sin2⊖23 < 0.697 (inverted hierarchy). The 90% CL allowed values for mass splitting are 2.03x10-3 eV2 < j m2 32j < 2.92x10-3 eV2 (normal hierarchy) and 2.03x10-3 eV2 < j m2 31j < 2.92x10-3 eV2(inverted hierarchy). This is the world's best measurement in sin2⊖23. A difference between neutrino and antineutrino survival probabilities could result from physics beyond the Standard Model, known as non-standard interactions. A simultaneous fit to the T2K neutrino-mode and antineutrino-mode datasets allows for a direct search for such interactions. We see no evidence for this hypothesis.
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