  T2K is a long-baseline neutrino oscillation experiment, in which a muon
neutrino beam is produced at J-PARC and detected 295 km away at the
Super-Kamiokande detector. The T2K experiment observed electron-neutrino
appearance in 2012. This observation enables T2K to explore CP violation in the
lepton sector by comparing electron-neutrino appearance and
electron-antineutrino appearance. Indeed, the number of observed electron
neutrino events up to 2012 is, though within statistical fluctuation, larger
than the expectation, which suggests maximal CP violation. Since 2013, T2K has
been accumulating data with a muon antineutrino beam. If the suggested maximal
CP violation is true, electron-antineutrino appearance would be suppressed. The
signal is further suppressed by the smaller cross section for antineutrinos
compared to neutrinos. Hence the observation of electron-antineutrino
appearance is an important next step. Furthermore, the CPT theorem imposes that
the muon disappearance rate must be the same for muon neutrinos and muon
antineutrinos; therefore the comparison between neutrinos and antineutrinos is
a good test of the CPT theorem, or else a probe for new non-standard
interactions of neutrinos with matter. We will report the result of the first
search for electron-antineutrino appearance in T2K, as well as a new
measurement of muon-antineutrino disappearance to compare with muon-neutrino
disappearance measurements.
