Abstract
The T2K experiment has observed the disappearance of muon neutrinos from the J-PARC muon-neutrino beam. Using data corresponding to 3.01 × 1020 protons on target, 58 muon neutrino event candidates were observed; 205 ± 17 (syst.) events are expected under the no-oscillation hypothesis. A fit to the observed rate and energy spectrum in a neutrino oscillation framework with three active neutrino flavours in constant density matter, assuming the normal mass hierarchy, yields a best fit mixing angle sin2 θ23 = 0.514 ± 0.082 and best fit mass squared splitting |Δm232| = 2.44+0.17−0.15 × 10-3 eV2/c4, which corresponds to the maximal disappearance probability.
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