The T2K experiment has submitted a paper to Physical Review Letters, reporting the definitive observation of electron neutrino appearance in a high-purity muon neutrino beam that travels 295 km, from the J-PARC accelerator complex to the Super-Kamiokande underground neutrino detector. A total of 28 electron neutrino events were detected, compared to 4.92 ± 0.55 background events expected if there were no neutrino oscillations of this type. This result rejects the no-oscillation scenario with a statistical significance of 7.3σ ; that is, the probability is less than 0.00000000001 that so many events would be detected if purely due to background. Thus, the new paper from T2K definitively confirms the existence of electron-neutrino appearance in a muon neutrino beam. For the relevant parameter that describes these neutrino oscillation phenomena, T2K reports a best-fit value of sin2(2θ13) = 0.140+0.038−0.032 for the “normal hierarchy” of mass states, or 0.170+0.045−0.037 for the “inverted hierarchy” assuming δCP=0 (see the Neutrino FAQ for more information). The submitted results are based on T2K data taken from January 2010 to May 2013. During this period the J-PARC proton beam power steadily increased, reaching 220 kW continuous operation with a world record of 1.2 × 1014 protons per pulse.
