The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, PRL 112, 061802 (2014) P
H Y S I C A L R E V I E W L E T T E R Sweek ending 14 FEBRUARY 2014 061802-2 corresponding to a significance of 7.3σ when compared to 4.92 AE 0.55 expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles θ 12 , θ 23 , θ 13 , a mass difference Δm 2 32 and a CP violating phase δ CP . In this neutrino oscillation scenario, assuming jΔm 2 32 j ¼ 2.4 × 10 −3 eV 2 , sin 2 θ 23 ¼ 0.5, and Δm −0.037 ) is obtained at δ CP ¼ 0. When combining the result with the current best knowledge of oscillation parameters including the world average value of θ 13 from reactor experiments, some values of δ CP are disfavored at the 90% C.L. DOI: 10.1103/PhysRevLett.112.061802 PACS numbers: 14.60.Pq, 14.60.Lm, 25.30.Pt, 29.40.Ka Introduction.-The discovery of neutrino oscillations using atmospheric neutrinos was made by SuperKamiokande in 1998 [1]. Since then, many other experiments have confirmed the phenomenon of neutrino oscillations through various disappearance modes of flavor transformations. However, to date, there has not been an observation of the explicit appearance of a different neutrino flavor from neutrinos of another flavor through neutrino oscillations. In 2011, the T2K collaboration published the first indication of electron neutrino appearance from a muon neutrino beam at 2.5σ significance based on a data set corresponding to 1.43 × 10 20 protons on target (POT) [2,3]. This result was followed by the publication of further evidence for electron neutrino appearance at 3.1σ in early 2013 [4]. This Letter presents new results from the T2K experiment that establish, at greater than 5σ, the observation of electron-neutrino appearance from a muon-neutrino beam.In a three-flavor framework, neutrino oscillations are described by the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix [5,6] which is parametrized by three mixing angles θ 12 , θ 23 , θ 13 , and a CP violating phase δ CP . In this framework, the probability for ν μ → ν e oscillation can be expressed [7] as where L is the neutrino propagation distance and E is the neutrino energy. The measurement of ν μ → ν e oscillations is of particular interest because this mode is sensitive to both θ 13 and δ CP . The first indication of nonzero θ 13 was published by T2K [3] based on the measurement of ν μ → ν e oscillations. More recently, indications of ν μ → ν e oscillations were also reported by the MINOS experiment [8]. The value of θ 13 is now precisely known to be 9.1°AE 0.6°from measurements ofν e disappearance in reactor neutrino experiments [9][10][11][12]. Using the reactor measurement of θ 13 , the ν μ → ν e appearance mode can be used to ...
