Recent Status of Neutrino Oscillation Study

It has been known that the atmospheric neutrino anomaly (See, e.g., Ref. ) can be accounted for by dominant νμ ↔ ντ oscillations with almost maximal mixing. According to the most up-to-date result of the two flavor analysis of νμ ↔ ντ with 1289 day SuperKamiokande data , the allowed region of the oscillation parameters at 90%CL is 0.88 < sin 2θatm ≤ 1, 1.6 × 10−3eV < ∆matm < 4× 10−3eV. Two flavor analysis of νμ ↔ νs has been also done by the SuperKamiokande group using the data of neutral current enriched multiring events, high energy partially contained events and upward going μ’s, and they have excluded the two flavor oscillation νμ ↔ νs at 99%CL . It has been also shown that the solar neutrino observations (See, e.g., Ref. ) suggest neutrino oscillations and the most up-to-date analyses 5,6,7 tell us that the large mixing angle (LMA) MSW solution (∆m⊙ ≃ 2 × 10−5eV2, sin 2θ ≃ 0.8) gives the best fit and it is followed by the LOW solution (∆m⊙ ≃ 1 × 10−7eV2, sin 2θ ≃ 1.0). The recent SNO data 8 prefer νe ↔ νactive to νe ↔ νs oscillations. On the other hand, it has been claimed by the LSND group that their data suggest neutrino oscillations with ∆mLSND ∼ O(1)eV2. If this anomaly as well as the atmospheric and solar neutrino data are to be interpreted as evidence of neutrino oscillations then we would need at least four flavors of neutrinos, since the mass squared differences ∆matm, ∆m 2 ⊙ and ∆m 2 LSND suggested by the atmospheric neutrino anomaly, the solar neutrino deficit and the LSND data have different orders of magnitudes.