On the Status of Three-neutrino Mixing

All existing solar and atmospheric neutrino oscillation data are described by the standard two-neutrino formulas with neutrino mass squared differences that satisfy the hierarchy relation ∆m2sol ≪ ∆m 2 atm. We discuss this phenomenon from the point of view of three-neutrino mixing. Possibilities to see effects of three-neutrino mixing and, in particular, effects of CP-violation in the lepton sector depend on the values of |Ue3| 2 and ∆m2sol. We present an upper bound of |Ue3| , which was obtained from a three-neutrino analysis of CHOOZ data in the case if the solar neutrino oscillation parameters lie in the LMA allowed region. There exist at present strong evidences in favor of neutrino masses and neutrino oscillations that were obtained in the atmospheric [1] and solar [2, 3, 4, 5, 6, 7, 8] neutrino experiments. From the analysis of the existing data it follows that 1. The data of all solar neutrino experiments can be described by twoneutrino oscillations, which are characterized by two parameters ∆m2sol and tan θsol. It is assumed also that the fluxes of initial νe are given by the Standard Solar Model [9]. After the Super-Kamiokande measurement of the day-night asymmetry and the spectrum of recoil electrons in the process νe → νe [7] and the measurement of flux of νe on the earth in the SNO experiment [8] it was established that the large mixing angle MSW allowed regions (LMA, LOW) are the most preferable ones [10, 11, 12]. For the best-fit values of the oscillation parameters in the LMA region it was found that [11] ∆m2sol = 4.5 · 10 −5 eV; tan θsol = 4.1 · 10 −1 . (1) 1 Report at the Meeting NO-VE, Neutrino oscillations at Venice, 24-26 July 2001