We discuss neutrino oscillations in the framework of the quantum field theory without introducing the concept of neutrino weak eigenstates. The external particles are described by wave packets and the different mass eigenstate neutrinos propagate between the production and detection interactions, which are macroscopically localized in space-time. The time-averaged cross section, which is the measurable quantity in the usual experimental setting, is calculated. It is shown that only in the extremely relativistic limit the usual quantum mechanical oscillation probability can be factored out of the cross section. GIUNTI@TORINO.INFN.IT;

It is shown that the singular seesaw mechanism can simultaneously explain all the existing data supporting nonzero neutrino masses and mixing. The three mass-squared differences that are needed to accommodate the atmospheric neutrino data (through νµ −νs oscillation), the solar neutrino data via MSW mechanism (through νe − ντ oscillation), and the positive result of νµ − νe oscillation from LSND can be generated by this mechanism, whereas the vacuum oscillation solution to the solar neutrino problem is disfavored. We find that the electron and tau neutrino masses are of order 10 −3 eV, and the muon neutrino and a sterile neutrino are almost maximally mixed to give a mass of order 1 eV. Two heavy sterile neutrinos have a mass of order 1 keV which can be obtained by the double seesaw mechanism with an intermediate mass scale ∼ 10 5 GeV. A possible origin of such a scale is discussed.

We investigate the implications of a possible excess of the electron neutrino events in the Super-Kamiokande atmospheric neutrino experiment. The excess, if real, leads to constraints on the ν µ ↔ ν e , ν µ ↔ ν τ and ν e ↔ ν τ oscillation parameters. Specifically, the excess implies that ν e − ν µ and ν µ − ν τ mixings are large whereas ν e − ν τ mixing is small. We show that the electron neutrino excess favors the large mixing angle MSW solution of the solar neutrino problem.PACS number(s): 14.60Lm, 14.60.Pq, 25.65 Typeset using REVT E X Recently, the Super-Kamiokande Collaboration has reported the evidence for the neu-