Sterile Neutrinos: Phenomenology and Theory

A sterile neutrino in addition to the three known neutrinos seems unavoidable if one wants a simultaneous understanding of the results of the LSND experiment indicating $\nu_{\mu}-\nu_e$ oscillation together with other evidences for neutrino oscillations such as those from solar and atmospheric neutrino deficits. A major theoretical challenge then is to understand why the sterile neutrino is so light. A simple solution appears to be to assume that it is the lightest neutrino of a mirror sector of the universe which has identical matter and gauge content as the standard model. After a brief review of the phenomenology, a recently constructed realistic grand unified model based on the gauge group $SO(10)\times SO(10)^\prime$ that embeds the mirror universe idea is presented. Detailed predictions for known and the sterile neutrino sector are given and their consequences for cosmology are mentioned.Comment: Invited talk presented at the workshop on "New trends in neutrino physics" held at the Ringberg Castle, Germany, May 24-28, 1998; 10 page

Seesaw Mechanism and Its Implications

The seesaw mechanism for neutrino masses is introduced and some of its different realizations and applications are discussed. It is pointed out how they can be used to understand the bi-large mixing patterns among neutrinos in combination with assumptions about high scale physics such as grand unification or quasi-degeneracy of neutrino masses.Comment: 14 pages; 2 figures; Invited talk presented at the SEESAW25 conference in Paris, June, 2004; a slightly shorter version will appear in the proceedings of SEESAW 25. This version contains a more references to original paper

Physics of Neutrino Mass

Recent discoveries in the field of neutrino oscillations have provided a unique window into physics beyond the standard model. In this lecture, I summarize how well we understand the various observations, what they tell us about the nature of new physics and what we are likely to learn as some of the planned experiments are carried out.Comment: Invited Lecture at the SLAC Summer Institute, August, 2004; 23 pages, 6 figure

Understanding the Standard Model

We explore to what extent the various assumptions behind the standard model as well as the observed quantization of electric charges of quarks and leptons can be understood using the Adler-Bell-Jackiw anomalies. The effect of including a non-zero neutrino mass on these considerations is also discussed.Comment: University of Maryland Report No. UMD-PP-94-116; To be published in the Robert E. Marshak memorial volume, edited by George Sudarshan. ( plain Tex; 17 pages

New vector-scalar contributions to neutrinoless double beta decay and constraints on R-parity violation

We show that in minimal supersymmetric standard model (MSSM) with R-parity breaking as well as in the left-right symmetric model, there are new observable contributions to neutrinoless double beta decay arising from hitherto overlooked diagrams involving the exchange of one W boson and one scalar boson. In particular, in the case of MSSM, the present experimental bounds on neutrinoless double beta decay lifetime improves the limits on certain R-parity violating couplings by about two orders of magnitude. It is shown that similar diagrams also lead to enhanced rates for $\mu^-\rightarrow e^+$ conversion in nuclei, which are in the range accessible to ongoing experiments.Comment: Latex file; 9 pages; 3 figures available on reques

Sterile neutrino decay as a common origin for LSND/MiniBooNe and T2K excess events

We point out that the excess of electron-like neutrino events recently observed by the T2K collaboration may have a common origin with the similar excess events previously reported by the LSND and MiniBooNE experiments and interpreted as a signal from the radiative decays of a sterile neutrino \nu_h with the mass around 50 MeV produced in muon neutrino neutral current (NC) interactions. In this work we assumed that the \nu_h can also be produced in tau neutrino NC reactions.Comment: 5 pages, 5 figures. Revised version accepted by PRD (Rapid Comm.) after referee's suggestions. Fig.1 added for illustratio