Constraint on Neutrino Decay with Medium-Baseline Reactor Neutrino Oscillation Experiments

The experimental bound on lifetime of nu_3, the neutrino mass eigenstate with the smallest nu_e component, is much weaker than those of nu_1 and nu_2 by many orders of magnitude to which the astrophysical constraints apply. We argue that the future reactor neutrino oscillation experiments with medium-baseline (~ 50 km), such as JUNO or RENO-50, has the best chance of placing the most stringent constraint on nu_3 lifetime among all neutrino experiments which utilize the artificial source neutrinos. Assuming decay into invisible states, we show by a detailed chi^2 analysis that the nu_3 lifetime divided by its mass, tau_3/m_3, can be constrained to be tau_3/m_3 > 7.5 (5.5) x 10^{-11} s/eV at 95% (99%) C.L. by 100 kt.years exposure by JUNO. It may be further improved to the level comparable to the atmospheric neutrino bound by its longer run. We also discuss to what extent nu_3 decay affects mass-ordering determination and precision measurements of the mixing parameters.Comment: 23 pages, 6 figures, clarification of some discussions, added some references, no change in results and conclusions, version accepted for publication in JHE

What can we learn about the lepton CP phase in the next 10 years?

We discuss how the lepton CP phase can be constrained by accelerator and reactor measurements in an era without dedicated experiments for CP violation search. To characterize globally the sensitivity to the CP phase \delta_{CP}, we introduce a new measure, the CP exclusion fraction, which quantifies what fraction of the \delta_{CP} space can be excluded at a given input values of \theta_{23} and \delta_{CP}. Using the measure we study the CP sensitivity which may be possessed by the accelerator experiments T2K and NOvA. We show that, if the mass hierarchy is known, T2K and NOvA alone may exclude, respectively, about 50%-60% and 40%-50% of the \delta_{CP} space at 90% CL by 10 years running, provided that a considerable fraction of beam time is devoted to the antineutrino run. The synergy between T2K and NOvA is remarkable, leading to the determination of the mass hierarchy through CP sensitivity at the same CL.Comment: Analyses and plots improved, conclusions unchanged, 23 pages, 8 figures, 1 tabl

Bounds on sterile neutrino mixing for cosmologically interesting mass range

This talk summarizes our recent work which studied the impact of resonant $\nu_e \to\nu_s$ and $\bar{\nu}_e\to\bar{\nu}_s$ ($\nu_s$ is a sterile neutrino) conversions on supernova physics, under the assumption that the mass of the sterile state is in the few eV -cosmologically significant range.Comment: Latex file, 3 pages including 4 ps figures, Talk given by H. Nunokawa in TAUP97, Gran Sasso, Italy, 7-11 September, 199

Resonant Conversion of Massless Neutrinos in Supernovae

It has been noted for a long time that, in some circumstances, {\sl massless} neutrinos may be {\sl mixed} in the leptonic charged current. Conventional neutrino oscillation searches in vacuum are insensitive to this mixing. We discuss the effects of resonant massless-neutrino conversions in the dense medium of a supernova. In particular, we show how the detected $\bar\nu_e$ energy spectra from SN1987a and the supernova $r$-process nucleosynthesis may be used to provide very stringent constraints on the mixing of {\sl massless} neutrinos.Comment: latex file, 20 pages, including 3 postscript figure

Three Generation Long-wavelength Vacuum Oscillation Solution to the Solar Neutrino Problem

We investigate the current status of the long-wavelength vacuum oscillation solution to the solar neutrino problem and to what extent the presence of a third neutrino can affect and modify it. Assuming that the smaller mass squared difference that can induce such oscillations, $\Delta m^2_{12}$, is in the range $10^{-11}-10^{-8}$ eV$^2$ and the larger one, $\Delta m^2_{23}$, in the range relevant to atmospheric neutrino observations, we analyze the most recent solar neutrino data coming from Homestake, SAGE, GALLEX, GNO and Super-Kamiokande experiments in the context of three neutrino generations. We include in our vacuum oscillation analysis the MSW effect in the Sun, which is relevant for some of the parameter space scrutinized. We have also performed, as an extreme exercise, the fit without Homestake data. % While we found that the MSW effect basically does not affect the best fitted parameters, it significantly modifies the allowed parameter space for $\Delta m^2_{12}$ larger than $\sim 3 \times 10^{-10}$ eV$^2$, in good agreement with the result obtained by A. Friedland in the case of two generations. % Although the presence of a third neutrino does not essentially improve the quality of the fit, the solar neutrino data alone can give an upper bound on $\theta_{13}$, which is constrained to be less than $\sim 60^\circ$ at 95 % C.L.Comment: 35 pages, 14 png figures: good quality postscript figures can be found in http://neutrinos.if.usp.br/gefan/papers/publicados/ps/GNZ

Potential of a Neutrino Detector in the ANDES Underground Laboratory for Geophysics and Astrophysics of Neutrinos

The construction of the Agua Negra tunnels that will link Argentina and Chile under the Andes, the world longest mountain range, opens the possibility to build the first deep underground labo- ratory in the Southern Hemisphere. This laboratory has the acronym ANDES (Agua Negra Deep Experiment Site) and its overburden could be as large as \sim 1.7 km of rock, or 4500 mwe, providing an excellent low background environment to study physics of rare events like the ones induced by neutrinos and/or dark matter. In this paper we investigate the physics potential of a few kiloton size liquid scintillator detector, which could be constructed in the ANDES laboratory as one of its possible scientific programs. In particular, we evaluate the impact of such a detector for the studies of geoneutrinos and galactic supernova neutrinos assuming a fiducial volume of 3 kilotons as a reference size. We emphasize the complementary roles of such a detector to the ones in the Northern Hemisphere neutrino facilities through some advantages due to its geographical location.Comment: 20 pages, 16 figures and 9 table