Earth matter effects on the supernova neutrino spectra

We explore the earth matter effects on the energy spectra of neutrinos from a supernova. We show that the observations of the energy spectra of $\nu_e$ and $\bar{\nu}_e$ from a galactic supernova may enable us to identify the solar neutrino solution, to determine the sign of $\Delta m^2_{32}$, and to probe the mixing matrix element $|U_{e3}|^2$ to values as low as $10^{-3}$. We point out scenarios in which the matter effects can even be established through the observation of the spectrum at a single detector.Comment: 8 pages LaTeX, 2 eps figures, uses Rinton-P9x6.cls. Talk given at CICHEP '2001, Cairo, Egypt, January 200

Signatures of heavy sterile neutrinos at long baseline experiments

Sterile neutrinos with masses $\sim 0.1$ eV or higher would play an important role in astrophysics and cosmology. We explore possible signatures of such sterile neutrinos at long baseline experiments. We determine the neutrino conversion probabilities analytically in a 4-neutrino framework, including matter effects, treating the sterile mixing angles $\theta_{14}, \theta_{24}, \theta_{34}$, the deviation of $\theta_{23}$ from maximality,as well as $\theta_{13}$ and the ratio $\Delta m^2_\odot/\Delta m^2_{atm}$ as small parameters for a perturbative expansion. This gives rise to analytically tractable expressions for flavor conversion probabilities from which effects of these parameters can be clearly understood. We numerically calculate the signals at a neutrino factory with near and far detectors that can identify the lepton charge, and point out observables that can discern the sterile mixing signals. We find that clean identification of sterile mixing would be possible for \theta_{24}\theta_{34} \gsim 0.005 and \theta_{14} \gsim 0.06 rad with the current bound of $\theta_{13} < 0.2$ rad; a better $\theta_{13}$ bound would allow probing smaller values of sterile mixing. We also generalize the formalism for any number of sterile neutrinos, and demonstrate that only certain combinations of sterile mixing parameters are relevant irrespective of the number of sterile neutrinos. This also leads to a stringent test of the scenario with multiple sterile neutrinos that currently is able to describe all the data from the short baseline experiments, including LSND and MiniBOONE.Comment: 28 pages, 12 figures, Revtex4 forma

Radiatively broken symmetries of nonhierarchical neutrinos

Symmetry-based ideas, such as the quark-lepton complementarity (QLC) principle and the tri-bimaximal mixing (TBM) scheme, have been proposed to explain the observed mixing pattern of neutrinos. We argue that such symmetry relations need to be imposed at a high scale $\Lambda \sim 10^{12}$ GeV characterizing the large masses of right-handed neutrinos required to implement the seesaw mechanism. For nonhierarchical neutrinos, renormalisation group evolution down to a laboratory energy scale $\lambda \sim 10^3$ GeV tends to radiatively break these symmetries at a significant level and spoil the mixing pattern predicted by them. However, for Majorana neutrinos, suitable constraints on the extra phases $\alpha_{2,3}$ enable the retention of those high scale mixing patterns at laboratory energies. We examine this issue within the Minimal Supersymmetric Standard Model (MSSM) and demonstrate the fact posited above for two versions of QLC and two versions of TBM. The appropriate constraints are worked out for all these four cases. Specifically, a preference for $\alpha_2 \approx \pi$ (i.e. $m_1 \approx -m_2$) emerges in each case. We also show how a future accurate measurement of $\theta_{13}$ may enable some discrimination among these four cases in spite of renormalization group evolution.Comment: 29 pages, 4 figures, revtex4. Minor changes in the Introduction, references added. Final version to be published in Phys. Rev.

2540 km: Bimagic baseline for neutrino oscillation parameters

We show that a source-to-detector distance of 2540 km offers multiple advantages for a low energy neutrino factory with a detector that can identify muon charge. At this baseline, for any neutrino hierarchy, the wrong-sign muon signal is almost independent of CP violation and $\theta_{13}$ in certain energy ranges. This reduces the uncertainties due to these parameters and allows the identification of the hierarchy in a clean way. In addition, part of the muon spectrum is also sensitive to the CP violating phase and $\theta_{13}$, so that the same setup can be used to probe these parameters as well.Comment: 4 pages, 4 figures, Revtex4. Text modified. Version to appear in PR

No-go for exactly degenerate neutrinos at high scale?

We show in a model independent manner that, if the magnitudes of Majorana masses of neutrinos are exactly equal at some high scale, the radiative corrections cannot reproduce the observed masses and mixing spectrum at the low scale, irrespective of the Majorana phases or the mixing angles at the high scale.Comment: 12 pages ReVTeX, A few typos corrected in the 2nd versio