We discuss big bang nucleosynthesis constraints on maximal νμ↔νs mixing. Vacuum νμ↔νs oscillation has been proposed as one possible explanation of the Super Kamiokande atmospheric neutrino data. Based on the most recent primordial abundance measurements, we find that the effective number of neutrino species for big bang nucleosynthesis (BBN) is Nv≲3.3. Assuming that all three active neutrinos are light (with masses ≪; 1 MeV), we examine BBN constraints on νμ↔νs mixing in two scenarios: (1) a negligible lepton asymmetry (the standard picture) and (2) the presence of a large lepton asymmetry which has resulted from an amplification by ντ↔νs′, mixing (νs′ being νs or another sterile neutrino species). The latter scenario has been proposed recently to reconcile the BBN constraints and large-angle νμ↔νs mixing. We find that the large-angle νμ↔νs mixing in the first scenario, which would yield Nv≈4, is ruled out as an explanation of the Super Kamiokande data. It is conceivably possible for the νμ↔νs solution to evade BBN bounds in the second scenario, but only if 200eV2≲mντ2-m νs′2≲104 eV2is satisfied, and if ντ decays non-radiatively with a lifetime ≲103 yr. This mass-squared difference implies 15eV≲mντ ≲100 eV if νs is much lighter than ντ. We conclude that maximal (or near maximal) νμ↔ντ mixing is a more likely explanation of the Super Kamiokande data. ©1999 The American Physical Society.

We propose a new and unique dark matter candidate: ∼ 100 eV to ∼ 10 keV sterile neutrinos produced via lepton-number-driven resonant Mikheyev-Smirnov-Wolfenstein conversion of active neutrinos. The requisite lepton number asymmetries in any of the active neutrino flavors range from 10-3to 10-1of the photon number. The unique feature here is that the adiabaticity condition of the resonance strongly favors the production of lower energy sterile neutrinos. The resulting nonthermal (cold) energy spectrum can cause these sterile neutrinos to revert to nonrelativistic kinematics at an early epoch, so that free-streaming lengths at or below the dwarf galaxy scale are possible. Therefore, the main problem associated with light neutrino dark matter can be circumvented in our model. © 1999 The American Physical Society.

We apply causality considerations to active-sterile neutrino transformation-based schemes for lepton number generation in the early Universe. These considerations necessarily lead to the creation of spatial domains of lepton number with opposite signs. Lepton number gradients at domain boundaries can open a new channel for Mikheyev-Smirnov-Wolfenstein resonant production of sterile neutrinos (νs). This enhanced νs production allows considerable tightening of big bang nucleosynthesis constraints on active-sterile neutrino mixing. 1999 © The American Physical Society.

We calculate the luminosity and energy spectrum of the neutrino emission from electron-positron pair annihilation during the collapse of a supermassive star (M ≳ 5 × 104 M⊙). We then estimate the cumulative flux and energy spectrum of the resulting neutrino background as a function of the abundance and redshift of supermassive stars and the efficiency of these objects in converting gravitational energy into neutrino energy. We estimate the expected signal in some of the new generation of astrophysical neutrino detectors from both a cumulative background of supermassive stars and single collapse events associated with these objects. © 1998. The American Astronomical Society. All rights reserved.

It has been proposed that an asymmetry in the electron neutrino sector may be generated by resonant active-sterile neutrino transformations during big bang nucleosynthesis (BBN). We calculate the change in the primordial 4He yield Y resulting from this asymmetry, taking into account both the time evolution of the νe and ν̄e distribution function and the spectral distortions in these. We calculate this change in two schemes: (1) a lepton asymmetry directly generated by νe mixing with a lighter right-handed sterile neutrino νs, and (2) a lepton asymmetry generated by a ντ↔νs or νμ↔νs transformation which is subsequently partially converted to an asymmetry in the νeν̄e sector by a matter-enhanced active-active neutrino transformation. In the first scheme, we find that the percentage change in Y is between -1% and 9% (with the sign depending on the sign of the asymmetry), bounded by the Majorana mass limit mνe≲1 eV. In the second scheme, the maximal percentage reduction in Y is 2%, if the lepton number asymmetry in neutrinos is positive; otherwise, the percentage increase in Y is ≲5% for mνμ,ντ2-mνs2≲104 eV. We conclude that the change in the primordial 4He yield induced by a neutrino-mixing-generated lepton number asymmetry can be substantial in the upward direction, but limited in the downward direction. ©1999 The American Physical Society.

We assess the effects on big bang nucleosynthesis (BBN) of lepton number generation in the early universe resulting from the two-doublet four-neutrino mass-mixing scheme. It has been argued that this neutrino mass-mixing arrangement gives the most viable fit to the existing data. We study full 4x4 mixing matrices and show how possible symmetries in these can affect the BBN 4He abundance yields. Though there is as yet no consensus on the reliability of BBN calculations with neutrino flavor mixing, we show that, in the case where the sign of the lepton number asymmetry is unpredictable, BBN considerations may pick out specific relationships between mixing angles. In particular, reconciling the observed light element abundances with a ν̄μ⇌ν̄e oscillation interpretation of LSND would allow unique new constraints on the neutrino mixing angles in this model. ©2000 The American Physical Society.

It has been suggested that the supermassive black holes, at the centers of galaxies and quasars, may initially form in single collapses of relativistic star clusters or supermassive stars built up during the evolution of dense star clusters. We show that it may be possible for ICECUBE (a planned 1 km3 neutrino detector in Antarctica) to detect the neutrino bursts associated with those collapses at redshift z ≲ 0.2 with a rate of ∼0.1-1 burst per year. Such detections could give new insights into the formation of structure in the Universe, especially when correlated with gravitational wave signatures or even gamma-ray bursts. © 1998 The American Physical Society.