Neutrino Mass and Dark Matter

Despite direct observations favoring a low mass density, a critical density universe with a neutrino component of dark matter provides the best existing model to explain the observed structure of the universe over more than three orders of magnitude in distance scale. In principle this hot dark matter could consist of one, two, or three species of active neutrinos. If all present indications for neutrino mass are correct, however, only the two-species (muon neutrino and tau neutrino) possibility works. This requires the existence of at least one light sterile neutrino to explain the solar electron neutrino deficit via nu(e)->nu(s), leaving nu(mu)->nu(tau) as the explanation for the anomalous nu(mu)/nu(e) ratio produced by atmospheric neutrinos, and having the LSND experiment demonstrating via anti-nu(mu)-> anti-nu(e) the mass difference between the light nu(e)-nu(s) pair and the heavier nu(mu)-nu(tau) pair required for dark matter. Other experiments do not conflict with the LSND results when all the experiments are analyzed in the same way, and when analyzed conservatively the LSND data is quite compatible with the mass difference needed for dark matter. Further support for this mass pattern is provided by the need for a sterile neutrino to rescue heavy-element nucleosynthesis in supernovae, and it could even aid the concordance in light element abundances from the early universe.Comment: 13 pages, 3 figures, IDM 98 conferenc

A Sterile Neutrino Needed for Heavy-Element Nucleosynthesis

A neutrino mass-mixing scheme which successfully avoids the "alpha effect," allowing r-process nucleosynthesis in the neutrino-heated ejecta of supernovae, quite independently requires the same parameters as the scheme which best fits all current indications for neutrino mass. The significance for particle physics is this independent evidence for (1) at least one light sterile neutrino, nu_s; (2) a near maximally-mixed nu_mu-nu_tau doublet split from a lower mass nu_mu-nu_s doublet; (3) nu_mu-nu_e mixing >~ 10^-4; and (4) a splitting between the doublets (measured by the nu_mu-nu_e mass difference) >~ 1 eV^2, favoring the upper part of the LSND range. If correct, it is tantalizing that neutrinos with tiny masses which mix with sterile species have profound effects on massive objects and the creation of the heaviest elements.Comment: 7 pages, 1 figure, PASCOS '99 conference tal

An Inverted Mass Hierarchy for Hot Dark Matter and the Solar Neutrino Problem.

The cosmological model in which 20% of the dark matter is shared by two nearly equal mass neutrinos fits the structure of the universe on all scales. This has been motivated a $\nu_\mu$-$\nu_{\tau}$ oscillation explanation of the deficit of atmospheric muon neutrinos. If the observed ratio of atmospheric $nu_\mu$ to $\nu_e$ has an alternative explanation, the cosmological model can be retained if the deficit of solar neutrinos is explained by $\nu_e$-$\nu_{\tau}$ oscillation. In this case an inverted mass hierarchy is required with $m_{\nu_{\mu}}\ll m_{\nu_e} \simeq m_{\nu_\tau}\approx 2.4$ eV. We show that if there exists an $L_e- L_{\tau}$ symmetry in nature, both the near mass degeneracy of \nue\ and \nut\ as well as the consistency of the above values for neutrino masses with the negative results for neutrinoless double beta decay search experiments are easily understood. We show that this symmetry implemented in the context of a high-scale left-right symmetric theory with the see-saw mechanism can lead to a simple theoretical understanding of the desired form of the mass matrix.Comment: Tex file; no figures; 10 page