Neutrinos and extra dimensions

A qualitative explanation of all present evidence for neutrino mass (the solar and atmospheric neutrino deficits, LSND, and significant dark matter) is provided by a neutrino mass-mixing scheme which also successfully avoids the "alpha effect," allowing r-process nucleosynthesis in the neutrino-heated ejecta of supernovae. The neutrino properties required to ensure production of heavy nuclei provides independent evidence for (1) at least one light sterile neutrino, nu (s); (2) a near maximally-mixed nu (e)-nu (s) doublet (which also explains the atmospheric anomaly and provides hot dark matter) split from a lower mass nu (e)-nu (s) doublet (needed also for the solar nu (e) deficit); (3) nu (mu)-nu (e) mixing greater than or similar to 10(-4); and (4) a splitting between the doublets (measured by the nu (mu)-nu (e) mass difference) greater than or similar to 1 eV(2), favoring the upper part of the LSND range. There is a quantitative problem with the solar observations, which do not in detail fit this or any other model. If, however, the Vs is a bulk neutrino in extra dimensions the solar data can be fit remarkably well with a combination of vacuum and MSW oscillations. Models with large extra dimensions lead to naturally light sterile neutrinos in the process of giving small mass to the known neutrinos. In this particular model radiative effects split a Dirac neutrino, giving a small mass difference between its Majorana constituents which provides vacuum oscillations of the nu (e) to the zero mode of the nu (s), while the nu (e) transitions to the Kaluza-Klein states of the nu (s) give MSW oscillations.