Experimental searches for hot and cold dark matter

A way to fix the overproduction by cold dark matter of small-scale structure in the universe is to add hot dark matter in the form of neutrinos. Unlike cold dark matter, such as supersymmetric neutralinos, which very likely could be detected directly, the nature of the neutrinos must be determined from terrestrial experiments. The LSND experiment may be providing such evidence, since in a anti νμ beam 22 events consistent with anti νep → e+n are seen, whereas only 4.6 ± 0.6 are expected from backgrounds; the probability of this being a fluctuation is < 10-7. If subsequent analysis shows a similar effect from the independent channel νμ → νe, then this would imply a neutrino mass difference which would contribute significantly to the dark matter of the universe. Explaining also the solar and atmospheric neutrino deficits results in a neutrino mass pattern which gives a cold + hot dark matter mix which fits the structure of the universe on all scales and requires a critical density universe and a universe age compatible with that of the oldest stars. Further evidence for this mass pattern comes from the production of heavy elements in supernovae. The net for cold dark matter must be cast more widely, but the new experiments just starting have a good chance of detecting neutralinos, if these are indeed the major constituent of the universe. (orig.)