Heavy triplet neutrinos as a new dark-matter option

Abstract

We propose a new scheme where dark matter is made of heavy stable neutral particles N, possibly even lighter than 1 2 m Z , but uncoupled to the Z 0. Such particles should be given an efficient way to annihilate, otherwise their fossil density would overclose the universe. In this scheme, the annihilation results from a delicate interplay between N and its heavier charged electroweak partner E ±. A closure density may be naturally reached if the mass splitting is ∼10 GeV, which suggests that the E ± may be discovered at LEP 200. The species N results from the mixing between a singlet and the neutral member of a triplet, here induced by the vacuum expectation value of a Higgs triplet. The latter is kept naturally small with respect to the electroweak scale, as a result of a new approximate discrete symmetry, the triplet-parity T p. We discuss various implications of this model for astrophysics and particle physics.