Cosmological constraints on an invisibly decaying Higgs

Working in the context of a proposal for collisional dark matter, we derive bounds on the Higgs boson coupling g' to a stable light scalar particle, w hich we refer to as phion (phi), required to solve problems with small scal e structure formation which arise in collisionless, dark matter models. We discuss the behaviour of the phion in the early universe for different rang es of its mass. We find that a phion in the mass range of 100 MeV is exclud ed and that a phion in the mass range of I GeV requires a large coupling co nstant, g' greater than or similar to 2, and m(h) less than or similar to 1 30 GeV in order to avoid overabundance, in which case the invisible decay m ode of the Higgs boson would be dominant. (C) 2001 Elsevier Science B.V. Al l rights reserved.