Invisible Υ decays into light dark matter

Invisible ψ and Υ decays into light neutralinos, within the minimal or next-to-minimal supersymmetric standard model, are smaller than for νν production, even if light spin-0 particles are coupled to quarks and neutralinos. In a more general way, light dark matter particles are normally forbidden, unless they can annihilate sufficiently through a new interaction stronger than weak interactions (at lower energies), as induced by a light spin-1 U boson, or heavy-fermion exchanges in the case of scalar dark matter. We discuss the possible contributions of U-boson, heavy-fermion, or spin-0 exchanges to invisible ψ and Υ decays. U exchanges could lead, but not necessarily, to significant branching fractions for invisible decays into light dark matter. We show how one can get the correct relic density together with sufficiently small invisible branching fractions, and the resulting constraints on the U couplings to ordinary particles and dark matter, in particular |cχfbV|<5x10-3 from Υ decays, for 2mχ smaller than a few GeV. We also explain why there is no model-independent way to predict ψ and Υ branching fractions into light dark matter, from dark matter annihilation cross sections at freeze-out time.