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Invisible Upsilon decays into Light Dark Matter

Invisible psi and Upsilon decays into light neutralinos, within the MSSM or N(n)MSSM, are smaller than for nu nubar 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 psi and Upsilon 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 |c_chi.f_bV| < 5 10^-3 from Upsilon decays, for 2 m_chi smaller than a few GeV. We also explain why there is no model-independent way to predict psi and Upsilon branching fractions into light dark matter, from dark matter annihilation cross sections at freeze-out time.