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.
