  The impact a new neutral light particle of spin 0, 1/2, 1, or 3/2 could have
on the tiny width of a light Higgs boson is systematically analyzed. To this
end, we include all the relevant effective interactions, whether renormalizable
or not, and review the possible signatures in the Higgs decay modes with
missing energy. This includes the fully invisible Higgs boson decay, as well as
modes with SM gauge boson or fermions in the final state. In many cases, simply
preventing these modes from being dominant suffices to set tight
model-independent constraints on the masses and couplings of the new light
states.
