  A model-independent analysis of decays of the form C -> l+ l- A (l=e,mu) is
presented, including the possibility that this three-body decay is preceded by
an additional decay step D -> j C. Here A, C and D are heavy new-physics
particles and j stands for a quark jet. It is assumed that A escapes direct
detection in a collider experiment, so that one cannot kinematically
reconstruct the momenta of the new particles. Instead, information about their
properties can be obtained from invariant-mass distributions of the visible
decay products, i.e. the di-lepton (ll) and jet-lepton (jl) invariant-mass
distributions. All possible spin configurations and renormalizable couplings of
the new particles are considered, and explicit expressions for the
invariant-mass distributions are derived, in a formulation that separates the
coupling parameters from the spin and kinematic information. In a numerical
analysis, it is shown how these properties can be determined independently from
a fit to the m(ll) and m(jl) distributions.
