  We outline a general strategy for measuring spins, couplings and mixing
angles in the case of a heavy partner decay chain terminating in an invisible
particle. We consider the common example of a new scalar or fermion D decaying
sequentially to other new particles C, B and A by emitting a quark jet j and
two leptons ln and lf. We derive analytic formulas for the dilepton {ln,lf} and
the two jet-lepton ({j,ln} and {j,lf}) invariant mass distributions for most
general couplings and mixing angles of the new partners. We then consider
various spin assignments for the particles A, B, C and D, and derive the
relevant functional basis for the invariant mass distributions which contains
the intrinsic spin information and does not depend on the couplings and mixing
angles. We propose a new method for determining the spins of the new partners,
using the three experimentally observable distributions {l+,l-}, {j,l+}+{j,l-}
and {j,l+}-{j,l-}. We show that the former two only depend on a single
model-dependent parameter alpha, while the latter may depend on two other
parameters beta and gamma. By fitting these distributions to our set of basis
functions, we are able to do a pure measurement of the spins per se. Our method
is also applicable at a pp-bar collider such as the Tevatron, for which the
previously proposed lepton charge asymmetry is identically zero and does not
contain any spin information. In the process of determining the spins, we also
obtain an independent measurement of the parameters alpha, beta and gamma,
which represent certain combinations of the couplings and the mixing angles of
the heavy partners A, B, C and D.
