  We propose a scheme to assign a 4-momentum to each WIMP in new physics event
producing a pair of mother particles each of which decays to an invisible
weakly interacting massive particle (WIMP) plus some visible particle(s). The
transverse components are given by the value that determines the event variable
M_T2, while the longitudinal component is determined by the on-shell condition
on the mother particle. Although it does not give the true WIMP momentum in
general, this M_T2-assisted on-shell reconstruction of missing momenta provides
kinematic variables well correlated to the true WIMP momentum, and thus can be
useful for an experimental determination of new particle properties. We apply
this scheme to some processes to measure the mother particle spin, and find
that spin determination is possible even without a good knowledge of the new
particle masses.
