  We propose an improved method for hadron-collider mass determination of new
states that decay to a massive, long-lived state like the LSP in the MSSM. We
focus on pair produced new states which undergo three-body decay to a pair of
visible particles and the new invisible long-lived state. Our approach is to
construct a kinematic quantity which enforces all known physical constraints on
the system. The distribution of this quantity calculated for the observed
events has an endpoint that determines the mass of the new states. However we
find it much more efficient to determine the masses by fitting to the entire
distribution and not just the end point. We consider the application of the
method at the LHC for various models and demonstrate that the method can
determine the masses within about 6 GeV using only 250 events. This implies the
method is viable even for relatively rare processes at the LHC such as
neutralino pair production.
