We clarify the relation between the variable MT2 and the method of kinematic constraints, both of which can be used for mass determination in events with two missing (dark matter) particles at hadron colliders. We identify a set of minimal kinematic constraints, including the mass shell conditions for the missing particles and their mother particles, as well as the constraint from the measured missing transverse momentum. We show that MT2 is the boundary of the mass region consistent with the minimal constraints. From this point of view, we also obtained a more efficient algorithm for calculating MT2. When more constraints are available in the events, we can develop more sophisticated mass determination methods starting from the MT2 constraint. In particular, we discuss cases when each decay chain contains two visible particles.
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