2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/199143I consider the two-body decay of a particle at a hadron collider into a visible and an invisible particle, generalizing $W \to e ν$, where the masses of the decaying particle and the invisible decay particle are, {\em a priori}, unknown. I prove that the transverse mass, when maximized over possible kinematic configurations, can be used to determine both of the unknown masses. I argue that the proof can be generalized to cover cases such as decays of pair-produced superpartners to the lightest, stable superpartner at the Large Hadron Collider.4 pages; version published in JHEPHigh Energy Physics - PhenomenologyHigh Energy Physics - ExperimentData Analysis, Statistics and Probabilitytext