The collider variable mT2: derivation, properties, and an application to SUSY GUTs
I derive the kinematical variable mT2 as a generalization of the mT variable, used at the CERN UA1 experiment to measure the W mass from the semi-invisible mode W $\to$ e nu. I then discuss a number of properties of mT2, that make it a very attractive tool for LHC events. For example its kink'', allowing to determine two new-particles masses simultaneously. Finally, I illustrate the above ideas in a concrete example, taken from SUSY GUTs. \\ \\ \\ \centerline{Note: UNUSUAL DATE and PLACE}