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We propose to use the MT2 concept to measure the masses of all particles inSUSY-like events with two unobservable, identical particles. To this end wegeneralize the usual notion of MT2 and define a new MT2(n,p,c) variable, whichcan be applied to various subsystem topologies, as well as the full eventtopology. We derive analytic formulas for its endpoint MT2{max}(n,p,c) as afunction of the unknown test mass Mc of the final particle in the subchain andthe transverse momentum pT due to radiation from the initial state. We showthat the endpoint functions MT2{max}(n,p,c)(Mc,pT) may exhibit three differenttypes of kinks and discuss the origin of each type. We prove that the subsystemMT2(n,p,c) variables by themselves already yield a sufficient number ofmeasurements for a complete determination of the mass spectrum (including theoverall mass scale). As an illustration, we consider the simple case of a decaychain with up to three heavy particles, X2 -> X1 -> X0, which is ratherproblematic for all other mass measurement methods. We propose three differentMT2-based methods, each of which allows a complete determination of the massesof particles X0, X1 and X2. The first method only uses MT2(n,p,c) endpointmeasurements at a single fixed value of the test mass Mc. In the second methodthe unknown mass spectrum is fitted to one or more endpoint functionsMT2{max}(n,p,c)(Mc,pT) exhibiting a kink. The third method is hybrid, combiningMT2 endpoints with measurements of kinematic edges in invariant massdistributions. As a practical application of our methods, we show that thedilepton W+W- and tt-bar samples at the Tevatron can be used for an independentdetermination of the masses of the top quark, the W boson and the neutrino,without any prior assumptions.

Using Subsystem MT2 for Complete Mass Determinations in Decay Chains with Missing Energy at Hadron Colliders