In this talk I will review common issues in the reconstruction of Lorentz invariants of the two body decay, in particular the problems that arise when the boost connecting the rest frame of the decaying particle and the observer (laboratory) frame is not known. par I will discuss how certain Lorentz -variant- quantities such as the Energy of one of the decay product encodes Lorentz -invariant- quantities of the decay in a simple and measurable way. I describe a method to access these invariants that allow to determine the mass of the decaying particle by observing only one of the two decay products. Not requiring the observation of both decay products, the described method can be applied when traditional quantities as the invariant mass cannot be used. I will show concrete results about mass measurements for the leptonic decay of the top quark and supersymmetric decay chains of the gluino. Possible uses of this observation to study the stabilization symmetry of the Dark Matter at colliders will be discussed as well.

University of Maryland