Dark matter particle spectroscopy at the LHC: generalizing M T2 to asymmetric event topologies

We consider SUSY-like missing energy events at hadron colliders and critically examine the common assumption that the missing energy is the result of two identical missing particles. In order to experimentally test this hypothesis, we generalize the subsystem M T2 variable to the case of asymmetric event topologies, where the two SUSY decay chains terminate in different “children” particles. In this more general approach, the endpoint M T2(max) of the M T2 distribution now gives the mass Mₚ( Mc⁽ᵃ⁾, Mc⁽ᵇ⁾ ) of the parent particles as a function of two input children masses Mc⁽ᵃ⁾ and Mc⁽ᵇ⁾ . We propose two methods for an independent determination of the individual children masses M (a) c and M (b) c . First, in the presence of upstream transverse momentum PUTM the corresponding function Mₚ( Mc⁽ᵃ⁾, Mc⁽ᵇ⁾,PUTM ) is independent of PUTM at precisely the right values of the children masses. Second, the previously discussed M T2 “kink” is now generalized to a “ridge” on the 2-dimensional surface Mₚ( Mc⁽ᵃ⁾, Mc⁽ᵇ⁾ ) . As we show in several examples, quite often there is a special point along that ridge which marks the true values of the children masses. Our results allow collider experiments to probe a multi-component dark matter sector directly and without any theoretical prejudice.