Precision Determination of Invisible-Particle Masses at the CERN LHC: II
  We further develop the constrained mass variable techniques to determine the mass scale of invisible particles pair-produced at hadron colliders. We introduce the constrained mass variable M_3C which provides an event-by-event lower bound and upper bound to the mass scale given the two mass differences between the lightest three new particle states. This variable is most appropriate for short symmetric cascade decays involving two-body decays and on-shell intermediate states which end in standard-model particles and two dark-matter particles. An important feature of the constrained mass variables is that they do not rely simply on the position of the end point but use the additional information contained in events which lie far from the end point. To demonstrate our method we study the supersymmetric model SPS 1a. We select cuts to study events with two Neutralino_2 each of which decays to Neutralino_1, and two opposite-sign same-flavor (OSSF) charged leptons through an intermediate on-shell slepton. We find that with 300 fb^-1 of integrated luminosity the invisible-particle mass can be measured to M=96.4 +/- 2.4 GeV. Combining fits to the shape of the M_3C constrained mass variable distribution with the max m_ll edge fixes the mass differences to +/- 0.2 GeV. 
Alan J. Barr and Alex Pinder and Mario Serna