Making The Most Of MET: Mass Reconstruction From Collimated Decays

At hadron colliders invisible particles $χ$ can be inferred only through observation of the transverse component of the vectorial sum of their momenta -- missing $E_T$ or MET -- preventing reconstruction of the masses of their mother particles. Here we outline situations where prior prejudice about the event kinematics allows one to make the most of MET by decomposing it into its expected sum of transverse contributions, each of which may be promoted to a full four-momentum approximating the associated $χ$. Such prejudice arises when all $χ$ in the event are expected to be light and (anti-)parallel to a visible object, due to spin-correlations, back-to-back decays or boosted decays. We focus on the last of these, with boosted semi-invisibly decaying neutralinos widely motivated in supersymmetry (in the presence of light gravitinos, singlinos, photini or pseudo-goldstini), and demonstrate our simple method's ability to reconstruct sharp mass peaks from the MET decomposition.