Reducing combinatorial uncertainties: a new technique based on M T2 variables

Abstract

We propose a new method to resolve combinatorial ambiguities in hadron collider events involving two invisible particles in the final state. This method is based on the kinematic variable M T 2 and on the M T 2-assisted-on-shell reconstruction of invisible momenta, that are reformulated as ‘test’ variables T i of the correct combination against the incorrect ones. We show how the efficiency of the single T i in providing the correct answer can be systematically improved by combining the different T i and/or by introducing cuts on suitable, combination-insensitive kinematic variables. We illustrate our whole approach in the specific example of top anti-top production, followed by a leptonic decay of the W on both sides. However, by construction, our method is also directly applicable to many topologies of interest for new physics, in particular events producing a pair of undetected particles, that are potential dark-matter candidates. We finally emphasize that our method is apt to several generalizations, that we outline in the last sections of the paper.