Reducing combinatorial uncertainties: a new technique based on MT2 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(T2) and on the M(T2)-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.