Resolving Combinatorial Ambiguities in Dilepton $t\bar t$ Event Topologies with Constrained $M_2$ Variables

• Main Authors: Debnath, Dipsikha, Kim, Doojin, Kim, Jeong Han, Kong, Kyoungchul, Matchev, Konstantin T
• Format: Journal Article
• Language: English
• Published: 15.06.2017
• Subjects: Physics - High Energy Physics - Phenomenology
• DOI: 10.48550/arxiv.1706.04995

Phys. Rev. D 96, 076005 (2017) We advocate the use of on-shell constrained $M_2$ variables in order to mitigate the combinatorial problem in SUSY-like events with two invisible particles at the LHC. We show that in comparison to other approaches in the literature, the constrained $M_2$ variables provide superior ansatze for the unmeasured invisible momenta and therefore can be usefully applied to discriminate combinatorial ambiguities. We illustrate our procedure with the example of dilepton $t\bar{t}$ events. We critically review the existing methods based on the Cambridge $M_{T2}$ variable and MAOS-reconstruction of invisible momenta, and show that their algorithm can be simplified without loss of sensitivity, due to a perfect correlation between events with complex solutions for the invisible momenta and events exhibiting a kinematic endpoint violation. Then we demonstrate that the efficiency for selecting the correct partition is further improved by utilizing the $M_2$ variables instead. Finally, we also consider the general case when the underlying mass spectrum is unknown, and no kinematic endpoint information is available.