Reconstruction and identification of pairs of collimated -leptons decaying hadronically using TeV $\textit{pp}$ collision data with the ATLAS detector

This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying -lepton pair with low transverse momentum. When two -leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the -leptons. A dedicated treatment attempting to tag the -lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in events using proton–proton collision data at TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of . The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.