Reconstruction and identification of pairs of collimated \(\tau \)-leptons decaying hadronically using \(\sqrt{s}=13\) TeV pp collision data with the ATLAS detector

This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying \(\tau \)-lepton pair with low transverse momentum. When two \(\tau \)-leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the \(\tau \)-leptons. A dedicated treatment attempting to tag the \(\tau \)-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 \(\tau ^+\tau ^-\) systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in \(Z\gamma \) events using proton–proton collision data at \(\sqrt{s}=13\) TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of \(139\,\text{ fb}^{-1}\). The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.