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

TL;DR

This paper presents a new algorithm for reconstructing and identifying highly collimated hadronic tau lepton pairs in 13 TeV proton-proton collision data, improving detection efficiency in overlapping decay scenarios.

Contribution

The paper introduces a novel reconstruction and identification method for collimated tau pairs using large radius jets and boosted decision trees, tailored for low transverse momentum events.

Findings

Identification efficiency close to simulation with 26-37% uncertainty.

Effective discrimination between tau pairs and QCD jets.

Validated in $Z\gamma$ events with 139 fb$^{-1}$ of data.

Abstract

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. This requires a dedicated treatment that attempts to tag it as a single object. 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 $\mbox{fb}^{-1}$. The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26% to 37%.