Estimation of backgrounds from jets misidentified as <i>τ</i>-leptons using the Universal Fake Factor method with the ATLAS detector

Processes with τ-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes $\tau$-leptons produced in proton-proton collisions only through their decay products. Data analyses involving hadronically decaying τ-leptons face challenges due to backgrounds from jets misidentified as $\tau$-leptons. These fake τ-leptons are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such 'fake' backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from control regions. This paper describes a new technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for any signal region by using fake factors from samples enriched in different sources of fake τ-leptons (light-quark, gluon, <i>b</i>-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors ranges from 15% to 35% depending on the $\tau$-lepton's transverse momentum and charged-particle decay multiplicity.