Estimation of backgrounds from jets misidentified as $\tau$-leptons using the Universal Fake Factor method with the ATLAS detector

Processes with $\tau$-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 $\tau$-leptons face challenges due to backgrounds from jets misidentified as $\tau$-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as $\tau$-leptons (light-quark, gluon, $b$-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, evaluated using $W(\mu \nu )$ enriched event sample, ranges from 15% to 35% depending on the $\tau$-lepton’s transverse momentum and charged-particle decay multiplicity.