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

Abstract Processes with $$\tau $$ 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 $$\tau $$ τ -leptons produced in proton–proton collisions only through their decay products. Data analyses involving hadronically decaying $$\tau $$ τ -leptons face challenges due to backgrounds from jets misidentified as $$\tau $$ τ -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 $$\tau $$ τ -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 $$W(\mu \nu )$$ W ( μ ν ) enriched event sample, ranges from 15 to 35% depending on the $$\tau $$ τ -lepton’s transverse momentum and charged-particle decay multiplicity.

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