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

Abstract

Processes with τ -leptons in the final state are important for Standard Model measurements and searchesfor physics beyond the Standard Model. The ATLAS experimentat the Large Hadron Collider observes τ -leptons producedin proton-proton collisions only through their decayproducts. Data analyses involving hadronically decayingτ -leptons face challenges due to backgrounds from jetsmisidentified as τ -leptons that are not modelled reliably byMonte Carlo simulations. Data-driven methods such as thefake-factor method allow such misidentified backgrounds tobe predicted by measuring transfer factors, known as fakefactors, in data from dedicated regions. This paper describesa refined technique for determining the fake factors, the UniversalFake Factor method. It evaluates the fake factors fora signal region by using fake factors from samples enrichedin different sources of jets misidentified as τ -leptons (lightquark,gluon, b-quark, and pile-up jets). Each fake factor iscalculated as a linear combination of fake factors measuredin these different enriched samples. For the full Run 2 dataset, the systematic uncertainty of the calculated fake factors,evaluated using W(μν) enriched event sample, ranges from15 to 35% depending on the τ -lepton's transversemomentum and charged-particle decay multiplicity.