A measurement of the differential cross-section for inclusive ττ production at high-mass in proton-proton collisions at the ATLAS detector

This thesis primarily focuses on describing the design and results of the high mass ττ measurement using the 139 fb−1 Run 2 data collected at 13 TeV during proton-proton collisions at the Large Hadron Collider (LHC) with the ATLAS detector. This analysis explores a kinematic region that was not previously probed for the τ lepton, with an emphasis on flexibility and reuseability through a model-independent approach. By utilising the visible final states with mℓℓ > 100 GeV, the study aims to explore the high mass regions that closely aligned with the detector’s acceptance, known as the fiducial region. The motivation for this high-mass ττ measurement stems from the anomalies observed in the rare decays of b mesons at the LHCb [1], which suggest deviations that could be more pronounced in third-generation leptons [2]. These anomalies could potentially be explained by the leptoquark model [3] and vector-like-leptons (VLL) model [4]. As an example of using such measurement to set limits and enhance re-interpretation power in searches for new physics, a CONTUR [5]-driven reinterpretation study is preformed using existing measurements to set constraints on the simplest VLL doublet model [4]. The result suggests an improvement from around 280 GeV to around 410 GeV on the third-generation doublet VLL mass CONTUR-limit by adding this new ττ measurement