Higgs Boson Production Cross-Section Measurements in the Di-Tau Final State at ATLAS

Abstract

The origin of masses for massive elementary particles in the Standard Model (SM) is explained by the Higgs mechanism. With the discovery of a SM-like Higgs boson resonance in 2012 a variety of measurements have been conducted to determine its exact nature. Properties like the spin and other quantum numbers are measured to ascertain whether the resonance resembles the Higgs boson predicted by the SM or another Higgs boson predicted by theories beyond the SM. <br> One of the Higgs boson's main characteristics is its coupling to elementary particles that grows proportional to the particle's mass. Deviations within this coupling structure could indicate hints of effects originating from physics beyond the SM. This thesis presents a measurement of the Higgs boson production cross-sections in the decay to two τ-leptons, <em>H</em>→ττ. The measurement uses the proton-proton collision dataset of 139 fb^-1 collected by the ATLAS experiment during Run II in the years 2015 to 2018 of the Large Hadron Collider (LHC). As the τ-lepton itself is not a stable particle and decays before interacting with the detector material, which always involves particles that cannot be measured at ATLAS, it is challenging to measure this decay channel. However, studying the <em>H</em>→ττ decay is worthwhile the efforts because it allows to deduce information on the Higgs boson's coupling to fermions in the Yukawa sector. <br> The measurement's final discriminating observable reconstructs the invariant mass of the two τ-lepton system and incorporates the knowledge about the produced τ-lepton-neutrinos that escape detection. Detailed studies regarding this algorithm have been performed that ultimately lead to a speedup by a factor of roughly two to four, depending on the τ-lepton decay. Multiple Higgs production cross-sections are measured, ranging from the inclusive production to the more detailed Simplified Template Cross Sections (STXS). Therefore, a complex fit model is needed to extract these information from the data. Validation methods for such complex models are discussed. In this context, a novel technique that is able to predict the behaviour of nuisance parameters and to provide an assessment of the overall agreement of the model with respect to the data is introduced. The measured cross-sections are in agreement with the SM predictions and provide the currently most precisely measured values in the <em>H</em>→ττ channel in an ATLAS standalone measurement. As an example, the inclusive Higgs production cross-section is measured to be 2.90 ± 0.21 (stat)^+0.37_-0.32 (syst) pb which agrees with the SM prediction of 3.15 ± 0.09 pb within the uncertainties. The vector-boson fusion production is observed with a significance of 5.3σ and there is evidence for gluon-gluon fusion production with a significance of 3.9σ.