Search For Lepton Flavour Violating Decays Of The Higgs Boson PDF Download

Abstract: This thesis presents a direct search for lepton-flavour violating (LFV) decays of the Higgs boson using data from proton-proton collisions at a center-of-mass energy of 13 TeV delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector at CERN. The dataset corresponds to an integrated luminosity of 138.4 1/fb collected during Run 2 of the LHC which lasted from 2015 to 2018. The search in this thesis is focused on the decays H -> e tau and H -> mu tau. Only leptonic decays of the tau-lepton leading to different-flavour final states are considered in order to suppress background contributions from Drell-Yan production. Two signal regions (VBF SR and nonVBF SR) are defined, targeting different production processes of the Higgs boson thereby allowing for exploiting the characteristic topology of the vector-boson-fusion (VBF) production. Individual neural networks are trained in each of the two signal regions aiming at classifying each collision event to be signal or background. This is achieved by assigning a probability value to each event quantifying how signal-like it is. The resulting probability distribution is used as final discriminant in the statistical analysis. For background estimation, the so-called "Symmetry Method" is used which exploits the fact that Standard-Model processes are symmetric with respect to an exchange of electrons with muons and vice versa at the high energies prevalent at the LHC. The two LFV decays H -> e tau and H -> mu tau, on the contrary, break this symmetry as long as their branching ratios (BR) are of different size since systematic differences of the kinematic properties of the electron and muon are implied within each decay. The differing branching ratios are motivated by a bound on the product of the two LFV couplings derived from mu -> e gamma. The usage of the "Symmetry Method" allows for an estimation of the background mainly based on data and hence reduces the dependence on simulated events. However, it also implies that the analysis is only sensitive to the difference in branching ratios of H -> e tau and H -> mu tau and does not allow for measuring their absolute values. The difference in branching ratios, Delta = BR(H -> mu tau) - BR(H -> e tau), is measured to be (0.25+/-0.10)% which indicates that the data favours a larger branching ratio for the decay H -> mu tau than for the decay H -> e tau with a significance of 2.5 sigma. An upper limit at 95% confidence level of 0.42% is set on the difference Delta, the expected limit is (0.19+0.08/-0.05)%. Combinations with partner analyses which use templates from simulations for background estimation are performed, assuming the respective other LFV signal to be zero. Observed (expected) upper limits at 95% confidence level on the branching ratios of the decays H -> e tau and H -> mu tau are derived to be 0.23% ((0.12+0.05/-0.03)%) and 0.17% ((0.09+0.04/-0.03)%), respectively