Search for lepton-flavour-violating decays of the Higgs boson and constraints on off-diagonal Yukawa couplings with the ATLAS detector

This thesis describes a search for evidence of lepton-flavour-violation (LFV) via non-diagonal Yukawa couplings, where the Higgs boson decays into a pair of leptons with different flavours. The search is performed with the ATLAS detector at the LHC using 138 fb−1 of p-p data at √s=13 TeV . The final result is presented as an upper limit on the decay branching ratios of B(H→eτ) and B(H→μτ). In addition, this thesis documents work to improve the reconstruction of τ -leptons for the ATLAS experiment via the adaptation of the Prompt-Lepton-Tagger (PLV) to include electrons and muons resulting from τ-lepton decays.