Searching for lepton-flavour-violating decays of the Higgs boson with the ATLAS detector

The observation of neutrino oscillations indicates that lepton flavour violation (LFV) occurs in nature and that lepton flavour is not an exact symmetry. However, no observation has been made in the charged sector, which would be a clear indication of physics Beyond the Standard Model (BSM). There are BSM models which predict LFV decays of the Higgs boson into a pair of leptons with different flavours such as models with more than one Higgs doublet, composite Higgs models, models with flavour symmetries, Randall–Sundrum models and many more. In this poster we present the search for Higgs Bosons decaying into a tau lepton and either an electron or a muon, using data collected by the ATLAS detector during 2015 and 2016, which corresponds to 36.1 fb-1, and it was taken at a centre-of-mass energy of 13 TeV. The searches for H->etau and H->mutau decays were performed independently and in both cases, the search was split into cases where the tau lepton decayed leptonically or hadronically. Multivariate techniques were used in this search to discern the LFV signal from the SM background. The uncertainties in the results were dominated by the systematic error related to the fake-tau background calculation, which involved a data-driven technique. No significant excess of events was found over the SM expectation and upper limits at 95% CL were placed in the branching ratios Br(H->tau) and Br(H->mutau) of 0.47% and 0.28%, respectively. These limits are a big improvement with respect to previous ATLAS results, from Run 1.