Milena Bajic

A thesis for the degree of Doctor of Philosophy defended in June 2018.

The PhD School of Science, Faculty of Science, Discovery Center, Niels Bohr Institute, University of Copenhagen

Supervisor:
Stefania Xella

A search for lepton-flavor-violating decays of the Z boson into a τ-lepton and a light lepton with the ATLAS detector in proton-proton collisions at √s=13 TeV at the LHC

In the original Standard Model with the massless neutrinos, the lepton flavour number is strictly conserved per each generation. Due to neutrino oscillations, lepton flavour number violation is possible, corresponding to tiny branching ratios at the order of 10-54. Additional sources of lepton flavour number violation are predicted in several Beyond the Standard Model theories such are certain Supersymmetric Models, multi Higgs doublet models and others. An observation of lepton-flavour violating process would be a sign of new Beyond the Standard Model physics.

The analysis presented in this thesis describes a search for lepton flavour violation in the Z →τe and Z τμ processes with hadronic τ-lepton decays. The search is performed using the 2015+2016 proton-proton collisions corresponding to 36:1 fb-1, recorded by the ATLAS detector at the centre-of-mass energy √s = 13TeV. The backgrounds are estimated using a data-driven fake-factor method for the processes where the -lepton is faked by a jet combined with the Monte Carlo simulation for the backgrounds contributing with a real τ-lepton or a τ-lepton faked by another lepton. The final fit is performed using a neural network score as a discriminating variable, in order to obtain a good separation between the signal and the backgrounds. The neural network score is fitted in the signal-enriched region and the upper limits on the branching ratios of 5:32 10-5 for the Z →τe and 2:69 10-5 for the Z →τ μ channel are obtained.