In pursuit of lepton flavour violation A search for the τ-> μγγ decay with atlas at √s=8 TeV

The Standard Model describes the subatomic particles and their interactions. The latest success of this theory was reached in 2012 with the discovery of the Higgs boson. Nevertheless, the Standard Model is not able to explain all the mysteries of our universe, such as dark matter, dark energy and the asymmetry between matter and antimatter. In addition, the observation of neutrino oscillations showed that neutrinos are massive particles and can change from one flavour (electron, muon or tau neutrino) to another. Can flavour violation be observed also among charged leptons? This would be a clear sign of physics beyond the Standard Model.
The Large Hadron Collider (LHC) at CERN is the most powerful accelerator in the world and is suitable to perform searches on charged lepton flavour violation. This thesis presents a search for the LFV τ-> μγγ channel at the center-of-mass energy of 8 TeV with the ATLAS detector, which is one of the detectors designed to study the high energy proton-proton collisions at the LHC. This channel has been analyzed here for the first time. No excess was observed with respect to the Standard Model and exclusion limits on the τ->μγγ branching fraction have been set on the order of 10^-4. The quest for charged lepton flavour violation will continue in the coming years.