Search for new physics using lepton flavor violating signatures in modern particle colliders.

Abstract

Charged lepton flavor violation is a clear signal of new physics. Such decays are not allowed in the Standard Model but highly anticipated in a large class of new physics models. A direct search for lepton flavor violation in decays of the Higgs boson with the ATLAS detector at the LHC is presented here. The analysis is performed in the H → l tau channel, where the leading lepton (l) can be either an electron or a muon, and the tau lepton decays into an opposite flavored lighter lepton or via the hadronic decay channel. Published results of this search are presented in this thesis based on a data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 36 inverse femtobarns (fb−1) at center-of-mass energy (√s) of 13 TeV during the 2015-2016 data-taking period. The analysis is found to be three times more sensitive than the previous analysis performed with 20 fb−1 of data collected at √s = 8 TeV during the 2012 data-taking period, and comparable to the one obtained by the CMS experiment with a similar-sized dataset at √s = 13 TeV during the 2015-2016 data-taking period. The direct search for lepton flavor violating decays of the Higgs boson obtained with the present analysis at the Large Hadron Collider is about twenty-five times more constraining than the indirect prediction. To complement my research goal of searching for new physics with lepton flavor violating signatures in final states containing the tau lepton, the generator level modeling of decays of the tau lepton decaying into Standard Model processes at the Belle II experiment at the world’s highest luminosity collider is also presented.