M.Sc Thesis
M.Sc Student Benjamin Gutelman Search for New Physics through Measurement of Lepton Universality Department of Physics Full Professor Rozen Yoram

Abstract

In the SM the coupling constant between charged leptons and weak gauge bosons is universal. The strength of the coupling is equal for the 3 leptons generations, moreover according to the SM the top quark decay almost exclusively to a b quark and a W boson. As a consequence, the branching ratios for the productions of taus and muons resulting from the leptonic decay of a top quark through a W are nearly equal. In Beyond the Standard Model scenarios, top quarks may decay through other mechanisms at a rate which may be significant enough to be measured at the LHC. For example Super Symmetric extensions of the SM with multi Higgs-doublet, models with exotic vector-like quarks or model arising from the extension of the electroweak symmetry of the SM with W' and Z' bosons all predict the decay of top quarks through additional mechanisms.

The branching ratio for the leptonic decays of those BSM contributions may then not be equal for the 3 lepton generations, effectively violating lepton universality in top decays. The simplest example would be a model with a charged higgs for which the leptonic decay would favor the heaviest tau lepton as the coupling would be proportional to the mass of the charged lepton. In this analysis we develop a tool for testing lepton universality in top decays considering only muons final states by using a multivariate analysis method to separate between prompt muons and tauonic muons. An estimator for measuring the ratio of these muons in data recorded in the ATLAS detector is developed and the statistical and systematics uncertainties on the estimator are derived. A case study for estimating the sensitivity of the ratio in the presence of a lepton universality violating charged higgs signal is conducted where deviations from a SM only hypothesis are measured for different charged higgs masses and for different branching ratios for a top quark to decay into a charged higgs and a b quark.