Measurement of the top quark pair production cross section with a tau lepton in the final state at the center of mass energy of 8 TeV

Abstract

This dissertation presents the measurement of the inclusive cross section of the top quark pair production in the final state of a tau lepton with associated jets. The dataset used in this measurement is collected by the ATLAS detector from proton-proton collisions during the 2012 operation of the Large Hadron Collider at the center-of-mass energy, $\sqrt{s} =$ 8~\tev. This dataset corresponds to an integrated luminosity of 20.2~\ifb. The inclusive \ttbar production cross section, $\sigma_{\ttbar}$ is found to be $\sigma_{\ttbar} = 231 \pm 3 {\rm (stat.)}^{+25}_{-25} {\rm (syst.)}\pm 3{\rm (lumi.)}~{\rm pb}$ for a top quark mass of 172.5~\gev. This result is consistent with the theoretical prediction $\sigma^{\mathrm{SM}}_{\ttbar} = 253^{+13}_{-15}~{\rm pb}$. Statistical analysis is performed to set a model independent upper limit on the visible cross section of any non Standard Model processes following frequentist probability. An upper limit on the branching ratio of the flavor changing neutral Higgs decay of the top quark to a charm quark is also calculated. The observed (expected) limit on the branching ratio at 95\% CL is $\text{BR}(t \rightarrow ch^0) <$ 10\% (15\%).