Search for flavor-changing neutral current top quark decays $\boldsymbol{t\to Hq}$, with $\boldsymbol{H \to b\bar{b}}$, in $\boldsymbol{pp}$ collisions at $\boldsymbol{\sqrt{s}=8}$ TeV with the ATLAS detector

In this dissertation a search for flavour-changing neutral currents in the decay of a top quark to an up-type quark ($q=u, c$) and the Standard Model Higgs boson, where the Higgs boson decays to $b\bar{b}$, is presented. The analysis searches for top quark pair events in which one top quark decays to $Wb$, with the $W$ boson decaying leptonically, and the other top quark decays to $Hq$. The search is based on $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3~fb$^{-1}$. Data are analyzed in the lepton+jets final state, characterized by an isolated electron or muon with moderately high transverse momentum and at least four jets. The search exploits the high multiplicity of $b$ jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by $t\bar{t} \rightarrow WbWb$ decays. No significant excess of events above the background expectation is found. The observed (expected) upper limits on the $t\to Hc$ and $t\to Hu$ branching ratios are 0.56\% (0.42\%) and 0.61\% (0.64\%) respectively, at the 95\% confidence level. The combination of this search with other ATLAS searches in the $H \to \gamma\gamma$ and $H\to WW^*, \tau\tau$ decay modes significantly improves the sensitivity yielding observed (expected) upper limits on the $t\to Hc$ and $t\to Hu$ branching ratios of 0.46\% (0.25\%) and 0.45\% (0.29\%) respectively, at the 95\% confidence level. The corresponding combined observed (expected) upper limits on the $|\lambda_{tcH}|$ and $|\lambda_{tuH}|$ couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds to date on $tqH$ interactions measured so far.