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

A search for flavour-changing neutral current decays 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$^{-1}$. No significant excess of events above the background expectation is found, and observed (expected) 95\% CL upper limits of 0.56\% (0.42\%) and 0.61\% (0.64\%) are derived for the $t\to Hc$ and $t\to Hu$ branching ratios respectively. The combination of this search with other ATLAS searches in the $H \to \gamma\gamma$ and $H \to W W^{*}$, $\tau \tau$ decay modes significantly improves the sensitivity, yielding observed (expected) 95\% CL upper limits on the $t\to Hc$ and $t\to Hu$ branching ratios of 0.46\% (0.25\%) and 0.45\% (0.29\%) respectively. 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 on $tqH$ interactions measured so far.