Search for a supersymmetric partner of the top quark using boosted top quark identification with the ATLAS detector at the LHC

A search for the top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum is presented, with a special focus on the high stop masses regime. The analysis is performed with proton-proton collision data at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC in 2012, corresponding to an integrated luminosity of $20$ fb$^{-1}$. The stop decay mode studied is mainly that to a top quark and the lightest neutralino, denoted as $\tilde{t}^{}_{1}\to t \tilde\chi^0_1$. The $\tilde\chi^0_1$ is assumed to be the lightest supersymmetric particle (LSP) and evades detection. A technique that reconstructs the decays of the boosted top quark is used to increase the experimental sensitivity. Other stop decay scenarios are also considered, including a mixed decay where the stop can in addition decay into a bottom quark and the lightest chargino. No significant excess over the Standard Model prediction is observed. Model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model. For models where the stop decays into a top quark and a massless $\tilde\chi^0_1$, stop masses between 210 and 640 GeV are excluded at 95% confidence level. Stringent exclusion limits are also derived for other stop decay modes considered.