Search for pair-produced top squarks decaying into a charm quark and the lightest neutralinos with 20.3 fb${}^{-1}$ of $pp$ collisions at $\sqrt{s}=8~$TeV with the ATLAS detector at the LHC

This note presents the search for direct top squark pair production in the decay channel to a charm quark and the lightest neutralino ($\tilde{t} \to c + \tilde{\chi}_{1}^{0}$), using $20.3$~fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 8$~TeV recorded by the ATLAS experiment at the LHC in 2012. The analysis is carried out in different signal regions according to the final state jet multiplicity. One of the regions uses charm-flavour identification to increase the signal purity. No excess above the Standard Model background expectation is observed. Limits are set on the visible cross-section of new physics within the requirements of the search. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the ($m_{\tilde{t}}$, $m_{\tilde{\chi}_{1}^{0}}$) plane. A top squark mass of 200~GeV is excluded at 95$\%$ confidence level for $m_{\tilde{t}}$--$m_{\tilde{\chi}_{1}^{0}} < 85$~GeV. Top squark masses up to 230~GeV are excluded for a neutralino mass of 200~GeV. This extends significantly previous Tevatron results.