Search for direct top squark pair production in final states with two leptons in \(\sqrt{s} = 13\) TeV pp collisions with the ATLAS detector

The results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using $$36.1~\hbox {fb}^{-1}$$ of integrated luminosity from proton–proton collisions at $$\sqrt{s}=13$$ TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark $$\tilde{t}$$ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay $$\tilde{t} \rightarrow b \tilde{\chi }_{1}^{\pm }$$ into a b-quark and the lightest chargino with $$\tilde{\chi }_{1}^{\pm } \rightarrow W \tilde{\chi }_{1}^{0}$$ , the decay $$\tilde{t} \rightarrow t \tilde{\chi }_{1}^{0}$$ into an on-shell top quark and the lightest neutralino, the three-body decay $$\tilde{t} \rightarrow b W \tilde{\chi }_{1}^{0}$$ and the four-body decay $$\tilde{t} \rightarrow b \ell \nu \tilde{\chi }_{1}^{0}$$ . No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the $$\tilde{t}$$ and $$\tilde{\chi }_{1}^{0}$$ masses. The results exclude at 95% confidence level $$\tilde{t}$$ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.