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

Abstract

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 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 $\tilde{t}\to b{\tilde{\chi }}_1^{\pm }$ into a $b$-quark and the lightest chargino with ${\tilde{\chi }}_1^{\pm }\to W{\tilde{\chi }}_1^0$, the decay $\tilde{t}\to t{\tilde{\chi }}_1^0$ into an on-shell top quark and the lightest neutralino, the three-body decay $\tilde{t}\to bW{\tilde{\chi }}_1^0$ and the four-body decay $\tilde{t}\to b\ell \nu {\tilde{\chi }}_1^0$. 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.

47 pages plus author list + cover pages (64 pages total), 11 figures, 19 tables, published version, all figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2016-17/