Search for new phenomena in events with two opposite-charge leptons, jets and missing transverse momentum in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

The results of a search for direct pair production of top squarks and for dark matter in events with two opposite-charge leptons (electrons or muons), jets and missing transverse momentum are reported, using 139 fb$^{-1}$ of integrated luminosity from proton-proton collisions at $\sqrt{s}=13$ TeV, collected by the ATLAS detector at the Large Hadron Collider during Run-2 (2015-2018). This analysis considers the pair production of top squarks and is sensitive across a wide range of mass differences between the $\tilde{t}$ and lightest neutralino $\tilde{\chi}_{1}^{0}$. Additionally, spin-0 mediator dark matter models are considered, in which the mediator is produced in association with a pair of top quarks. The mediator subsequently decays to a pair of dark-matter particles. No significant excess of events is observed above the Standard Model background, and limits are set at $95\%$ confidence level. The results exclude $\tilde{t}$ masses up to about 1 TeV, and $\tilde{\chi}_{1}^{0}$ masses up to about 500 GeV. Limits on Dark Matter production are set for scalar (pseudoscalar) mediator masses up to about 250~(300) GeV.