Search for top squarks in events with a Higgs or Z boson using 139 fb ${}^{-1}$ of pp collision data at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton–proton collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb ${}^{-1}$ . No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark ( ${\stackrel{~}{t}}_{1}$ ) or the heavier top squark ( ${\stackrel{~}{t}}_{2}$ ), excluding at 95% confidence level ${\stackrel{~}{t}}_{1}$ and ${\stackrel{~}{t}}_{2}$ masses up to about 1220 and 875 GeV, respectively.