Search for top squarks in events with a Higgs or Z boson using 139 fb(-1) of pp collision data at root 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 root 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 ((t) over tilde (1)) or the heavier top squark ((t) over tilde (2)), excluding at 95% confidence level (t) over tilde (1) and (t) over tilde2 masses up to about 1220 and 875 GeV, respectively.