Search for direct pair production of top squarks in $pp$ collisions at $\sqrt{s}= 13$ TeV and $13.6$ TeV in events with two oppositely charged leptons using the ATLAS detector

Abstract

This paper presents the search for direct pair production of top squarks decaying into two on-shell top quarks and two neutralinos in final states with two oppositely charged leptons (electrons or muons), $b$-jets and large missing transverse momentum. The search uses the full Run 2 dataset, corresponding to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV collected by the ATLAS detector from 2015 to 2018, and the early Run 3 dataset, corresponding to an integrated luminosity of 53 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} = 13.6$ TeV collected in 2022 and 2023. Machine-learning-based classifiers are used to distinguish the signal from Standard Model backgrounds, to enhance signal discrimination across a wide kinematic phase space. No significant excess is observed with respect to the Standard Model prediction and 95% confidence level limits are set on top squark and neutralino mass combinations reaching up to 1060 GeV for the former and 560 GeV for the latter, improving mass limits by about 10% compared to previous ATLAS analyses in the same channel.