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Abstract

This paper presents a search for top-squark pair production in fnal states with a top quark, a charm quark and missing transverse momentum. The data were collected with the ATLAS detector during LHC Run 2 and correspond to an integrated luminosity of 139 fb−1 of proton-proton collisions at a centre-of-mass energy of √ s = 13 TeV. The analysis is motivated by an extended Minimal Supersymmetric Standard Model featuring a non-minimal favour violation in the second- and third-generation squark sector. The top squark in this model has two possible decay modes, either t˜1 → cχ˜ 0 1 or t˜1 → tχ˜ 0 1 , where the χ˜ 0 1 is undetected. The analysis is optimised assuming that both of the decay modes are equally probable, leading to the most likely fnal state of tc + Emiss T . Good agreement is found between the Standard Model expectation and the data in the search regions. Exclusion limits at 95% CL are obtained in the m(t˜1) vs. m(χ˜ 0 1 ) plane and, in addition, limits on the branching ratio of the t˜1 → tχ˜ 0 1 decay as a function of m(t˜1) are also produced. Top-squark masses of up to 800 GeV are excluded for scenarios with light neutralinos, and top-squark masses up to 600 GeV are excluded in scenarios where the neutralino and the top squark are almost mass degenerate.