A search for top-squark pair production, in final states containing a top quark, a charm quark and missing transverse momentum, using the 139 fb${}^{-1}$ of $pp$ collision data collected by the ATLAS detector

This paper presents a search for top-squark pair production in final 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 corresponds to an integrated luminosity of 139fb${}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. The analysis is motivated by an extended Minimal Supersymmetric Standard Model featuring a non-minimal flavour violation in the second- and third-generation squark sector. The top squark in this model has two possible decay modes, either ${\tilde{t}}_1\to c{\tilde{\chi }}_1^0$ or ${\tilde{t}}_1\to t{\tilde{\chi }}_1^0$, where the ${\tilde{\chi }}_1^0$ is undetected. The analysis is optimised assuming that both of the decay modes are equally probable, leading to the most likely final state of $tc+E_{\text{T}}^{\text{miss}}$. 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 $m({\tilde{t}}_1)$ vs $m({\tilde{\chi }}_1^0)$ plane and, in addition, limits on the branching ratio of the ${\tilde{t}}_1\to t{\tilde{\chi }}_1^0$ decay as a function of $m({\tilde{t}}_1)$ 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.