A search for top-squark pair production, in final states containing a $t$-quark, $c$-quark and missing transverse momentum, using the full Run 2 dataset collected by the ATLAS detector

This paper presents a search for top-squark pair production in final states with a top quark, charm quark and missing transverse momentum using a dataset collected with the ATLAS detector during LHC Run 2, corresponding to an integrated luminosity of 139 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s} = 13$ TeV proton-proton collisions. The analysis is motivated by an extended MSSM model featuring a non-minimal flavour violation in the 2nd and 3rd generation squark sector. The top squark in these models has two possible decay modes, either $\tilde{t}_1\rightarrow c\tilde{\chi}_1^0$ or $\tilde{t}_1\rightarrow t\tilde{\chi}_1^0$. The analysis is optimised assuming both decay modes are equally probable, leading to the most likely final state of $tc + E_{\mathrm{T}}^{\mathrm{miss}}$. Good agreement is found between the post-fit SM expectation and the data in the search regions. Limits are presented in the m($\tilde{t}_1$) vs m($\tilde{\chi}_1^0$) plane and, in addition, limits on the BR($\tilde{t}_1\rightarrow t\tilde{\chi}_1^0$) 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 top squark are almost mass degenerate.