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

A bstract 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 correspond to an integrated luminosity of 139 fb − 1 of proton-proton collisions at a centre-of-mass energy of $$ \sqrt{s} $$ 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 $$ {\tilde{t}}_1\to c{\overset{\sim }{\chi}}_1^0 $$ t ~ 1 → c χ ~ 1 0 or $$ {\tilde{t}}_1\to t{\overset{\sim }{\chi}}_1^0 $$ t ~ 1 → t χ ~ 1 0 , where the $$ {\overset{\sim }{\chi}}_1^0 $$ χ ~ 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 $$ tc+{E}_T^{\textrm{miss}} $$ tc + E T 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 $$ m\left({\tilde{t}}_1\right) $$ m t ~ 1 vs. $$ m\left({\overset{\sim }{\chi}}_1^0\right) $$ m χ ~ 1 0 plane and, in addition, limits on the branching ratio of the $$ {\tilde{t}}_1\to t{\overset{\sim }{\chi}}_1^0 $$ t ~ 1 → t χ ~ 1 0 decay as a function of m ( $$ {\tilde{t}}_1 $$ 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.

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