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

Abstract

Abstract 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.