UC Berkeley

A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic tt¯$$t{\bar{t}}$$ plus missing transverse momentum final state is presented. The analysis of 139 fb-1$$\hbox {fb}^{-1}$$ of s=13$${\sqrt{s}=13}$$ TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via t~→t(∗)χ~10$${\tilde{t}} \rightarrow t^{(*)} {\tilde{\chi }}^0_1$$, with t(∗)$$t^{(*)}$$ denoting an on-shell (off-shell) top quark and χ~10$${\tilde{\chi }}^0_1$$ the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where mt~>mt+mχ~10$$m_{{\tilde{t}}}> m_t+m_{{\tilde{\chi }}^0_1}$$, top squark masses are excluded in the range 400–1250 GeV for χ~10$${\tilde{\chi }}^0_1$$ masses below 200 GeV at 95% confidence level. In the situation where mt~∼mt+mχ~10$$m_{{\tilde{t}}}\sim m_t+m_{{\tilde{\chi }}^0_1}$$, top squark masses in the range 300–630 GeV are excluded, while in the case where mt~