Search for a scalar partner of the top quark in the all-hadronic $t\overline{t}$ plus missing transverse momentum final state at $\sqrt{s}=13$ TeV with the ATLAS detector

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 $t\overline{t}$ plus missing transverse momentum final state is presented. The analysis of 139 ${\text{fb}}^{-1}$ of $\sqrt{s}=13$ 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 $\stackrel{~}{t}\to {t}^{\left(\ast \right)}{\stackrel{~}{\chi }}_{1}^{0}$ , with ${t}^{\left(\ast \right)}$ denoting an on-shell (off-shell) top quark and ${\stackrel{~}{\chi }}_{1}^{0}$ the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where ${m}_{\stackrel{~}{t}}>{m}_{t}+{m}_{{\stackrel{~}{\chi }}_{1}^{0}}$ , top squark masses are excluded in the range 400–1250 GeV for ${\stackrel{~}{\chi }}_{1}^{0}$ masses below 200 GeV at 95% confidence level. In the situation where ${m}_{\stackrel{~}{t}}\sim {m}_{t}+{m}_{{\stackrel{~}{\chi }}_{1}^{0}}$ , top squark masses in the range 300–630 GeV are excluded, while in the case where ${m}_{\stackrel{~}{t}}<{m}_{W}+{m}_{b}+{m}_{{\stackrel{~}{\chi }}_{1}^{0}}$ (with ${m}_{\stackrel{~}{t}}-{m}_{{\stackrel{~}{\chi }}_{1}^{0}}\ge 5$ GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.