Search for the Supersymmetric Partner of the Top Quark in the Jets+Emiss Final State at sqrt(s) = 13 TeV

A search for direct pair production of a scalar partner to the top quark in events with four or more jets plus missing transverse momentum is presented. An analysis of 13.3 fb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collisions collected using the ATLAS detector at the LHC yielded no significant excess over the Standard Model background expectation. In the supersymmetric interpretation, the top squark is assumed to decay via $\tilde{t} \to t \tilde{\chi}_{1}^{0}$, $\tilde{t}\to b\tilde{\chi}^{\pm}_{1} \to b W^{\left(\ast\right)} \tilde{\chi}_{1}^{0}$, or $\tilde{t}\to b W \tilde{\chi}_{1}^{0}$, where $\tilde{\chi}_{1}^{0}$ ($\tilde{\chi}^{\pm}_{1}$) denotes the lightest neutralino (chargino). Exclusion limits are reported in terms of the top squark and neutralino masses. Assuming branching fractions of 100% to $t \tilde{\chi}_{1}^{0}$, top squark masses in the range 310–820 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 160 GeV. In the case where $m_{\tilde{t}} ∼ m_t + m_{\tilde{\chi}_{1}^{0}}$ top squark masses between 23–380 GeV are excluded. Limits are also reported in terms of simplified models describing the associated production of dark matter ($\chi$) with top quark pairs through a (pseudo)scalar mediator; models with a global coupling of 3.5, mediator masses up to 300 GeV, and $\chi$ masses below 40 GeV are excluded.