Search for new phenomena in $t\bar{t}$ final states with additional heavy-flavour jets in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for new phenomena in $t\bar{t}$ final states with additional heavy-flavour jets is presented. The search targets a variety of signals, including the pair production of a vector-like top quark ($T$) with a significant branching ratio to a top quark and either a Standard Model Higgs boson or a $Z$ boson; four-top-quark production, both within the Standard Model and in several new physics scenarios; and heavy Higgs bosons (neutral and charged) produced in association with, and decaying into, third generation quarks. The analysis uses $pp$ collisions at $\sqrt{s}=13\tev$ recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 13.2 fb$^{-1}$. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets, as well as the jets+$E_{\text{T}}^{\text{miss}}$ final state, characterised by multiple jets and large missing transverse momentum. The search exploits the high multiplicity of $b$-jets, the high scalar sum of transverse momenta of all final state objects, and the presence of boosted, hadronically-decaying resonances reconstructed as large-radius jets characteristic of signal events. In the absence of a significant excess above the Standard Model expectation, 95\% CL upper limits are derived for the signal models in a number of benchmark scenarios, in most cases significantly extending the reach of previous searches.