  A search for dark mesons originating from strongly-coupled, SU(2) dark flavor
symmetry conserving models and decaying gaugephobically to pure Standard Model
final states containing top and bottom quarks is presented. The search targets
fully hadronic final states and final states with exactly one electron or muon.
The analyzed dataset corresponds to an integrated luminosity of 140 $fb^{-1}$
of proton-proton collisions collected at $\sqrt{s}=13$ TeV with the ATLAS
detector at the Large Hadron Collider. No significant excess over the Standard
Model background expectation is observed and the results are used to set the
first direct constraints on this type of model. The two-dimensional signal
space of dark pion masses $m_{\pi_D}$ and dark rho-meson masses $m_{\rho_D}$ is
scanned. For $m_{\pi_D}/m_{\rho_D}=0.45$, dark pions with masses
$m_{\pi_D}<943$ GeV are excluded, while for $m_{\pi_D}/m_{\rho_D}=0.45$ masses
of $m_{\pi_D}<738$ GeV are excluded.
