Search for dark mesons decaying to top and bottom quarks with the ATLAS detector

The Standard Model can be extended by a new strongly-coupled, SU(2) dark flavour symmetry conserving model that gives rise to a familiar-looking dark sector as direct analogue to the QCD meson and baryon sector. Dark matter could thus manifest itself as composite dark mesons free of constraints from precision Higgs coupling or electroweak measurements. We present the first dedicated search at any collider experiment for such dark mesons decaying promptly and gaugephobically to pure Standard Model final states. In this case dark mesons are always pair produced, predominantly decay to top and bottom quarks resulting in either tttb or ttbb final states and are searched for in the all-hadronic channel. The dominant background originates from QCD multijets and is estimated by a sophisticated 4-dimensional extrapolation technique that takes undesired correlations between discriminating variables into account. All remaining backgrounds are estimated from Monte Carlo simulation. The presence of a signal is evaluated by performing a binned likelihood fit simultaneously in all search regions. The analysis relies on the full Run 2 proton–proton collision dataset at √s = 13 TeV collected with the ATLAS detector at the CERN Large Hadron Collider.