Search for a new pseudoscalar decaying into a pair of bottom and antibottom quarks in top-associated production in SQUARE ROOTs = 13 TeV proton-proton collisions with the ATLAS detector

Abstract

A search for a pseudoscalar a produced in association with a top-quark pair, or in association with a singletop quark plus a W boson, with the pseudoscalar decayinginto b-quarks (a RIGHTWARDS ARROW b-b), is performed using the full Run 2data sample using a dileptonic decay mode signature. Thesearch covers pseudoscalar boson masses between 12 and100 GeV and involves both the kinematic regime where thedecay products of the pseudoscalar are reconstructed as twostandard b-tagged small-radius jets, or merged into a largeradiusjet due to its Lorentz boost. No significant excess relativeto expectations is observed. Assuming a branching ratioBR(a RIGHTWARDS ARROW b-b ) = 100%, the range of pseudoscalar massesbetween 50 and 80 GeV is excluded at 95% confidence levelfor a coupling of the pseudoscalar to the top quark of 0.5,while a coupling of 1.0 is excluded at 95% confidence levelfor the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.