Search for a light CP-odd Higgs boson decaying into a pair of $τ$-leptons in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Abstract

This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13 TeV proton-proton collision data with an integrated luminosity of 140 fb$^{-1}$ collected with the ATLAS detector at the Large Hadron Collider. The analysis assumes the resonance is produced via gluon-gluon fusion and decays into a $τ^{+}τ^{-}$ pair which subsequently decays into a fully leptonic $μ^{+}ν_μ \barν_τ e^{-} \barν_{e} ν_τ$ or $e^{+}ν_{e}\barν_τ μ^-\barν_μν_τ$ final state. No significant excess of events above the predicted Standard Model background is observed. The results are interpreted within a flavour-aligned two-Higgs-doublet model, and a model-independent cross-section interpretation is also given. Upper limits at 95$%$ confidence level between 3.0 pb and 68 pb are set on the cross-section for producing a CP-odd Higgs boson that decays into a $τ^+τ^-$ pair.