Search for Lorentz-boosted di-$\tau$ resonances produced in association with top quark pairs in $\sqrt{s}=13$ TeV pp collisions with the ATLAS detector

Abstract

A search for a Lorentz-boosted pseudoscalar resonance $a$ in the mass range between 20 GeV and 85 GeV, produced in association with a top-antitop quark pair and decaying into a $\tau$-lepton pair, is presented. Both $\tau$-leptons are reconstructed in the hadronic decay mode. The boosted production of $a$ results in a small angular separation between its decay products, requiring the use of dedicated boosted di-$\tau$ objects with custom reconstruction, energy calibration, and identification procedures. The analysis uses proton-proton collision data at a centre-of-mass-energy of $\sqrt{s}=13$ TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{-1}$. No significant deviation is observed over the expected Standard Model background, and a model independent upper limit of 0.19 fb is placed at the 95% confidence level on the visible cross-section. Limits on $\sigma(t\bar{t}a) \times BR(a \to \tau\tau)$ are also obtained for a Two-Higgs-Doublet Model signal scenario, ranging from 0.4 pb at $m_a=20$ GeV to 0.05 pb at $m_a=85$ GeV.