Does the Top Quark Embrace its Antipode? - Unveiling a Pseudoscalar Excess near the ttbar Threshold
ORAL · Invited
Abstract
The top quark — the heaviest known elementary particle — plays a unique role at the energy frontier of the Large Hadron Collider (LHC). In this talk, I will present a search for resonances in top–antitop quark pair (ttbar) production using ATLAS and CMS data collected from 2016 to 2018 at a center-of-mass energy of √s = 13 TeV. The analyses examine the invariant mass of the ttbar system and sensitive angular observables that capture subtle effects arising from correlations between the spins of the top and antitop quarks. A significant excess of events near the kinematic ttbar threshold — deviating from fixed-order perturbative QCD predictions — was observed by CMS and independently confirmed by ATLAS. This enhancement is compatible with the production of a color-singlet, pseudoscalar quasi-bound "toponium" state — long thought unobservable — as predicted by non-relativistic QCD. I will also discuss alternative explanations, including potential resonances arising from new elementary scalar or pseudoscalar particles.
*I acknowledge support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany‘s Excellence Strategy – EXC 2121 “Quantum Universe” – 390833306.
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Publication:1) A. Hayrapetyan et al. [CMS Collaboration], ``Observation of a pseudoscalar excess at the top quark pair production threshold,'' Rept. Prog. Phys. 88, no.8, 087801 (2025) [arXiv:2503.22382 [hep-ex]].
2) A. Hayrapetyan et al. [CMS Collaboration], ``Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton-proton collisions at sqrt(s) = 13 TeV,'' submitted to Rept. Prog. Phys., arXiv:2507.05119 [hep-ex].
3) ATLAS Collaboration, ``Observation of a cross-section enhancement near the ttbar production threshold in sqrt(s) =13 TeV pp collisions with the ATLAS detector,'' ATLAS-CONF-2025-008.
