Toponia at the HL-LHC and FCC-ee

TL;DR

This paper investigates the properties, detection prospects, and physics implications of toponium states at the HL-LHC and FCC-ee, highlighting challenges and potential for new physics searches.

Contribution

It provides detailed calculations of toponium masses and decay widths, assesses detection strategies at future colliders, and explores their use in probing top-quark properties and new physics.

Findings

Detection of vector toponium at HL-LHC is challenging due to Landau-Yang theorem.

Lepton colliders like FCC-ee offer better sensitivity for toponium detection.

Precise toponium measurements can inform top-quark property studies and new physics searches.

Abstract

The hint of a pseudoscalar toponium state at the Large Hadron Collider (LHC) opens a new avenue for studying a novel class of QCD (quasi-)bound states with comparable formation and decay times. Compared with charmonium and bottomonium, toponium is a quasi-bound state, resembling a hydrogen atom of the strong interaction, although it appears as a broader resonance. We compute the masses and annihilation decay widths of the lowest $S$-wave ($\eta_t$, $\psi_t$) and $P$-wave ($\chi_{t0}$, $\chi_{t1}$) toponium states, and assess their discovery prospects at the High-Luminosity LHC (HL-LHC) and future lepton colliders, such as the $e^+e^-$ stage of the Future Circular Collider (FCC-ee). Detecting the vector $\psi_t$ state at the HL-LHC is hindered by the Landau-Yang theorem and the gluon-dominated production environment of the collider, whereas lepton colliders offer promising sensitivity through both constituent and two-body decays. A more precise measurement of the $\eta_t$ mass, approximately equal to that of $\psi_t$, at the LHC could help determine the optimal $t\bar{t}$ threshold center-of-mass energy for FCC-ee. The $P$-wave states remain challenging to observe at both the HL-LHC and future lepton colliders. We also discuss how toponium measurements can be used to probe top-quark properties and to conduct indirect searches for new physics, including light scalars that couple to the top quark.