New physics in toponium's shadow?

TL;DR

The paper addresses enhancements in $pp\to t\bar{t}$ production near threshold, where toponium dynamics become important, and investigates whether a top-philic neutral pseudoscalar with mass near $2 m_t$ could contribute. It develops an effective model with a pseudoscalar $a$ coupling to gluons and tops, and embeds NRQCD bound-state effects into both SM and BSM amplitudes via a Green's-function reweighting scheme using $\widetilde{G}(E,p^*)$, implemented in FeynRules and MG5_aMC. The analysis shows that interference between SM and BSM amplitudes and the threshold non-perturbative dynamics reshape the viable $(M_a,c_t)$ parameter space, especially for narrow widths near $M_a\approx 2 m_t$, with the threshold region providing strong sensitivity to both short-distance and bound-state physics. While SM threshold effects explain a significant portion of the observed enhancement, a pseudoscalar contribution remains compatible, highlighting the need for a fully consistent treatment of BSM-modified threshold dynamics to interpret threshold observables and to guide future explorations of top-philic resonances.

Abstract

ATLAS and CMS have recently reported enhancements in the top-antitop production rate near threshold, a region where non-perturbative QCD dynamics associated with toponium formation become relevant. We investigate how this behaviour is modified in the presence of a neutral pseudoscalar that couples to gluons and top quarks, using an effective description that consistently incorporates perturbative Standard Model and new physics contributions, their interference and non-perturbative threshold effects. We show that the combined effect of those ingredients markedly shapes the viable region of the pseudoscalar parameter space, particularly for narrow resonances with masses close to twice the top mass. While Standard Model threshold effects could explain a sizeable part of the measured enhancements, the current data remain compatible with additional contributions from pseudoscalar interactions.

Figures (3)

• Figure 1: Feynman diagram representing the contribution of a pseudoscalar resonance $a$ to $t\bar{t}$ production in the gluon fusion channel. This amplitude interferes with the SM one and affects both the total rate and the spin-correlation structure.
• Figure 2: Invariant mass $m_{t\bar{t}}$ distribution (left) and total cross-section shifts (right) induced by the presence of the pseudoscalar state in the theory. Left -- Predictions for a scenario with $c_t = 0.01$ and $M_a = 340$ GeV. We show perturbative SM (dashed black) and perturbative BSM (dashed blue) results, together with predictions including toponium effects through NRQCD matrix-element re-weighting (solid black and solid red, respectively). These are compared to the naive combination of perturbative BSM rates with SM toponium effects (solid blue), and to results for $M_a = 360$ GeV (solid green). Right -- Dependence of $\sigma_{\mathrm{BSM}}-\sigma_{\mathrm{pQCD}}$ on $M_a$ for $c_t = 0.01$, $m_{t\bar{t}}<400$ GeV and assuming width-to-mass ratios of 1% (blue) and 5% (orange). The predictions are compared with the toponium expectation from NRQCD (dashed grey).
• Figure 3: Exclusion contours in the $(M_a, c_t)$ plane for $\Gamma/M_a = 1\%$ (left) and $5\%$ (right). The three rows correspond to different theoretical assumptions: the full perturbative SM and BSM contributions supplemented by the non-perturbative corrections (top row), predictions in which the non-perturbative corrections are applied only to the SM part (middle row), and predictions with only perturbative contributions (bottom row).