Observation of a cross-section enhancement near the $t\bar{t}$ production threshold in $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

TL;DR

This ATLAS study probes tt̄ production near the threshold at $\sqrt{s}=13$ TeV to search for NRQCD-predicted colour-singlet quasi-bound states. By reconstructing the $t\bar{t}$ system in dileptonic events and exploiting spin-sensitive angular observables, the analysis compares a baseline pQCD model (hvq/bb4l) with an extended NRQCD-based model that includes $S$-wave quasi-bound-state formation near threshold via Green's-function reweighting ($t\bar{t}_{\text{GFRW}}$). A profile-likelihood fit across 13 regions (nine SRs plus control regions) yields a fitted quasi-bound-state cross-section of $\sigma(t\bar{t}_{\text{GFRW}})=9.3^{+1.4}_{-1.3}$ pb, significantly exceeding the perturbative baseline and yielding an observed significance $>8\,\sigma$. The results are broadly consistent with NRQCD expectations for colour-singlet, spin-singlet $S$-wave states near threshold, though they also motivate further development of NRQCD-perturbative matching and higher-order corrections for more precise interpretation and cross-experiment comparisons.

Abstract

A measurement of $t\bar{t}$ production is presented in the invariant-mass region near the pair production threshold, $m_{t\bar{t}} \sim 345$ GeV, in final states with two charged leptons and multiple jets. The measurement is based on $140\,\mathrm{fb}^{-1}$ of proton-proton collision data collected at $\sqrt{s} = 13$ TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of $t\bar{t}$ production: a baseline model including only perturbative QCD predictions for the hard process, and an extended model that, in addition, incorporates non-relativistic QCD simulations of colour-singlet quasi-bound-state formation near the $t\bar{t}$ threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed $m_{t\bar{t}}$ distributions, in bins of two angular observables sensitive to spin-correlations in the $t\bar{t}$ system. An excess of events is observed over the baseline perturbative QCD prediction, with an observed significance over $8$ standard deviations. This excess is consistent with the formation of colour-singlet and spin-singlet $S$-wave quasi-bound $t\bar{t}$ states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of $9.3^{+1.4}_{-1.3}$ pb.

Figures (12)

• Figure 1: Parton-level distributions of (a) $c_\mathrm{hel}$ and (b) $c_\mathrm{han}$ for pQCD and $t\bar{t}_{\text{GFRW}}$ production, emphasizing their different shapes. The distributions for a pure spin-singlet state ($^1S_0$) and a pure spin-triplet state ($^3P_0$), obtained analytically, are shown as well. The $t\bar{t}_{\text{GFRW}}$ sample includes the full spectrum of $^1S_0$ states, and therefore has a shape similar to the analytical $^1S_0$ distribution.
• Figure 2: Definition of the nine signal regions based on the angular variables $c_\mathrm{hel}$ and $c_\mathrm{han}$.
• Figure 3: Pre-fit distributions of $m_{t\bar{t}}$ in the nine SRs (upper panel), together with a comparison between the quasi-bound-state prediction and the data, from which the pQCD contribution and background processes were subtracted (middle panel), and the ratio of the data and the extended model including the $t\bar{t}_{\text{GFRW}}$ contribution (lower panel). The error bars on the data markers represent the statistical uncertainty in the measurement, while the grey hashed and shaded bands represent the total systematic uncertainty in the prediction.
• Figure 4: Post-fit distributions of $m_{t\bar{t}}$ in the nine SRs (upper panel), together with a comparison between the quasi-bound-state prediction and the data, from which the pQCD contribution and background processes were subtracted (middle panel), and the ratio of the data and the extended model including the $t\bar{t}_{\text{GFRW}}$ contribution (lower panel). The error bars on the data markers represent the statistical uncertainty in the measurement, while the grey hashed and shaded bands represent the total systematic uncertainty in the prediction. The dashed line represents the data/MC ratio before the fit.
• Figure 5: Ranking of the 15 most impactful individual NPs on $\mu(t\bar{t}_{\text{GFRW}}\xspace)$ in the profile-likelihood fit with free-floating $\mu(t\bar{t}_{\text{GFRW}}\xspace)$. The covariance-matrix approximation of the shifted observables method is used to evaluate the impact of a given NP Pinto:2023yob. The impact is taken as the corresponding off-diagonal element of the fit covariance matrix divided by its pre-fit uncertainty and is given in percent on the upper scale. The black marker shows the shifts of the NPs relative to the nominal value $\theta_0$. They are shown together with their corresponding uncertainty on the lower scale. The SR numbers correspond to the bins introduced in Figure \ref{['fig:results:SRsplit']}. For the flavour-tagging uncertainties, individual eigenvariations (EVs) are shown.