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

ATLAS Collaboration

Report

PUBDB-2025-05444

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

ATLAS Collaboration (Collaboration author)DESY*

2025

50 pp. (2025) [10.3204/PUBDB-2025-05444]

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Please use a persistent id in citations: doi:10.3204/PUBDB-2025-05444

Report No.: ATLAS-CONF-2025-008

Abstract: A measurement is presented of $t\bar{t}$ production in the invariant-mass region near the pair production threshold,$m_{t\bar{t}}$∼345GeV, in final states with two charged leptons and multiple jets. The measurement is based on 140fb$^{-1}$ of proton–proton collision data collected at $\sqrt{s}$=13TeV with the ATLAS detector. The data are compared to two models of $t\bar{t}$ production: a baseline model including only perturbative QCD (pQCD) predictions for the hard process, and an extended model that, in addition to the pQCD predictions, incorporates state-of-the-art Monte Carlo 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 pQCD prediction, with an observed significance of 7.7 standard deviations. This excess is consistent with the formation of colour-singlet, S-wave, quasi-bound $t\bar{t}$ states as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.0$\pm$1.3pb.

Keyword(s): Production threshold ; Quasi-bound-state ; Dilepton channel ; ttbar ; TOP