In this episode, we detail experimental particle physics research conducted by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC), specifically focusing on proton-proton (pp) collisions at a centre-of-mass energy of 13 TeV. Both collaborations investigate top-antitop (tt) pair production, with a particular emphasis on the kinematic threshold region where quasi-bound tt states are expected to form. The ATLAS experiment reports an observed excess of events over standard perturbative Quantum Chromodynamics (pQCD) predictions, consistent with the formation of these colour-singlet, S-wave quasi-bound tt states, and quantifies this excess with a significance of 7.7 standard deviations. The CMS experiment also notes a similar enhancement at the tt threshold and reproduces mild tension regarding spin correlation measurements, ultimately establishing the presence of a pseudoscalar excess at the tt production threshold with a significance exceeding five standard deviations. Both papers describe their detector apparatus, event reconstruction methods, background estimations, and systematic uncertainties, utilising Monte Carlo simulations to model various processes and validate their findings.

ATLAS paper: https://cds.cern.ch/record/2937636/files/ATLAS-CONF-2025-008.pdf

CMS paper: https://arxiv.org/pdf/2503.22382