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

This episode details an experiment conducted by the ATLAS Collaboration using Large Hadron Collider data from 2015-2018. The research investigates the CP properties of the Higgs boson in its vector-boson fusion production mode, specifically focusing on its decay into two tau leptons. Utilising the Optimal Observable method and an effective field theory framework, the study aims to detect CP-violating interactions between the Higgs boson and electroweak gauge bosons. While no deviations from the Standard Model (which predicts a CP-even Higgs boson) were observed, the analysis provides stringent limits on strength parameters that describe potential beyond-Standard Model physics. The paper compares these findings with previous ATLAS and CMS Collaboration results, contributing to the ongoing effort to understand the fundamental nature of the Higgs boson.

Link to arxiv: https://arxiv.org/abs/2506.19395

The CMS experiment at CERN has conducted a search for light-weight hypothetical particles, specifically axion-like particles (ALPs), that may result from Higgs boson decays. These ALPs are theorised to decay into electron-positron pairs, which present a significant challenge for detection due to their merged experimental footprints. To overcome this, the CMS Collaboration developed a new multivariate identification technique using machine learning and precise tracking data. While no significant excess beyond the Standard Model was observed, the research established new stringent limits on exotic Higgs boson decays, thus providing crucial constraints on theoretical models of ALPs and demonstrating the LHC's unique capacity for exploring new physics.

CMS Physics Analysis Summary (EXO-24-031): "Search for light pseudoscalar a bosons in H→aa→4e decays in pp collisions at √s = 13 TeV"