The Standard Model of particle physics is an extremely successful description of known fundamental particles and interactions, but it remains incomplete. It does not explain, for example, the origin of the Higgs-boson mass scale, the nature of dark matter, or the observed matter–antimatter asymmetry of the universe. Many theories beyond the Standard Model predict new particles or interactions, referred to as new physics, that couple strongly to the top quark, the heaviest known elementary particle. Processes involving several top quarks are therefore especially powerful probes of new physics.
This project focused on the production of four top quarks, tttt, using data collected by the ATLAS experiment at the Large Hadron Collider (LHC). Four-top-quark production is one of the rarest and most energetic processes accessible at the LHC. It can reveal new physics in several ways: through a production rate larger than predicted, through altered event kinematics, through new heavy particles that decay into top quarks, or through indirect effects described by effective field theories.
The original objective of the project was to use tttt production to search for new physics through three connected steps: discovery and inclusive cross-section measurement, differential measurements of kinematic distributions, and interpretation of the results in terms of possible new physics. The project also planned to develop improved reconstruction tools for complex multi-top-quark events.
During the fellowship, the scientific context changed significantly. Four-top-quark production was discovered before the start of the project in a previous analysis led by the researcher using LHC Run 2 data. The project was therefore re-focused towards the next scientific step: using the established tttt process as a platform for dedicated new-physics searches and preparing the analysis tools needed for LHC Run 3 data at the new energy frontier of 13.6 TeV.
The project pathway to impact was therefore to improve the experimental and methodological foundations for future tttt measurements and searches. This included preparing Run 3 analysis infrastructure, improving object selection and electron calibration, implementing improved simulations for rare backgrounds, developing machine-learning tools, contributing to new-physics searches, and producing sensitivity projections for the High-Luminosity LHC (HL-LHC). These activities contribute to the long-term European strategy for particle physics by clarifying what future LHC and HL-LHC data can reveal about the fundamental structure of matter.
