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oai:www.repository.cam.ac.uk:1810/3936522025-12-10T01:43:42Zcom_1810_205871com_1810_256064col_1810_206446

Global Standard Model analyses for new physics searches https://doi.org/10.17863/CAM.123890 Hammou, Elie 0009000456127729 Ubiali, Maria The discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 completed the Standard Model (SM), one of the most successful physical theories ever formulated. Since then, the SM has withstood extensive experimental scrutiny, with theoretical predictions showing remarkable agreement with measurements. Nevertheless, there are compelling theoretical and observational arguments for the existence of physics beyond the SM (BSM). Yet, no sign of it has been directly observed at colliders, suggesting the existence of a mass gap between the SM particles and the undiscovered ones. This has motivated the systematic use of effective field theories (EFTs) to search indirectly for potential signs of new physics, both in the current data and in future measurements from programmes such as the High-Luminosity upgrade of the LHC (HL-LHC) and the next proposed CERN flagship experiment, the Future Circular Collider (FCC). EFTs, such as the SMEFT, enable model-agnostic new physics searches via global analyses of collider data. This thesis focuses on a possible pitfall in such studies: the risk of introducing a bias toward the SM by absorbing potential new physics signals within the parametrisation of the Parton Distribution Functions (PDFs), which describe the structure of the proton. We explore the interplay between PDFs and the SMEFT in global analyses, presenting a realistic risk assessment that demonstrates how energy-growing BSM effects can be absorbed within the PDFs and thus missed. To mitigate this, we propose and evaluate two complementary strategies. First, we add new constraints on the large-x PDFs using lower-energy projected measurements from future experiments such as the Electron-Ion Collider (EIC) and the proposed Forward Physics Facility (FPF). This approach reduces the excessive flexibility of the antiquark PDFs that allows them to mimic BSM effects. Second, we perform a simultaneous fit of PDFs and SMEFT parameters, using our publicly available tool, SIMUnet. Both methodologies are shown to be effective in disentangling the new physics signals from the PDFs. We discuss the strengths and limitations of both methods and highlight clear recommendations for the experimental and theoretical high-energy physics community. We conclude by outlining directions for future work, motivated by the precision requirements of robust SMEFT analyses in the HL-LHC era. University of Cambridge 2025-07-31 Thesis Doctoral Doctor of Philosophy (PhD) eng https://www.repository.cam.ac.uk/handle/1810/393652 https://www.repository.cam.ac.uk/bitstreams/ed61c542-55c8-49ec-a3ea-c346266bba62/download 16a558bc807b70988facdcbd03371028 https://www.repository.cam.ac.uk/bitstreams/1e43f783-af5c-4691-8c00-12eea6c45a0f/download 87eda9de84448d1f82354d60eee3eb5f http://purl.org/NET/rdflicense/allrightsreserved Particle physics Standard Model New physics Indirect searches SMEFT PDFs