Two Higgses, The Statistical Wall, and The Chronometer: Probing the Higgs Potential and Preparing for the High-Luminosity Era at the LHC

by Arthur Lafarge (Université Clermont Auvergne (FR), LPCA (Clermont-Ferrand))

Thursday Nov 6, 2025, 2:00 PM → 4:00 PM Europe/Paris

Amphi Recherche (LPCA)

   The Standard Model's crowning achievement, the Higgs boson, holds the key to the universe’s fundamental stability. Yet, the detailed shape of the Higgs potential, which governs the mechanism stabilizing the electroweak vacuum, remains largely unknown. This uncertainty leaves open profound questions about the dynamics of electroweak symmetry breaking and possible physics beyond the Standard Model.

   This thesis tackles one of the most challenging measurements in particle physics: the direct search for Higgs boson pair (HH) production, a process that provides a unique experimental access to the Higgs self-coupling shaping this potential. This production mode is exceedingly rare, around a thousand times rarer than single Higgs production, requiring the highest possible experimental sensitivity. The first ATLAS search for the HH→bbγγ final state using the full Run 2 and Run 3 dataset (308 fb⁻¹) achieves the highest single-channel sensitivity obtained so far in this decay mode and is presented in this defense.

   Complementing this effort, the first dedicated ATLAS search for ttHH production in the bbγγ channel is presented. Although roughly 38 times rarer than gluon-gluon fusion HH production, this process provides a unique handle on the Higgs coupling structure by probing the double top–Higgs vertex.

   Looking ahead to the High-Luminosity LHC (HL-LHC), the thesis further addresses the experimental challenges posed by the extreme particle densities expected. The validation of the readout electronics (ALTIROC) for the High-Granularity Timing Detector (HGTD) confirms their timing precision and radiation tolerance, important steps toward ensuring the HL-LHC’s readiness for precision Higgs measurements.

   While the current HH sensitivity is still limited by available statistics, the HL-LHC, with 3 ab⁻¹ of expected data, should offer a realistic opportunity for discovery. This increase in luminosity is projected to yield substantially tighter constraints on the Higgs self-coupling, and is also illustrated in the results from the bbγγ channel.

Supervision: Djamel Boumediene
Co-supervision: Louis D'Eramo

Lafarge_PhD_HH-and-HGTD_2025.pdf