The Higgs field – an invisible quantum field that permeates the entire Universe – gives masses to fundamental particles. The discovery of the Higgs boson confirmed its existence and kick-started the effort to measure its potential energy. The shape of the Higgs potential provides crucial information about the long-term stability of the Universe and depends on the strength of the Higgs "self-coupling" (where multiple Higgs bosons interact).

While the ATLAS Collaboration has previously studied pairs of Higgs bosons (di-Higgs production), research into three Higgs bosons (tri-Higgs production) offers unique opportunities to explore the Higgs self-coupling. In particular, it allows researchers to study the as-yet-unexplored quartic Higgs self-coupling (λ₄), which governs the interaction of four Higgs bosons. Additionally, they can test Beyond the Standard Model (BSM) theories which predict tri-Higgs production at higher rates through the decay of new, heavy bosons.

At this week’s Higgs2024 conference, the ATLAS Collaboration unveiled the first LHC search for tri-Higgs production – a process over 600,000 times rarer than the production of a single Higgs boson. Due to its extremely short lifetime, the Higgs boson cannot be directly detected. Instead, physicists look for its decay products, most commonly pairs of bottom quarks. Each bottom quark produces a shower of particles that can be readily detected and reconstructed as a “b-jet”.

To search for tri-Higgs production, physicists studied collison events with six b-jets (see event display). This signal is extremely challenging to identify, as there are 15 different ways to assign six b-jets to three Higgs bosons and many other physics processes can also produce this signal (such as gluons splitting into b-jet pairs). To spot tri-Higgs production, ATLAS researchers paired the b-jets with a combined reconstructed mass close to that of the Higgs boson (125 GeV). This enabled them to explore interesting properties of the individual Higgs bosons, as well as the entire collision event. A dedicated machine learning algorithm exploited these properties, allowing genuine signal events to be statistically differentiated from background events. The background model was carefully studied and validated to ensure its accuracy.

Although no evidence of tri-Higgs production was observed in the data, physicists set upper limits on the current sensitivity to tri-Higgs production at the ATLAS experiment. Additionally, the strengths of 2 types of the Higgs self-coupling were directly probed for deviations from Standard Model predictions. The constraints on the tri-linear (λ₃) and quartic (λ₄) Higgs self-couplings are shown in Figure 1, including the first-ever constraints on λ₄!

However, the work is far from complete. These initial results serve as an important stepping stone for future searches for rare multi-Higgs processes, which will become increasingly accessible as more data are collected. With the High-Luminosity LHC set to amass over 20 times the current LHC dataset and with ever-advancing analysis techniques, ATLAS physicists aim to gain a deeper understanding of the Higgs self-coupling and probe BSM theories by studying the tri-Higgs production.

Learn more

• A search for triple Higgs boson production in the 6b final state using proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2411.02040, see figures)
• Higgs 2024 presentation by William Balunas: Probing the nature of electroweak symmetry breaking with Higgs boson pairs in ATLAS
• Higgs 2024 presentation by Maggie Chen: Searches for resonances decaying to pairs of Higgs bosons in ATLAS
• Combination of searches for Higgs boson pair production in proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Rev. Lett. 133 (2024) 101801, arXiv:2406.09971, see figures)
• Constraints on the Higgs boson self-coupling from single- and double- Higgs production with the ATLAS detector using proton-proton collisions at 13 TeV (Phys. Lett. B 843 (2023) 137745, arXiv:2211.01216, see figures)
• A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery (Nature 607 52 (2022), arXiv:2207.00092, see figures)