What if a Higgs boson could interact with another Higgs boson? The Standard Model predicts the existence of this "self-coupling", which regulates the strength of the interaction of the Higgs boson with other Higgs bosons. The self-coupling plays a pivotal role in shaping the potential energy of the Higgs boson, and is ultimately responsible for giving mass to elementary particles. While physicists have been diligently investigating the properties of the Higgs boson for over a decade, measurements of the Higgs self-coupling are more challenging.

To directly study the Higgs self-coupling, physicists are searching for the simultaneous production of two Higgs bosons ("di-Higgs production"). These processes are extremely rare – according to the Standard Model, the probability of seeing di-Higgs production in a proton-proton collision at the LHC is 1000 times smaller than that of producing a single Higgs boson! However, new physics processes beyond the Standard Model could greatly increase this probability – making the search for di-Higgs production in the current LHC data also a key tool in the search for new physics.

The ATLAS Collaboration has released two brand-new results searching for di-Higgs events in the full LHC Run-2 dataset (recorded 2015-2018). In one search, researchers looked for events where one Higgs boson decays into two bottom quarks and the other decays into two photons (bbγγ). In the other search, they looked for events where one Higgs boson decays into two bottom quarks, and the other into two tau leptons or W/Z bosons, which subsequently decay into leptons (bbll).

While previous studies of these decays only focused on Higgs bosons produced via gluon-gluon fusion (the most common production mode), ATLAS’ new results also looked at Higgs bosons produced via vector-boson fusion (VBF). Studying this production mode is the only direct way to probe the interaction of Higgs-boson pairs and two vector bosons and look for any possible deviations from Standard Model. This interaction is measured by the “κ_2V” coupling modifier and normalised to the Standard-Model prediction, where k_2V is equal to one in the Standard Model.

Many other Standard Model physics processes can create events that look very similar to HH→bbγγ and HH→bbll production. To tackle the extremely challenging task of isolating the rare signal events from an overwhelming background, ATLAS researchers used advanced machine-learning algorithms trained on the kinematic quantities that describe the di-Higgs events. In particular, VBF production leaves a distinctive signature in the ATLAS detector involving two highly energetic “jets” of particles in the forward region; both analyses used algorithms trained to look for this topology.

Neither search found any hint of a signal above the expected background, or any deviation from Standard-Model expectations. The HH→bbll analysis placed an upper limit on the di-Higgs production cross-section to 9.6 times the Standard-Model prediction and constrained the Higgs self-coupling between −6.2 and 13.3, normalised to its Standard-Model value. The HH→bbγγ analysis placed an upper limit on the di-Higgs production cross-section that is 4 times the Standard Model prediction and constrained the self-coupling between −1.4 and 6.9 times its Standard-Model value.

By optimising both analyses to VBF-produced Higgs boson pairs, ATLAS physicists greatly improved their ability to study the interaction between two vector-bosons and two Higgs bosons. Researchers set the first constraints on the strength of this interaction in the bbll channel (see Figure 1) and new constraints in the bbγγ channel (see Figure 2). Finally, for the first time in the HH→bbγγ analysis, researchers set limits on anomalous interactions which might alter the di-Higgs production cross-section and kinematics using the Effective Field Theory approach, which is typically used to investigate potential new physics effects hidden in small deviations from the Standard Model.

Researchers have only begun to scratch the surface. By exploring new final states, combining different analyses statistically and studying new collision data from LHC Run 3, ATLAS physicists will further improve their ability to measure this unique property of the Higgs boson.

Learn more

• Studies of new Higgs boson interactions through nonresonant HH production in the bbγγ final state in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2023-050)
• Search for non-resonant Higgs boson pair production in the 2b + 2l + E^T_miss final state in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2023-064)
• Higgs Hunting 2023 presentation by Lidija Zivkovic: Higgs Self Coupling - ATLAS
• Higgs Hunting 2023 presentation by Tom Ingebretsen Carlson: Search for Higgs boson pairs in the bbɣɣ final state in ATLAS
• EPS-HEP2023 presentation by Tatsuya Masubuchi: Recent results in Higgs Physics
• EPS-HEP2023 presentation by Viviana Cavaliere: Probing the nature of electroweak symmetry breaking with Higgs boson pairs in ATLAS
• MPI 2023 presentation by Abraham Tishelman-Charny: Studies of new Higgs boson interactions through nonresonant HH production in the bb𝛾𝛾 final state in pp collisions at 13 TeV with the ATLAS detector