Combination of searches for Higgs boson pair production in pp collisions at √s = 13 TeV with the ATLAS detector

This Letter presents results from a combination of searches for Higgs boson pair production using 126−140 fb⁻¹ of proton−proton collision data at √s = 13 TeV recorded with the ATLAS detector. At 95% confidence level (CL), the upper limit on the production rate is 2.9 times the Standard Model (SM) prediction, with an expected limit of 2.4 assuming no Higgs boson pair production. Constraints on the Higgs boson self-coupling modifier κ_λ = λ_HHH/λ^SM_MHHH, and the quartic HHVV coupling modifier κ_2V=g_HHVV/g^SM_MHHVV, are derived individually, fixing the other parameter to its SM value. The observed 95 CL intervals are −1.2 < κ_λ < 7.2 and 0.6 < κ_2V < 1.5, while the expected intervals are −1.6 < κ_λ < 7.2 and 0.4 < κ_2V < 1.6 in the SM case. Constraints obtained for several interactions within Higgs effective field theory are the strongest to date, offering insights into potential deviations from SM predictions.