Combination of searches for nonresonant Higgs boson pair production in proton-proton collisions at sqrt(s) = 13 TeV

This paper presents a combination of searches for the nonresonant production of Higgs boson pairs (HH) in proton–proton collisions at a centre-of-mass energy of 13 TeV. The dataset was collected by the CMS experiment at the LHC from 2016 to 2018 and corresponds to a total integrated luminosity of 138 fb−1. The observed (expected) upper limit on the inclusive HH production cross-section relative to the standard model (SM) prediction is found to be 3.5 (2.5). Assuming all other Higgs boson couplings are equal to their SM values, the Higgs boson trilinear self-coupling modifier κλ = λ3/λSM 3 is constrained in the range −1.35 ⩽ κλ ⩽ 6.37 at 95% confidence level (CL). Similarly, for the coupling modifier κ2V, which governs the interactions between two vector bosons and two Higgs bosons, we have excluded κ2V = 0 at more than five standard deviations for all values of κλ. At 95% CL, assuming that the other couplings are equal to their SM values, κ2V is constrained in the range 0.64 ⩽ κ2V ⩽ 1.40. This work also investigates HH production in several new physics scenarios, using the Higgs effective field theory (HEFT) framework. The HEFT framework is further exploited to study various ultraviolet complete models with an extended Higgs sector and set constraints on specific parameters. An extrapolation of the results in the expected integrated luminosity following the high-luminosity upgrade of the LHC is also reported.