Combination of ATLAS and CMS Searches for Higgs Boson Pair Production at √s=13 TeV

This Letter presents a combination of searches for Higgs boson pair (𝐻⁡𝐻 ) production performed by the ATLAS and CMS Collaborations using proton-proton collision datasets recorded at √𝑠 =13 TeV during the Large Hadron Collider Run 2, corresponding to integrated luminosities ranging between 126 and 140 fb⁻¹. The upper limit at the 95% confidence level on the total 𝐻⁡𝐻 signal strength, defined as the ratio of the measured cross section to the standard model (SM) prediction, corresponds to 2.5, with an expected value of 1.7 (2.8) assuming the absence (presence) of the SM 𝐻⁡𝐻 signal. The strength of the 𝐻⁡𝐻 signal is measured to be0.8+0.9−0.7 relative to the SM prediction. The observed significance is found to be 1.1 standard deviations whereas 1.3 are expected for the SM 𝐻⁡𝐻 signal. Constraints are set on the Higgs boson trilinear self-coupling and on the couplings of two Higgs bosons to two vector bosons, both normalized to the SM predictions and denoted as 𝜅_𝜆 and 𝜅2⁢𝑉, respectively. The observed individual constraints at the 95% confidence level are −0.71<𝜅_𝜆<6.1 and 0.73 <𝜅2⁢𝑉 <1.3 , while the expected constraints assuming the presence of the SM 𝐻⁡𝐻 signal are −1.3 <𝜅_𝜆<6.7 and 0.66 <𝜅2⁢𝑉 <1.4. .