Search for the nonresonant production of Higgs boson pairs via gluon fusion and vector-boson fusion in the 𝑏𝑏ˉ𝜏⁺𝜏⁻ final state in proton-proton collisions at √s = 13 TeV with the ATLAS detector

A search for the non-resonant production of Higgs boson pairs in the HH→bb¯τ⁺τ⁻ channel is performed using 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimised to probe anomalous values of the Higgs boson self-coupling modifier κ_λ and of the quartic HHVV (V=W,Z) coupling modifier κ_2V. No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limit μ_HH < 5.9 (3.3) is set at 95% confidence-level on the Higgs boson pair production cross-section normalised to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of −3.1 < κ_λ < 9.0 (−2.5 < κ_λ < 9.3) and −0.5 < κ_2V < 2.7 (−0.2 < κ_2V < 2.4), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross-sections assuming different kinematic benchmark scenarios.