Search for nonresonant pair production of Higgs bosons in the bb¯bb¯ final state in pp collisions at √s = 13 TeV with the ATLAS detector

A search for nonresonant Higgs boson pair production in the bb¯bb¯ final state is presented. The analysis uses 126 fb⁻¹ of pp collision data at √s 
= 13 TeV collected with the ATLAS detector at the Large Hadron Collider, and targets both the gluon-gluon fusion and vector-boson fusion production modes. No evidence of the signal is found and the observed (expected) upper limit on the cross-section for nonresonant Higgs boson pair production is determined to be 5.4 (8.1) times the Standard Model predicted cross-section at 95% confidence level. Constraints are placed on modifiers to the HHH and HHVV couplings. The observed (expected) 2σ constraints on the HHH coupling modifier, κλ, are determined to be [−3.5,11.3] ([−5.4,11.4]), while the corresponding constraints for the HHVV coupling modifier, κ2V, are [−0.0,2.1] ([−0.1,2.1]). In addition, constraints on relevant coefficients are derived in the context of the Standard Model effective field theory and Higgs effective field theory, and upper limits on the HH production cross-section are placed in seven Higgs effective field theory benchmark scenarios.