Search for non-resonant pair production of Higgs bosons in the $b\bar{b}b\bar{b}$ final state in $pp$ collisions at $\sqrt{s}= 13$ TeV with the ATLAS detector

"A search for non-resonant Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state is presented. The analysis uses 126 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}={13}$ 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 for signal is found and the observed (expected) upper limit on the cross section for non-resonant 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 upon modifiers to the $HHH$ and $HHVV$ couplings. The observed (expected) constraints on the $HHH$ coupling modifier, $\kappa_\lambda$, are determined to be [-3.9, 11.1] ([-4.6, 10.8]) at 95% confidence level, while the corresponding constraints for the $HHVV$ coupling modifier, $\kappa_{2V}$, are [-0.03, 2.11] ([-0.05, 2.12])."