Combination of searches for Higgs boson pairs in Collisions at with the ATLAS detector

This letter presents a combination of searches for Higgs boson pair production using up to 36.1${\text{fb}}^{-1}$ of proton–proton collision data at a centre-of-mass energy $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the $b\overline{b}b\overline{b}$, $b\overline{b}{W}^{+}{W}^{-}$, $b\overline{b}{\tau }^{+}{\tau }^{-}$, ${W}^{+}{W}^{-}{W}^{+}{W}^{-}$, $b\overline{b}\gamma \gamma $ and ${W}^{+}{W}^{-}\gamma \gamma $ final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (${\kappa }_{\lambda }$) of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to $-5.0<{\kappa }_{\lambda }<12.0$ ($-5.8<{\kappa }_{\lambda }<12.0$). In addition, limits are set on the production of narrow scalar resonances and spin-2 Kaluza–Klein Randall–Sundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model.