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Description
A search for nonresonant triple Higgs boson production in the six b quark final state is performed using proton-proton collisions at √s = 13 TeV corresponding to an integrated luminosity of 138 fb−1 recorded by the CMS experiment. Each Higgs boson is reconstructed either from two small-radius jets (resolved) or a single large-radius jet (merged). No significant excess of events over the Standard Model background prediction is observed. Observed (expected) 95% confidence level upper limits on the signal cross section relative to the Standard Model expectation are set at 588 (572), corresponding to a signal cross section upper limit of 44 (43) fb. Assuming the quartic coupling modifier κ4 = 1, the observed (expected) constraint on the trilinear coupling modifier κ3 is −7.4 < κ3 < 12.4 (−6.4 < κ3 < 11.2) and, assuming κ3 = 1, the observed (expected) constraint on the quartic coupling modifier is −177 < κ4 < 185 (−177 < κ4 < 183). These are the most stringent constraints to date on nonresonant triple Higgs boson production and its quartic self-coupling, and provide the first probe, in a direct nonresonant triple Higgs boson search, of the perturbative unitarity boundary in the (κ3, κ4) plane.