In proton collisions at the Large Hadron Collider, sometimes not one but two Higgs bosons are produced. The rate of such events is a direct imprint of how the Higgs field interacts with itself. In the Standard Model, this coupling is rigidly fixed, but extended theories allow deviations.
The authors calculated one such variant, scNMSSM, with record two-loop precision. Out of a million random parameter sets, only 66 passed all filters: they do not contradict collider data, dark matter, or rare particle decays. All surviving points fell into a narrow Higgs mass band of 122–125 GeV, and their "singlet fraction" (how much the particle resembles the Standard Model single Higgs) turned out to be above 95%.
The cross section for non-resonant double Higgs production at 13.6 TeV lies in the range 30.3–36.9 femtobarns, which almost matches the Standard Model prediction (0.88–1.05 of it). The resonant contribution through a heavy third Higgs is negligible — less than 0.004 fb. All numbers fit within current ATLAS and CMS constraints, but the future HL-LHC will be able to test them.
A quantitative calculation of the transition strength at finite temperature is left for the future. But it is already clear: double Higgs production is not just an exotic curiosity but a window into the first moments after the Big Bang.
A femtobarn is a unit of area. One femtobarn is roughly the cross-sectional area of a proton. An event with a cross section of 1 fb at the LHC occurs about once every few years of continuous operation.