Search for Nonresonant Pair Production of Highly Energetic Higgs Bosons Decaying to Bottom Quarks

A search for nonresonant Higgs boson ($H$) pair production via gluon and vector boson ($V$) fusion is performed in the four-bottom-quark final state, using proton-proton collision data at 13 TeV corresponding to $138{\mathrm{fb}}^{-1}$ collected by the CMS experiment at the LHC. The analysis targets Lorentz-boosted $H$ pairs identified using a graph neural network. It constrains the strengths relative to the standard model of the $H$ self-coupling and the quartic $VVHH$ couplings, ${\kappa }_{2V}$, excluding ${\kappa }_{2V}=0$ for the first time, with a significance of 6.3 standard deviations when other $H$ couplings are fixed to their standard model values.

Figure 1

The data and fitted signal and background distributions for the $D_{b\overline{b}}$-subleading jet regressed mass are shown for the ggF BDT event category 1, the category accounting for most of the sensitivity to the ggF $HH$ signal. The SM $HH$ (${\kappa }_{2V}={\kappa }_V={\kappa }_{\lambda }=1$) signal is shown scaled to the best fit signal strength $\mu =3.5$. The lower panel shows the ratio of the data and the total prediction, with its uncertainty represented by the shaded band. The error bars on the data points represent the statistical uncertainties.

Figure 2

The distributions of the invariant mass of the $HH$ system after a background-only fit to the data, for the VBF low-purity (LP), medium-purity (MP), and high-purity (HP) categories. The VBF signal for ${\kappa }_{2V}=0$, ${\kappa }_V={\kappa }_{\lambda }=1$, is shown in red with the vertical error bar indicating the prefit uncertainty. The lower panel shows the ratio of the data and the total background prediction, with its uncertainty represented by the shaded band. The error bars on the data points represent the statistical uncertainties.

Figure 3

Two-parameter profile likelihood test statistic ($-2\mathrm{\Delta }\ln \mathcal{L}$) scan in data as a function of ${\kappa }_{\lambda }$ and ${\kappa }_{2V}$. The black cross indicates the minimum, while the red diamond marks the SM expectation (${\kappa }_{\lambda }={\kappa }_{2V}=1$). The gray solid, dashed, dotted, and dash-dotted contours enclose the 1, 2, 3, and $5\sigma$ CL regions, respectively.