14.7 Summary and Outlook

In this chapter, we described two sensitive and complementary searches for SM nonresonant double Higgs boson production ($\mathrm{\text{H}}\mathrm{\text{H}}$) and the BSM nonresonant massive scalar resonances X and Y ($\mathrm{\text{X}}\to \mathrm{\text{H}}\mathrm{\text{Y}}$), into the $\mathrm{\text{b}}\overline{\mathrm{\text{b}}}$ and $\mathrm{\text{V}}\mathrm{\text{V}}$ all-hadronic final states, at high $m_{\mathrm{\text{H}}\mathrm{\text{H}}}$, and high $m_X$ and $m_Y$, respectively. The former aims to constrain the ${\kappa }_{2\mathrm{\text{V}}}$ quartic coupling modifier, while the latter is motivated by the high branching fraction predicted for the $\mathrm{\text{Y}}\to \mathrm{\text{V}}\mathrm{\text{V}}$ decay and the potential for new physics in the $\mathrm{\text{X}}\to \mathrm{\text{H}}\mathrm{\text{Y}}$ sector.

In both cases we search for boosted $\mathrm{\text{H}}\mathrm{\text{H}}∕\mathrm{\text{Y}}$ with fully merged jets; i.e., where all H and Y daughter quarks are contained within a single wide-radius AK8 jet. The established ParticleNet mass-decorrelated tagger is used to select for $\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}$ jets and we develop a new high performing Particle Transformer tagger for $\mathrm{\text{H}}∕\mathrm{\text{Y}}\to \mathrm{\text{V}}\mathrm{\text{V}}$ jets.

We extract the SM $\mathrm{\text{H}}\mathrm{\text{H}}$ signal from the $\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}$ regressed mass and the resonant $\mathrm{\text{X}}\to \mathrm{\text{H}}\mathrm{\text{Y}}$ signal from the $\mathrm{\text{Y}}\to \mathrm{\text{W}}\mathrm{\text{W}}$ regressed mass and dijet X mass, using control regions with tagger scores inverted to obtain a data-driven estimate of the shape and normalization of the QCD multijet background via a parametric transfer function. Other minor backgrounds including top quark and vector boson plus jets are estimated using MC simulations. We observe (expect) an upper limit at 95% CL of $142\times$ ($69\times$) the SM for nonresonant $\mathrm{\text{H}}\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}(\mathrm{\text{V}}\mathrm{\text{V}}\to 4\mathrm{\text{q}})$ production and a constraint on ${\kappa }_{2\mathrm{\text{V}}}$ of $[-0.04,2.05]$ ($[0.05,1.98]$), which is the second-highest constraint set by the CMS experiment. We expect exclusion limits as low as 0.3fb for resonant $\mathrm{\text{X}}\to (\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}})(\mathrm{\text{Y}}\to \mathrm{\text{V}}\mathrm{\text{V}}\to 4\mathrm{\text{q}})$ production for various $m_X$ and $m_Y$ mass points.

As discussed in Section 14.3.1, a significant limitation of both the Run 2 boosted $\mathrm{\text{H}}\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}\mathrm{\text{V}}\mathrm{\text{V}}$ and $\mathrm{\text{b}}\overline{\mathrm{\text{b}}}\mathrm{\text{b}}\overline{\mathrm{\text{b}}}$ analyses were the lack of dedicated triggers for boosted Higgs jets, leading to very low trigger efficiencies for AK8 jet $p_T<400\text{GeV}$. This has been improved in Run 3 with dedicated triggers boosted $\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}$ triggers using ParticleNet [397], which have the potential to significantly improve our constraints on the $\mathrm{\text{H}}\mathrm{\text{H}}$ cross section and ${\kappa }_{2\mathrm{\text{V}}}$. Additionally, as the boosted regime is currently statistically limited, with the statistical uncertainty on the background estimate dominating the uncertainties on the signal strength, such boosted $\mathrm{\text{H}}\mathrm{\text{H}}$ analyses stand to gain significantly from the increased luminosity of Run 3 and HL-LHC as well. Thus, the future is very bright for high energy searches of Higgs pair production!

Acknowledgements

This chapter, in part, is currently being prepared for the publication of the material by the CMS collaboration. The dissertation author was the primary investigator and author of these papers.