Unravelling an extended quark sector through multiple Higgs production?

TL;DR

The paper investigates whether adding heavy quarks beyond the SM can significantly modify gluon-fusion double-Higgs production while keeping the single-Higgs rate SM-like. It analyzes two explicit quark-extended models—a vector-like singlet top partner and a mirror-fermion framework—using exact LO calculations for $gg\to HH$, LET-based insights, and an EFT mapping to dimension-6 operators $\mathcal{O}_1$ and $\mathcal{O}_2$. The results show that electroweak precision data and the measured $gg\to H$ rate strongly constrain possible deviations in $gg\to HH$, with only modest enhancements in certain parameter regions and minor shifts in $H\to\gamma\gamma$. Framing the findings in terms of $c_H=c_1+c_2$ and $c_{HH}=c_1-c_2$ clarifies why large beyond-SM effects are difficult to realize in these models and guides how combined Higgs measurements can probe the origin of fermion masses.

Abstract

In many new physics scenarios, the particle content of the Standard Model is extended and the Higgs couplings are modified, sometimes without affecting single Higgs production. We analyse two models with additional quarks. In these models, we compute double Higgs production from gluon fusion exactly at leading-order, and present analytical results in the heavy-quark mass ap- proximation. The experimental bounds from precision electroweak measurements and from the measured rate of single Higgs production combine to give significant restrictions for the allowed deviation of the double Higgs production rate from the Standard Model prediction as well as on the branching ratio for the Higgs decay into photons. The two models analysed eventually present a similar Higgs phenomenology as the Standard Model. We connect this result to the magnitude of the dimension six operators contributing to the gluon-fusion Higgs production.

Figures (15)

• Figure 1: Feynman diagrams for $gg \to HH$ in the Standard Model.
• Figure 2: Double Higgs production cross section as a function of the hadronic center of mass energy $\sqrt{S}$ in the infinite top mass approximation, LET, (solid lines) and retaining the ${\cal O} ({s\over m_t^{2}})$ corrections (dashed lines), normalized to the exact result. The black (red) curves choose as the renormalization and factorization scales $\mu=2m_H$ ($\mu=M_{HH}=\sqrt{s}$).
• Figure 3: Total cross sections for $HH$ production using CTEQ6L LO PDFS and CT10 NLO PDFs. The renormalization/factorization scale is $\mu=2m_H$ in (a) and $\mu=M_{HH}=\sqrt{s}$ in (b). For all curves, $\alpha_s$ is evaluated at NLO.
• Figure 4: Invariant mass distributions for Higgs pair-production at $\sqrt{S}=8$ TeV and $\sqrt{S}=14$ TeV, for terms in the large mass expansion up to ${\cal O}(m_t^{-4})$ (Eq. \ref{['eq:gghh_SM_expansion']}) and with the full mass dependence.
• Figure 5: Transverse momentum distribution for double Higgs production cross section. The Standard Model exact result, the LET and the heavy top mass approximations up to ${\cal O} ( m_t^{-4})$ are shown. We choose as the renormalization and factorization scales $\mu=M_{HH}=\sqrt{s}$ and use the CT10 NLO PDFs.