Sensitivity to new physics: single-Higgs couplings vs. the trilinear Higgs coupling

Abstract

The trilinear Higgs self-coupling provides a unique probe of the structure of the Higgs potential and of the nature of the electroweak phase transition, and constitutes a key target for future collider experiments. Recent studies have shown that confronting theoretical predictions for the trilinear Higgs coupling with current experimental bounds offers a powerful and complementary way to test effects of physics beyond the Standard Model (BSM), in particular those arising from extended Higgs sectors. Meanwhile, substantial progress has been achieved in the precise calculation and automation of the trilinear Higgs coupling in a wide class of BSM models. This contribution discusses several BSM scenarios, compatible with existing constraints, in which sizeable deviations in the trilinear Higgs coupling w.r.t. the Standard Model (SM) value are predicted, while other Higgs observables remain close to their SM expectations and are therefore difficult to probe experimentally. These results highlight the strong physics motivation for a precise measurement of the trilinear Higgs coupling at a future Higgs factory.

Figures (5)

• Figure 1: Powers of the large coupling $g_{hh\Phi\Phi}$ (marked in red) contributing at the one-loop order to the couplings involving different numbers of SM-like Higgs bosons. In columns from left to right: the trilinear Higgs coupling, $\lambda_{hhh}$, the coupling of a Higgs boson to two gauge bosons, $g_{hVV}$, and the couplings of a Higgs boson to two fermions $g_{hff}$. Below each diagram we indicate the proportionality to the couplings of interest, the loop functions and the order of contribution of $g_{hh\Phi\Phi}$ in the limit where mass splitting effects are large, namely where $g_{hh\Phi\Phi}v^2 \gg \mathcal{M}^2$ (which implies $m_\Phi^2\sim g_{hh\Phi\Phi}v^2/2$).
• Figure 2: Left: Parameter scan results for the $\mathbf{Z}_2$SSM in the plane of $\kappa_\lambda$ and $\delta g_{hZZ}$. The colour of the points indicates the perturbative order (one or two loops) at which the two quantities are computed, as specified in the legend. The black dashed line shows the expected $2\sigma$ sensitivity of FCC-ee to the $g_{hZZ}$ coupling. Right: Parameter scan results in the IDM in the plane of $\kappa_\lambda^{(2)}$ (at two loops) and $\delta^{(1)}g_{hZZ}$ (at one loop). Points are grouped into hexagonal bins, whose colour indicates the minimal BSM deviation in $\Gamma(h\to\gamma\gamma)$ among the points in each bin. The red solid line in the colour bar corresponds to the expected $2\sigma$ FCC-ee sensitivity to $h\to\gamma\gamma$Selvaggi:2025kmd. Additional reference lines are described in the legend and in the text.
• Figure 3: One-loop OS results for the $g_{hZZ}$ coupling in the RxSM, normalised to the SM one-loop value, as a function of the one-loop OS prediction for $\kappa_\lambda$.
• Figure 4: Parameter scan results in the plane {$\kappa_{\lambda}$, $g_{hZZ}$} in the 2HDM-I (left) and 2HDM-II (right). Blue points indicate the tree-level results for these couplings, while red points show for the same parameter points the one-loop predictions including also for $g_{hZZ}$ the leading higher-order corrections involving insertions of one or two powers of the trilinear coupling, see text for details. The black dashed lines indicate the projected sensitivity at FCC-ee.
• Figure 5: Same as Fig. \ref{['thdmeff']}, but restricted to parameter points featuring a SFOEWPT. The colour coding indicates the strength of the transition $\xi_n$.