Real Singlet Scalar Benchmarks in the Multi-TeV Resonance Regime

TL;DR

This work analyzes a real singlet extension of the SM to assess how a heavy scalar $h_2$ can resonantly boost di-Higgs production and modify the SM Higgs trilinear coupling $\lambda_{111}$. By scanning the parameter space under theoretical constraints (vacuum stability, perturbative unitarity, global minimum) and experimental inputs (Higgs signal strengths, direct searches, narrow-width limit), the authors identify maximum resonant rates and the corresponding $\sin\theta$, $a_2$, $b_3$, $b_4$ values for four collider scenarios, including multi-TeV resonance reach. They find that current data allow resonant di-Higgs rates up to an order of magnitude above the SM and that HL-LHC can yield $\lambda_{111}$ enhancements up to about $3\times$ SM for certain $m_2$ ranges, with even larger possibilities in future lepton-collider scenarios. The study also reveals regions where $\lambda_{111}$ is large while the resonant rate is suppressed, underscoring the complementary roles of nonresonant and resonant di-Higgs measurements as probes of the real singlet model.

Abstract

Scalar extensions of the Standard Model are of much interest at the LHC and future colliders. In particular, these models can give rise to resonant di-Higgs production and alter the Higgs trilinear coupling. In this paper, we study di-Higgs production in the Standard Model extended by a real scalar singlet with no additional symmetries. We determine how large the resonant di-Higgs rate and variation in the Higgs trilinear coupling can be in four scenarios: current LHC results and projected results at the HL-LHC, the HL-LHC combined with a circular $e^-e^+$ collider such as the CEPC or FCC-ee, and the HL-LHC combined with a linear $e^-e^+$ collider such as the ILC. While these are updated results from a previous study using current LHC data, we go further and find benchmark points in the multi-TeV resonance regime for future colliders beyond the HL-LHC. Considering current LHC results, the resonant di-Higgs rate can still be an order of magnitude larger than the SM predicted di-Higgs rate. In the HL-LHC scenario, the Higgs trilinear coupling can still be a factor of three larger than the SM prediction for resonance masses in the $1.5-3.5$ TeV range, where resonant searches may have less reach. This enhancement is just at the projected 2$σ$ sensitivity of the HL-LHC. We find there are resonance masses for which the change in the Higgs trilinear is maximized while the resonant rate is negligible. We provide an analytical understanding of these effects with a discussion on the interplay of various constraints on the parameter space and the Higgs trilinear coupling.

Figures (7)

• Figure 1: Heat maps of ${\rm BR}(h_2\rightarrow h_1h_1)$ in the allowed $b_3-a_2$ plane for $m_2 =260$ GeV and $\sin^2\theta = 0.14$. In (a) $b_4 = 1.0$ with a maximum BR of $0.66$, and in (b) $b_4 = 4.2$ with a maximum BR of $0.78$.
• Figure 2: (a) Mixing angle constraints for scenario S1 from (black solid) Higgs precision measurement, (red dashed) hard cut bounds from scalar searches, and (blue dashed) a $\Delta\chi^2$ combination of scalar searches and Higgs precision measurements. (b) Mixing angle constraints for scenarios (black solid) S2, (blue dot dashed) S3, and (red dashed) S4. The shaded region is excluded by the narrow width requirement.
• Figure 3: Maximum allowed cross section $\sigma(pp \to h_2 \to h_1h_1)$ in scenario S1 at lab frame energy $\sqrt{S}=14$ TeV for (a) gluon fusion and (b) VBF, with (c) the corresponding branching ratio ${\rm BR}(h_2\rightarrow h_1h_1)$. Two possibilities for $\sin\theta$ limits from Higgs precision and scalar searches are shown: blue dashed lines for the $\Delta\chi^2$ combination and red dashed for the hard cut limits. In (a,b) the light grey line is for the SM di-Higgs rate and the black solid the SM production rate at mass $m_2$ for (a) gluon fusion and (b) VBF.
• Figure 4: (a) Normalized maximum double Higgs resonant production rates and (b) the corresponding branching ratio ${\rm BR}(h_2\rightarrow h_1h_1)$ for scenarios (black solid) S2, (blue dashed) S3, and (red dot-dashed) S4. In (c) the maximum rates are compared to (grey solid) current di-Higgs upper bounds. (d) The values of $|\sin\theta|$ corresponding to maximum rates.
• Figure 5: Values of (a) $\lambda_{111}$ normalized to the SM prediction $\lambda_{111}^{\rm SM}$ and (b) $\lambda_{112}$ corresponding to the maximum di-Higgs rate. The (solid) maximum and (dashed) minimum allowed $\lambda_{111}$ are shown in (c), and the corresponding $pp\rightarrow h_2\rightarrow h_1h_1$ rates in (d). All results are presented for current LHC constraints using the (blue) $\Delta\chi^2$ and (red) hard cut combinations of Higgs precision scalar searches. In (a,c) the 1$\sigma$ and 2$\sigma$ projected bounds on $\lambda_{111}$ at the HL-LHC ATL-PHYS-PUB-2022-005 are shown in dark and light grey shaded regions, respectively.