Dark scalar reinterpretation of searches for Higgs boson decays into long-lived particles at the LHC

TL;DR

This work reinterprets ATLAS and CMS searches for Higgs decays into long-lived particles within the scalar portal BC5 using public HEPData results to map sensitivity in the parameter space spanned by $m_S$ and $\sin^2\theta$. By recasting both leptonic and hadronic LLP signatures, it demonstrates that different experimental strategies probe complementary regions, revealing gaps in phase-space coverage. The study then extrapolates current constraints to the HL-LHC era under several scenarios and compares these with projected sensitivities of dedicated LLP experiments (SHiP, CODEX-b, MATHUSLA, ANUBIS), showing these experiments can extend the reach beyond what is achievable by luminosity scaling alone. The results underscore the importance of background control for LLP searches and provide guidance for optimizing future LLP reach across a broad mass range.

Abstract

We present a reinterpretation of selected ATLAS and CMS searches for Higgs boson decays into long-lived particles (LLPs) within the scalar portal framework, where the standard model is extended by a light scalar that mixes with the Higgs boson. Using public HEPData results, we evaluate the sensitivity of several experimental strategies targeting different final states. The analysis shows how published searches probe complementary regions of the model parameter space, and it provides a comparative assessment of their coverage. The study also identifies the existing gaps in phase space coverage and highlights key areas for future improvement in LLP searches, which will be crucial to maximize the discovery potential for exotic decays of the Higgs boson into LLPs. Finally, we extrapolate the constraints to the integrated luminosity of the High-Luminosity LHC under different assumptions and compare them with the projected sensitivities of proposed dedicated LLP experiments at CERN, showing that these experiments will further extend the experimental reach independently of the assumed extrapolation.

Figures (8)

• Figure 1: Branching fractions of $\text{S}$ as a function of $m(\text{S})$ for various final states.
• Figure 2: Predicted ${\cal B}(\text{Z}_\text{D} \to \mu^{+}\mu^{-})$ in the HAHM (red), ${\cal B}(\text{S} \to \mu^{+}\mu^{-})$ in the scalar portal model (blue), and their ratio, $R_{\mu\mu}$ (green).
• Figure 3: The sum of the predicted ${\cal B}{(\text{S}})$ in the considered hadronic final states in Eqs. \ref{['eq:scale_pipi']}–\ref{['eq:scale_qq']} and the corresponding rescaling factors.
• Figure 4: 95% CL observed upper limits on ${\cal B}({\text{h}\to\text{S}\text{S}})$ as a function of $\sin^{2}\theta$ in the BC5 scalar portal model, for $m(\text{S})$ ranging from [55,60] $\text{GeV}$ (upper left) to 0.4 $\text{GeV}$ (lower right), obtained from the reinterpretation of selected LHC LLP searches.
• Figure 5: 95% CL observed upper limits in the $\{m(\text{S}), \sin^{2}\theta\}$ plane, assuming ${\cal B}({\text{h}\to\text{S}\text{S}}) = 1\%$, in the BC5 scalar portal model. Shown are the reinterpretation of LHC LLP Summaries including Refs. ATLAS:2024qooCMS:2024xzbATLAS:2024ocvATLAS:2018tupATLAS:2022gbwCMS:2024bvl (blue), EXO-23-014 CMS:2024qxz (green), EXO-20-014 CMS:2021sch (orange), and EXO-21-008 CMS:2024bvl (red). The results are compared with current constraints from Ref. PBC:2025sny (gray), which include results from E949, NA62, MicroBooNE, KOTO, ICARUS, LHCb, and Belle II as well as reinterpretations of PS191, CHARM and LSND.