Search for Light Scalars in the TRSM at the LHC

Abstract: We study the production of Beyond the Standard Model light scalar states in association with a vector boson ($Vh_2$, with $V = W^\pm, Z$) at the LHC. We consider the scenario where the Standard Model scalar sector is extended by two real scalar singlets, where these additional scalars have mass $M_i \leq M_{h_{125}}$. In this work, the scalar boson $h_2$ decays via $h_2 \to h_1 h_1 \to 4b$, while the associated vector boson decays either into a pair of oppositely charged leptons or into a single charged lepton and a neutrino. We analyze the signal using LHC detector parameterizations and evaluate its statistical significance at a center-of-mass energy of 13.6~TeV for integrated luminosities of 300~fb$<sup>{-1}$ and 3000~fb$<sup>{-1}$ corresponding to the LHC Run 3 and High Luminosity LHC, respectively. Our preliminary results indicate promising discovery prospects for this channel serving as a complementary probe of extended scalar sectors.\ RBI-ThPhys-2026-04, COMETA-2026-04

• The paper presents discovery potential for light scalars in the Two Real Singlet Model through Vh2 production channels and a focused analysis of multijet final states.
• It employs simulations using MadGraph5_aMC@NLO, Pythia8, and Delphes with optimized b-tagging to effectively reduce SM backgrounds.
• Statistical analysis indicates strong signal significance (up to 7.4σ at 300 fb⁻¹), underscoring promising avenues for probing extended scalar sectors.

Search for Light Scalars in the TRSM at the LHC: Formal Analysis

Motivation and Theoretical Framework

The paper addresses the phenomenology of an extended scalar sector realized through the Two Real Singlet Model (TRSM), where the Standard Model (SM) Higgs sector is augmented by two real scalar singlet fields. This minimal extension is motivated by theoretical considerations including dark energy, CP violation, electroweak vacuum metastability, and the possibility of new scalar states beyond the SM. The TRSM yields three CP-even states ($h_1$, $h_2$, $h_3$), with the SM-like Higgs identified as the heaviest, $h_3 \equiv h_{125}$.

The model is constructed with two discrete $\mathbb{Z}_2$ symmetries to ensure the stability of the scalar potential and suppress unwanted couplings. The scalar spectrum arises from mixing among the doublet and two singlets, governed by three independent mixing angles $(\theta_{hS},\theta_{hX},\theta_{SX})$ and seven physical parameters including the singlet VEVs.

Three benchmark points are selected after rigorous theoretical and experimental filtering, relying on tools such as ScannerS (Mühlleitner et al., 2020) and HiggsTools (Bahl et al., 2022). Constraints include perturbative unitarity, vacuum stability, LEP and LHC Higgs limits, and kinematic thresholds ensuring dominant $h_1 \to b\bar{b}$ branching. Selected points exhibit $M_1 \ll M_{h_{125}}$, moderate $M_2 - 2M_1$ mass splitting, and sizable $h_2 \to h_1 h_1$ branching ratios.

Signal Topology and Event Generation

The primary search is the associated production of a light scalar ($h_2$0) with an electroweak boson ($h_2$1), yielding final states $h_2$2. Vector bosons decay either leptonically ($h_2$3 or $h_2$4). The analysis targets events with four $h_2$5-jets and one or two isolated leptons, exploiting the inherently suppressed backgrounds and optimizing sensitivity to light $h_2$6 masses.

Signal and background generation is performed at LO with MadGraph5_aMC@NLO (Alwall et al., 2014), showered using Pythia8 (Sjöstrand et al., 2014), and reconstructed with Delphes (Favereau et al., 2013). $h_2$7-tagging leverages DeepCSV efficiencies, with fixed $h_2$8 tagging, $h_2$9 $h_3$0-mis-tag, and $h_3$1 light flavor mis-tag. Preselection requires $h_3$2 GeV, $h_3$3 GeV, $h_3$4, $h_3$5.

Kinematic Discriminants and Selection Strategy

Signal discrimination leverages low missing transverse energy and modest hadronic activity, reflecting the soft nature of the multi-$h_3$6-jet final state. As shown in the normalized distributions,

Figure 1: Missing transverse energy (left) and total hadronic transverse energy (right) for the signal and backgrounds; signal events prefer low $h_3$7 and $h_3$8 regions.

the signal is concentrated at $h_3$9 GeV and $h_3 \equiv h_{125}$0 GeV, increasing separation from SM backgrounds such as $h_3 \equiv h_{125}$1 and $h_3 \equiv h_{125}$2+jets.

Further selection exploits the reconstruction of the $h_3 \equiv h_{125}$3 resonance from $h_3 \equiv h_{125}$4-jet pairs; invariant mass windows $h_3 \equiv h_{125}$5 GeV and $h_3 \equiv h_{125}$6 are applied to capture boosted $h_3 \equiv h_{125}$7 decays. Additional requirements on $h_3 \equiv h_{125}$8-jet $h_3 \equiv h_{125}$9 ($\mathbb{Z}_2$0, $\mathbb{Z}_2$1 GeV for leading/subleading) and object multiplicity (at least four jets, $\mathbb{Z}_2$22 $\mathbb{Z}_2$3-tagged jets) are imposed. The dilepton channel reconstructs $\mathbb{Z}_2$4 via $\mathbb{Z}_2$5 GeV.

The efficacy of these cuts is evidenced by the characteristic invariant mass distributions,

Figure 2: Reconstructed invariant mass for leading/subleading $\mathbb{Z}_2$6-jets (left) and leading/subleading leptons (right), showing $\mathbb{Z}_2$7 and $\mathbb{Z}_2$8 resonance structures.

which illustrate clear signal peaks distinct from background shapes.

Statistical Analysis and Discovery Prospects

Significance is estimated with the profile likelihood approach (Cowan et al., 2010), using the formula:

$\mathbb{Z}_2$9

Results demonstrate strong discovery sensitivity in single-lepton ($(\theta_{hS},\theta_{hX},\theta_{SX})$0) channels. At $(\theta_{hS},\theta_{hX},\theta_{SX})$1, significances for BP1/2/3 range from $(\theta_{hS},\theta_{hX},\theta_{SX})$2 to $(\theta_{hS},\theta_{hX},\theta_{SX})$3 (for $(\theta_{hS},\theta_{hX},\theta_{SX})$4/$(\theta_{hS},\theta_{hX},\theta_{SX})$5), well above the $(\theta_{hS},\theta_{hX},\theta_{SX})$6 threshold, with $(\theta_{hS},\theta_{hX},\theta_{SX})$7 channel below $(\theta_{hS},\theta_{hX},\theta_{SX})$8 at this luminosity. Scaling to $(\theta_{hS},\theta_{hX},\theta_{SX})$9, $h_1 \to b\bar{b}$0 yields reach $h_1 \to b\bar{b}$123 for BP3, with $h_1 \to b\bar{b}$2 channel remaining subdominant but increasing to $h_1 \to b\bar{b}$33.7.

Backgrounds are dominated by $h_1 \to b\bar{b}$4, $h_1 \to b\bar{b}$5jets, and $h_1 \to b\bar{b}$6, but the optimized selection substantially reduces their impact. Uncertainties from background normalization are not considered in this preliminary estimate; further studies incorporating systematic effects and higher-order corrections remain ongoing.

Implications and Outlook

The study establishes the $h_1 \to b\bar{b}$7 topology as a sensitive and complementary probe of extended scalar sectors, unconstrained by typical SM exotic decay searches that focus on $h_1 \to b\bar{b}$8-mediated processes. The high single-lepton significance at moderate luminosities provides a robust avenue for TRSM discovery, and optimized $h_1 \to b\bar{b}$9-tagging plus kinematic selections are crucial for suppressing backgrounds.

Theoretical implications center on the validation or exclusion of minimal scalar extensions with singlet fields, informing model building in cosmological and CP-violation frameworks. Practically, these results motivate dedicated triggers and reconstruction strategies for very low mass scalars, especially in the context of HL-LHC.

Future directions will require:

• Inclusion of systematic uncertainties and NLO corrections,
• Consideration of detector granularity and pileup for low-$M_1 \ll M_{h_{125}}$0 jets,
• Exploration of additional channels ($M_1 \ll M_{h_{125}}$1, $M_1 \ll M_{h_{125}}$2, $M_1 \ll M_{h_{125}}$3) as complementary avenues,
• Integration with global fits and limits from cosmological and astroparticle searches.

Conclusion

This analysis rigorously explores the search for light CP-even scalars in the TRSM via $M_1 \ll M_{h_{125}}$4 associated production at the LHC. The study identifies benchmark scenarios that offer strong discovery potential, particularly in single-lepton channels with multijet final states. The results underscore the unique sensitivity of this channel to hidden scalar sectors and highlight its value in broadening the phenomenological coverage of BSM Higgs extensions. Further refinement and incorporation of full statistical and detector modeling are necessary for quantitative projections, but the channel remains highly competitive and complementary to traditional exotic Higgs boson searches.