Search for Light Neutral Scalar in the Georgi-Machacek Model with Forward Detectors at the LHC

TL;DR

This paper investigates a light, long-lived neutral scalar H_5^0 in the Georgi–Machacek model, exploiting loop-induced couplings that enable production via meson decays in the forward LHC region. An effective Lagrangian is developed to capture H_5^0 couplings to WW, ZZ, γγ, and fermions through loop effects, with production dominated by meson decays and decays often governed by H_5^0 → γγ. The study computes forward-detector sensitivities for AL3X, FACET, and FASER/FASER2, showing that these detectors can probe s_H down to O(10^-5) for sub-GeV m_5, providing a valuable, complementary probe to existing terrestrial and astrophysical constraints. The results highlight cancellations and parameter dependencies that shape the reach, underscoring the forward-detector program as a crucial component in testing extended Higgs sectors with light, long-lived states.

Abstract

Long-lived particle (LLP) is one of the well-motivated targets for current collider experiments searching for the physics beyond the Standard Model. In recent years, many dedicated detectors have been developed for such scenarios which are designed to extend the sensitivity to weakly coupled particles with macroscopic $cτ$. In this work, we investigate the LLP signatures of the neutral component of the fermiophobic fiveplet $H_5^0$ in the Georgi-Machacek model. Due to its fermiophobic nature at tree level, it possesses suppressed decay widths in the low mass region and can naturally be long-lived over a wide region of parameter space. We show that $H_5^0$ can be produced with an appreciable flux in the forward region from the meson decay through loop-induced couplings leading to observable LLP signatures in forward detectors. We evaluate the sensitivity of representative forward detectors to this scenario and compare the result with existing constraints from terrestrial experiments and astrophysical observations. Our results demonstrate that the forward detectors can probe the $s_H$ down to $\mathcal{O}(10^{-5})$ for sub-GeV scalar masses, and hence providing a powerful and complementary probe of the extended Higgs sectors that is inaccessible to conventional searches.

Figures (8)

• Figure 1: The generic Feynman diagrams contributing to the FCNC coupling of two fermions and GM neutral fiveplet from renormalizable Lagrangian.
• Figure 2: Decay branching ratios of different channels for the production of light CP-even fiveplet $H_5^0$ from meson decays as a function of $s_H$/$m_5$. The legend corresponding to the four figures is presented in the second figure. Notably, $Br(B^{0} \rightarrow K^{0} H_5^0)$ represented by the red line and $Br(B^{\pm} \rightarrow K^{\pm} H_5^0)$ represented by the orange line are found to be comparable, with the orange line slightly higher in value.
• Figure 3: The cross-section corresponding to the process of hadron collision producing mesons and mesons further decay into light CP-even fiveplet $H_5^0$.
• Figure 4: The production cross-section of the light CP-even fiveplet $H_5^0$ in the $m_5$-$s_H$ parameter plane.
• Figure 5: Decay widths of the light CP-even fiveplet $H_5^0$ into various final states as a function of $s_H$/$m_5$