Dark Higgs Bosons at FASER

TL;DR

FASER investigates the discovery potential for dark Higgs bosons produced via Higgs-portal mixing, focusing on forward, long-lived decays from B and K mesons at the HL-LHC. Using forward-mphysics simulations and detector geometries, the study maps the sensitivity in the (m_phi, theta) plane and demonstrates that a compact forward detector can probe new regions of parameter space, including m_phi ~ 0.2–3.5 GeV and theta ~ 10^-5–10^-3, with potential to observe hundreds to thousands of events. It also assesses the trilinear coupling h-phi-phi via b -> s h^* -> s phi phi, showing complementary reach to h -> phi phi searches and direct collider constraints. Beyond particle physics, the work ties dark Higgs searches to cosmology, highlighting implications for dark matter mediation and inflation, and emphasizing FASER's role as a low-cost, high-impact probe of light, weakly-coupled hidden sectors.

Abstract

FASER, ForwArd Search ExpeRiment at the LHC, has been proposed as a small, very far forward detector to discover new, light, weakly-coupled particles. Previous work showed that with a total volume of just $\sim 0.1 - 1~\rm{m}^3$, FASER can discover dark photons in a large swath of currently unconstrained parameter space, extending the discovery reach of the LHC program. Here we explore FASER's discovery prospects for dark Higgs bosons. These scalar particles are an interesting foil for dark photons, as they probe a different renormalizable portal interaction and are produced dominantly through $B$ and $K$ meson decays, rather than pion decays, leading to less collimated signals. Nevertheless, we find that FASER is also a highly sensitive probe of dark Higgs bosons with significant discovery prospects that are comparable to, and complementary to, much larger proposed experiments.

Figures (7)

• Figure 1: Left: Dark Higgs decay length $\bar{d} = c \tau_{\phi} \beta \gamma$ as a function of $m_{\phi}$ for various energies $E_\phi$ and $\theta = 10^{-4}$. The decay length scales as $\bar{d} \propto \theta^{-2}$. Adapted from Ref. Bezrukov:2013fca. Right: Dark Higgs decay length $\bar{d}$ in the $(m_{\phi}, \theta)$ plane for $E_{\phi} = 1~\text{TeV}$. The decay length scales as $\bar{d} \propto E_{\phi}$ for large $E_{\phi}$. The gray shaded regions are experimentally excluded.
• Figure 2: Dark Higgs branching fractions as a function of $m_{\phi}$. Adapted from Ref. Bezrukov:2013fca.
• Figure 3: Distribution of particles produced at the 13 TeV LHC with an integrated luminosity of $3~\text{ab}^{-1}$ in the $(\theta, p)$ plane, where $\theta$ and $p$ are the particle's angle with respect to the beam axis and momentum, respectively. The panels show the number of particles produced in one hemisphere ($0 < \cos \theta \le 1$). The bin thickness is $1/5$ of a decade along each axis. The top row shows the distributions of $B$ mesons (left), dark Higgs bosons produced in $B$ decays (center), and dark Higgs bosons produced in $B$ decays that themselves decay after traveling a distance in the range $(L_{\text{min}}, L_{\text{max}}) = (390~\text{m}, 400~\text{m})$ (right) for model parameters $(m_\phi, \theta) = (2~\text{GeV}, 10^{-4})$. The bottom row shows the analogous distributions for $K_L$ and $(m_\phi, \theta) = (300~\text{MeV}, 5 \times 10^{-4})$. The black dashed lines corresponds to $p_T = p \sin \theta = m_B$ in the top row and $m_K$ in the bottom row, and the gray dashed vertical lines in the right panels show the angular coverage of two representative configurations of FASER in the far location.
• Figure 4: Number of dark Higgs bosons produced at the 13 TeV LHC with $3~\text{ab}^{-1}$ that decay in FASER at the far location as a function of detector radius $R$. Results are given for dark Higgs bosons produced in $B$, $K_L$, $K_S$, and $K^{\pm}$ decay for model parameters $(m_{\phi},\theta)= (300~\text{MeV}, 5\times 10^{-4})$, and for dark Higgs bosons produced in $B$ decay for model parameters $(2~\text{GeV}, 10^{-4})$. The vertical lines at $R = 20~\text{cm}$ and 1 m are the radii for two representative far location detectors. Results for the near location with fixed $R = 4~\text{cm}$ are also shown.
• Figure 5: Number of signal events $N_{\text{sig}}$ in dark Higgs parameter space for the far detector location with $R=20~\text{cm}$ (top left) and $R=1~\text{m}$ (top right) and for the near detector location (bottom left), given an integrated luminosity of $3~\text{ab}^{-1}$ at the 13 TeV LHC. As indicated, the contours are for $N_{\text{sig}} = 3, 10, 100, 1000, \ldots$ from the processes $b\to s \phi$ (green) and $K \to \pi \phi$ (red). The gray shaded regions are excluded by current experimental bounds. The black stars correspond to the representative parameter-space points discussed in the text. The bottom right panel shows the exclusion reach for FASER at the far location for $R=1~\text{m}$ (solid black line) along with the projected reaches of other proposed experiments that search for long-lived particles.