Scalar Rayleigh Dark Matter: current bounds and future prospects

TL;DR

This work analyzes Rayleigh Dark Matter, a real scalar DM that interacts with electroweak gauge bosons through loop-suppressed dimension-6 operators, using an EFT framework that maps loop-induced couplings to potential tree-level UV completions. It conducts a comprehensive collider survey across LHC, HL-LHC, FCC-hh, FCC-ee, CEPC, and a future muon collider, recasting DY and VBS channels and enforcing EFT validity with momentum-transfer scales, while also presenting direct and indirect detection perspectives from LZ/XLZD, FERMI-LAT, and CTA. The study finds that, thanks to collider-cosmology complementarity, thermally produced Rayleigh DM at the hundreds-of-GeV scale can be thoroughly tested by next-generation experiments, and lighter candidates will explore largely uncharted parameter space beyond the thermal benchmark. Indirect detection, especially γ-ray line searches, already provides competitive constraints, with CTA offering significant reach at higher masses, while direct detection remains challenging due to loop suppression; UV completions play a critical role in interpreting EFT bounds, underscoring the need to consider both loop- and tree-level UV scenarios in future analyses.

Abstract

Dark Matter can interact with electroweak gauge bosons via higher-dimensional operators, in spite of being neutral under gauge interactions, much like neutral atoms interact with photons through Rayleigh scattering. This study explores effective interactions between a real scalar Dark Matter particle, singlet under the SM gauge group, and electroweak gauge bosons. We present a comprehensive analysis of current constraints and projected sensitivities from both lepton and hadron colliders as well as direct and indirect detection experiments in testing Rayleigh Dark Matter interactions. We find that, thanks to the complementarity between collider experiments and cosmological probes, thermally produced Rayleigh Dark Matter at the hundreds of GeV scale can be thoroughly tested with the next generation of experiments. For lighter candidates, upcoming forecasts will explore uncharted parameter space, significantly surpassing the thermal Dark Matter benchmark.

Figures (14)

• Figure 1: Bounds on ${\tilde{\cal C}}^{-1/2}_{\mathcal{B,W}}$, as defined in Eq. \ref{['eq:strong_matching']}, for a scalar Rayleigh DM candidate in the ${\tilde{\cal C}}_{\cal B} = 0$ (Left panel) and ${\tilde{\cal C}}_{\cal W} = 0$ (Right panel) planes. Solid lines represent current experimental constraints, while dashed lines illustrate future projections. The solid blue line represents the bounds from the LHC, while the dashed blue line corresponds to the high-luminosity phase projection. Brown, magenta, and green dashed lines indicate the projections for FCC-ee at the $Z$-pole, FCC-hh and the $\mu$C, respectively. Regarding LHC, HL-LHC, FCC-hh, $\mu$C the lines (both solid and dashed) represent the most stringent constraints obtained from the DY and VBS searches. Additionally, the dashed black line marks the thermal relic target, and the solid purple line indicates the bound from the FERMI experiment. Gray lines represent direct detection constraints, with the solid line for LZ-2022 and the dashed line for the projected next-generation XLZD experiment. The diagonal gray gradient represents the region allowed by the requirement $\Lambda_{\rm tree} > 2m_{\rm DM}$. The darker the region, the closer the coupling $g_{\rm tree}$ approaches its maximum value allowed by perturbative unitarity, $g_{\rm tree} \sim 4\pi$.
• Figure 2: Representative Feynman diagrams for DY mono$-\gamma$ pair-production (left) and VBS production (right) at an hadron collider of scalar $\phi$ particles through the operators of Eq. \ref{['eq:ops_1']}. The black dot represents an insertion of the effective operator.
• Figure 3: Constraints in the $({\tilde{\cal C}}_{\cal B},{\tilde{\cal C}}_{\cal W})$ plane for DY process at the LHC (left panel) and the high-luminosity LHC (right panel), shown for various DM mass values. The dashed lines indicate the directions associated with the couplings defined in Eq. \ref{['eq:EWSB-coeff']}. The orange, green, and brown lines correspond to the directions where ${\tilde{\cal C}}_{\gamma\gamma}$, ${\tilde{\cal C}}_{\rm ZZ}$, and ${\tilde{\cal C}}_{\gamma \rm Z}$ vanish, respectively.
• Figure 4: Constraints in the ${\tilde{\cal C}}_{\cal B}-{\tilde{\cal C}}_{\cal W}$ plane for the DY process at an $e^\pm$ collider operating at the Z-pole, comparing a CEPC-like machine (left panel) and an FCCee-like machine (right panel), shown for various DM mass values. The dashed lines indicate the directions associated with the couplings defined in Eq. \ref{['eq:EWSB-coeff']}. As expected, the ellipses are tilted toward the ${\tilde{\cal C}}_{\gamma \rm Z}=0$ direction, since at the Z-pole, the strongest sensitivity arises from the on-shell Z boson.
• Figure 5: Left Panel: The DY signal cross-section as a function of the DM mass at the 10 TeV muon collider. The solid and dashed lines for $\tilde{\mathcal{C}}_\mathcal{B}=0$ and $\tilde{\mathcal{C}}_\mathcal{W}=0$, respectively, with the other coefficient put at $1 \,{\rm TeV}^{-2}$. Right panel: Unit-normalized $p_T^\gamma$ distribution for the main background and the signal for several illustrative values of the DM mass for $\Tilde{C}_\mathcal{W}=0$.