Probing Freeze-In Dark Matter via a Spin-2 Portal at the LHC with Vector Boson Fusion and Machine Learning

Abstract

The persistent absence of signals in traditional dark matter searches has intensified interest in scenarios beyond the canonical weakly interacting massive particle paradigm. In this work, we investigate the collider phenomenology of feebly interacting dark matter produced via the freeze-in mechanism through a spin-2 portal. We consider a framework in which a massive graviton-like mediator couples minimally and universally to the energy--momentum tensor of both the Standard Model (SM) and the dark sector. Such interactions arise naturally in extra-dimensional constructions and effective theories of gravity, providing a theoretically well-motivated and predictive setup. We systematically connect early-Universe cosmology with collider observables by identifying regions of parameter space consistent with freeze-in conditions and the observed dark matter relic abundance, and examining their testability at the Large Hadron Collider (LHC). Focusing on bosonic fusion production channels, which are particularly sensitive to spin-2 interactions, we analyze invisible mediator decay signatures and assess current and projected experimental sensitivities. To enhance sensitivity in this challenging regime of feeble couplings, we develop a search strategy based on machine-learning algorithms. Our results demonstrate that collider searches can probe substantial regions of the cosmologically viable freeze-in parameter space, highlighting the high-luminosity LHC as a powerful laboratory for feebly interacting dark sectors. This study establishes a concrete and complementary pathway to test freeze-in dark matter scenarios through spin-2 portals, thereby bridging gravitationally motivated new physics, cosmology, and high-energy collider experiments.

Figures (12)

• Figure 1: Ratio of decay width to spin-2 mediator mass for dark matter masses of $m_\chi = 0.1~\text{GeV}$ and $m_\chi = 100~\text{GeV}$. The solid line represents the contour of constant relative decay width $\Gamma / m_G = 0.01$.
• Figure 2: Theoretical constraints on the mediator couplings to dark matter and photons, derived from resonant and off-resonant freeze-in production, as well as the requirement that the relic abundance satisfies $\Omega_\chi h^2 \leq 0.12$ for reheating temperatures of $10~\mathrm{MeV}$, $100~\mathrm{MeV}$, and $1~\mathrm{GeV}$. For the line contours, the region above each line denotes the allowed parameter space.
• Figure 3: Representative Feynman diagrams of $pp\to \nu\bar{\nu}+$jets SM background production at the LHC.
• Figure 4: Representative Feynman diagram of $pp\to \chi\chi jj$ signal production via photon-photon fusion at the LHC.
• Figure 5: Dijet transverse momentum distributions for $m_G=100\text{ GeV}$ and $m_G=1000\text{ GeV}$ signals and dominant SM backgrounds