Vector Higgs-Portal Dark Matter: How UV Completion Reopens Viable Parameter Space

Abstract

The particle nature of dark matter (DM) remains one of the central open problems in modern physics. Among the most extensively studied candidates are weakly interacting massive particles, whose parameter space is now under strong pressure from direct detection, indirect detection, and collider searches. In this work we revisit the Higgs-portal scenario with vector DM, first in an effective-field-theory description and then in a renormalizable UV-complete realization. We show that the effective Higgs-portal model with a Proca vector coupled quadratically to the Standard Model Higgs is essentially excluded over almost all of its parameter space by current direct-detection limits, with only a narrow region near the Higgs resonance surviving with a required fine tuning of the DM to Higgs mass that should at the permille level. We then consider a UV completion based on an additional gauged $U(1)_X$ symmetry, in which the DM candidate is a massive vector boson $V$ and the scalar sector is extended by a dark Higgs that mixes with the Standard Model Higgs. In this framework, the presence of a second scalar mediator opens an additional resonant annihilation channel and can substantially weaken the direct-detection constraints. In particular, when the DM mass lies sufficiently close to the heavy-scalar resonance, $m_V \simeq m_{H_2}/2$, the coupling required to reproduce the observed relic abundance can lie up to about two orders of magnitude below current direct-detection bounds, thereby opening viable parameter space that is absent in the effective description. Our results highlight the importance of going beyond the effective-field-theory approximation in Higgs-portal vector DM models and show that UV-complete realizations can qualitatively change the phenomenological conclusions.

Figures (6)

• Figure 1: Tree-level annihilation topologies for vector DM $V$ in the Higgs-portal setup. Left: $s$-channel Higgs exchange, $VV\to h^\ast\to XX$, where $X$ denotes any kinematically accessible SM final state (e.g. $f\bar{f}$, $W^+W^-$, $ZZ$, or $hh$). Middle: $t/u$-channel $V$ exchange contributing to $VV\to hh$. Right: contact interaction $VVhh$ arising after electroweak symmetry breaking.
• Figure 2: Loop-induced annihilation $VV\to gg$ mediated by an off-shell Higgs $h^\ast$ through a quark loop.
• Figure 3: Fractional contribution $\mathcal{F}_i=\langle\sigma v\rangle_i/\langle\sigma v\rangle_{\rm tot}$ of each annihilation channel as a function of the DM mass, computed for a fixed $\lambda_{HV}=0.01$.
• Figure 4: Combined constraints for the Higgs-portal EFT with vector DM $V$. The black solid curve shows the model parameters that satisfy the relic-density condition. The blue dashed curve shows the LZ upper limit on the SI scattering cross section recast as a bound on $\lambda_{HV}$LZ:2024zvo, the brown dashed curve shows the projected DARWIN sensitivity recast in the same plane DARWIN:2016hyl, the red dotted curve shows the ATLAS bound from invisible Higgs decays $h\to VV$ (relevant only for $m_V<m_h/2$) ATLAS:2022yvh, and the green dot-dashed curve shows the dSph $\gamma$-ray limits recast as bounds on $\lambda_{HV}$McDaniel:2023bju. The gray shaded region indicates parameter values excluded by the perturbative-unitarity requirement discussed in Sec. \ref{['sec:validity']}. The lower panel shows a zoom into the resonant region $m_V\simeq m_h/2$, where the relic-density requirement can still be compatible with present direct-detection limits.
• Figure 5: Combined constraints for the UV-complete vector Higgs-portal model. For each benchmark choice of the heavy scalar mass $m_{H_2}$ (rows) and mixing angle $\sin\theta$ (columns), the black solid curve shows the value of $\lambda_{HS}$ that reproduces the observed relic abundance $\Omega_{\rm DM}h^2\simeq 0.12$. The blue dashed curve is the current LZ limit on the spin-independent DM--nucleon scattering cross section, recast as an upper bound on $\lambda_{HS}$LZ:2024zvo. The red dashed curve shows the projected DARWIN sensitivity, also recast as a bound on $\lambda_{HS}$DARWIN:2016hyl. The green dot-dashed curve shows the dwarf-spheroidal $\gamma$-ray limits from Fermi-LAT, recast as bounds on $\lambda_{HS}$McDaniel:2023bju. Where visible, the red dotted curve shows the ATLAS bound from invisible Higgs decays, relevant for $m_V<m_{H_1}/2$ATLAS:2022yvh. In each panel, viable thermal-relic solutions require the relic-density curve to lie below the relevant experimental bounds.