Dark Z-mediated dark matter with verifiable exotic scalars

TL;DR

This work proposes a WIMP DM framework where a dark $Z_D$ mediator mixes with the SM $Z$ via an extra dark Higgs doublet and a scalar singlet, enabling DM annihilation into exotic scalars and scalar/vector final states during freeze-out. The authors perform a comprehensive phenomenological analysis, enforcing scalar potential perturbativity/positivity and oblique constraints, Higgs signal-strength and exotic-decay limits, $Z_D$ interactions, and DM relic density and direct detection bounds, using a suite of tools ($\text{SARAH}$, $\text{micrOMEGAs}$, $\text{HiggsTools}$, $\text{DarkCast}$). They identify viable parameter regions with DM in the 10 GeV–5 TeV range and a rich scalar sector, where direct detection can be suppressed by a small mixing angle $\theta_X$, and the relic density is set by $Z_D$-mediated annihilations into bosons rather than secluded channels. The study also highlights HL-LHC prospects for probing light exotics through exotic Higgs decays such as $H_1\to SS$ (with $S=H_3,A$) and resonant heavy scalar channels $gg\to H_2\to H_1H_1$ or $gg\to A\to ZH_1$, offering complementary handles to collider searches, direct detection, and DM relic abundance constraints. Overall, the model demonstrates a viable, testable dark sector scenario where a connected scalar and gauge sector governs both freeze-out dynamics and collider phenomenology.

Abstract

In this work, we study a dark matter scenario where a dark Z boson possessing mass mixing with the SM Z boson couples to the DM candidate and serves as the portal to the SM. The UV origin of the mass mixing in the form of an extra dark Higgs doublet and a scalar dark singlet provides new exotic scalars which can constitute the final state of DM annihilation during freeze-out. We find that existing constraints on the observed Higgs coupling strength, exotic Higgs searches and dark matter observables complement each other, while future searches for exotic Higgs decays and resonant heavy scalars at HL-LHC will be sensitive to part of the allowed parameter space.

Figures (7)

• Figure 1: Dark matter annihilation channels. The left channel is the leading diagram for annihilation into two scalars. The possible combinations of $S_A,S_B$ are $\{H^+H^-,AH_i\}$. The diagram on the right side is the leading contribution to annihilation into a scalar and a vector boson. The possible combinations of $S_A,V_B$ are $\{H^+W^-,H_iZ,H_iZ_D\}$.
• Figure 2: Allowed parameter space in the plane of $\tan\beta$ and $\cos(\theta_{12}-\beta)$. The gray region corresponds to points satisfying the $Z$ boson mass, perturbative scalar couplings, scalar potential positivity and SM Higgs-like couplings constraints. Orange points additionally satisfy tree-level perturbative unitarity constraints of the scalar couplings. The red horizontal band represents the limits from charged Higgs-induced flavor-violating processes. The region below the band is excluded for any charged Higgs mass, while points inside the band can be allowed if the charged Higgs is heavy enough to avoid the constraint. Points above the band are allowed for the masses obtained in our scan.
• Figure 3: Left: Parameter space in the $m_{Z_D}-\sin\theta_X$ plane of the scan defined in eq. \ref{['eq:scanlimits']} in blue, together with the limits from dark photon searches at LHCb LHCb:2019vmc recast to the dark $Z$ model Ilten:2018crwBaruch:2022esd in green. Right: Oblique parameter constraints on the generated benchmark points. The red oval corresponds to the $90\%$ CL contour of the global fit ParticleDataGroup:2024cfk when taking $U=0$.
• Figure 4: Scalar spectrum of generated points. These points satisfy perturbativity, perturbative unitarity, scalar positivity, $H_1$ SM-like properties, oblique parameters and $Z$ and $H_1$ decay width constraints. Left: Scalar spectrum in the $m_{H^+}-m_A$ plane. Right: Scalar spectrum in the $m_{H^+}-m_{H_3}$ plane. For all points, $m_{H_2}\approx m_{H^+}$.
• Figure 5: Dark Matter constraints on $m_{DM}$ and $m_{Z_D}$. Left: Benchmark points satisfying theoretical, electroweak, Higgs signal strengths, exotic scalar and relic density constraints in the $m_{DM}-m_{Z_D}$ plane. Blue, green and red diagonal lines correspond to $m_{Z_D}/m_{DM}=5,\,2$ and $1$, respectively. Right: Same set of benchmark points in the $m_{DM}-\sigma_{SI}$ plane. Magenta region is excluded by the latest results of the LZ collaboration LZ:2024zvo. Blue dashed line corresponds to projected XLZD limits for 200 ton-years XLZD:2024nsu of exposure, representative of a conservative projection for next-generation experiments. In solid green, we show the neutrino floor for Xenon OHare:2021utq, the material used in both LZ and XLZD. In both panels, we show the final state of the largest annihilation cross section at freeze-out according to the shape of the point as indicated in the insets. Here $H_i=H_1,\,H_2$ or $H_3$.