LHC Mono-$W/Z$ Signatures as a Probe for Dark Matter Explanations of Astrophysical Excesses

Abstract: The inert two-Higgs doublet model (IDM) is a compelling framework for weakly interacting massive particles (WIMPs) linked to electroweak symmetry breaking. It can account for both the Galactic Center gamma-ray excess (GCE) and the AMS-02 antiproton anomaly while also satisfying relic density and direct detection constraints for dark matter (DM) masses in the $55-75$ GeV range. Three specific DM annihilation channels can be identified: Higgs resonance, $SA$ co-annihilation, and $SS\to WW<sup>{\ast}$ annihilation. Among these, the DM mass range of $70-75$ GeV with dominant $SS\to WW<sup>{\ast}$ annihilation has received less attention in collider searches. To validate this parameter space, we combine LHC searches for mono-$W/Z$ signatures. In particular, we develop a channel-separation strategy to disentangle the contributions of charged mass splitting ($Δ<sup>{\pm}$) and neutral mass splitting ($Δ<sup>0$) in the inert scalar sector at the LHC. Our results indicate that most of the parameter space consistent with these astrophysical anomalies in the $SS\to WW<sup>{\ast}$ annihilation regime will be testable at the High-Luminosity LHC. Specifically, from the leptonic channel we obtain a $2σ$ exclusion limit of $80 \lesssim Δ⁰ \lesssim 260$ GeV, while the hadronic channel yields $30 \lesssim Δ⁰ \lesssim 150$ GeV and $70 \lesssim Δ^{\pm} \lesssim 230$ GeV for $m_S = 70$ GeV.

• The paper demonstrates that LHC mono-W/Z searches can effectively probe the inert two-Higgs doublet model by exploiting resonant production in gauge-dominated regimes.
• It employs detailed Monte Carlo simulations at √s=14 TeV to optimize kinematic selections, achieving 2σ exclusions for key mass thresholds in the dark sector.
• The analysis bridges astrophysical excesses and collider tests, highlighting the complementarity of indirect detection and high-luminosity searches in dark matter investigations.

LHC Mono-$W/Z$ Signatures and the IDM Explanation for Astrophysical Dark Matter Excesses

Theoretical Motivation and Model Framework

This study targets the tension between observed indirect astrophysical dark matter (DM) signals—the Galactic Center gamma-ray excess (GCE) and the AMS-02 antiproton anomaly—and the limited reach of direct detection and standard relic density probes. The inert two-Higgs doublet model (IDM) extends the Standard Model (SM) with a $Z_2$-odd scalar doublet, yielding a stable, neutral scalar $S$ as a DM candidate. For $m_S$ in the interval 55–75 GeV, the IDM is strongly favored by combined fits to the GCE and AMS-02 anomalies, realizing the correct relic density and satisfying direct detection bounds via three dominant processes: Higgs resonance, $SA$ co-annihilation, and, uniquely, $SS \to WW^*$ annihilation.

In the preferred region $m_S \approx 70$–75 GeV, $SS \to WW^*$ is the dominant thermal freeze-out channel but is weakly constrained by direct detection owing to the necessity of a suppressed Higgs-portal coupling ($\lambda_S \lesssim 10^{-3}$). In this regime, gauge interactions govern both annihilation and collider signatures, while loop-induced direct detection rates are below the neutrino floor. Consequently, this astrophysically-motivated region requires collider exploration for definitive testing.

Collider Strategy and Parameter Space

The analysis proposes a dedicated LHC search strategy based on mono-$W$ and mono-$Z_2$0 signatures arising from associated production of DM pairs: $Z_2$1 with $Z_2$2. The mono-$Z_2$3 process probes charged-neutral mass splitting $Z_2$4 via the $Z_2$5 resonance, while mono-$Z_2$6 accesses the neutral splitting $Z_2$7 through the $Z_2$8 resonance. The model's structure ensures that the mono-$Z_2$9 cross sections, for $S$0–75 GeV and $S$1, are almost entirely insensitive to $S$2; gauge couplings and mass splittings dictate collider phenomenology.

Key resonance thresholds—$S$3 and $S$4—amplify signal rates sharply via on-shell production of $S$5 and $S$6. Beyond these thresholds, cross sections are suppressed due to parton luminosity limitations at high mass.

Simulation, Backgrounds, and Kinematic Discriminants

Detailed Monte Carlo simulations are carried out at $S$7 TeV (HL-LHC), including both leptonic ($S$8) and hadronic ($S$9) final states. The leptonic channel, focusing on $m_S$0, leverages clean OSSF dilepton signals and exploits kinematic features such as invariant mass windows around $m_S$1, missing transverse energy ($m_S$2), lepton $m_S$3, and back-to-back angularity between visible and invisible systems. Backgrounds ($m_S$4, $m_S$5, $m_S$6, $m_S$7, Drell-Yan, triboson) are suppressed by optimized selections exploiting the harder $m_S$8 and sharper $m_S$9 peaks in the signal near resonance.

The hadronic channel, with contributions from both $SA$0 and $SA$1 decays, employs stringent cuts on $SA$2, jet kinematics, and an adaptive dijet-mass window tuned to on-shell and off-shell regimes. The separation of signal from backgrounds ($SA$3+jets, $SA$4+jets) is further enhanced using the $SA$5 ratio. This is crucial in the compressed mass region, which is otherwise elusive in direct searches.

Numerical Results and Projected Sensitivity

The mono-$SA$6 leptonic channel achieves strong sensitivity near and above the $SA$7 threshold. For an integrated luminosity of $SA$8, there is 2$SA$9 exclusion for $SS \to WW^*$0 GeV; at $SS \to WW^*$1 (HL-LHC), this extends to $SS \to WW^*$2 GeV, corresponding to $SS \to WW^*$3 GeV. The signal statistical significance peaks sharply near the resonance, facilitating focused experimental tests.

The hadronic channel substantially enlarges the accessible parameter space by independently probing $SS \to WW^*$4 and $SS \to WW^*$5. For $SS \to WW^*$6 data, $SS \to WW^*$7 up to $SS \to WW^*$8 GeV is accessible, with $SS \to WW^*$9 sensitivity extending over much of the $m_S \approx 70$0 window preferred by indirect detection. This further expands at $m_S \approx 70$1, covering $m_S \approx 70$2 GeV and $m_S \approx 70$3 GeV.

Crucially, most of the IDM parameter space consistent with the astrophysical excesses and yielding a viable relic density—particularly the gauge-interaction-dominated region otherwise hidden from direct detection—becomes testable at the HL-LHC via these channels.

Implications and Future Directions

The paper demonstrates that collider-based mono-$m_S \approx 70$4 searches provide an essential, complementary probe of dark sector physics when direct detection loses sensitivity due to suppressed Higgs-portal couplings. The robustness of the mono-$m_S \approx 70$5 strategies—insensitivity to $m_S \approx 70$6, clear kinematic discriminators, and sharp resonance thresholds—positions them as necessary tools for testing WIMP DM models motivated by astrophysical anomalies.

The results indicate that much of the traditionally "blind-spot" region, where DM annihilates predominantly via gauge bosons and escapes detection in direct search experiments, can be conclusively explored with current-generation colliders once luminosity upgrades are complete. Combining LHC constraints with cosmic-ray and gamma-ray observables could be critical for unambiguously attributing observed excesses to DM rather than astrophysical backgrounds.

Looking forward, further exploration of the soft-lepton and jet-substructure techniques could enhance sensitivity to the most compressed regions of parameter space. Additionally, the model's collider signatures provide benchmarks for next-generation $m_S \approx 70$7 machines and future precision experiments.

Conclusion

This work presents a detailed strategy to probe, via LHC mono-$m_S \approx 70$8 signatures, the inert two-Higgs doublet model as a unified DM interpretation of both Galactic Center gamma-ray and AMS-02 antiproton excesses. By systematically mapping lepton and jet kinematic regimes, and exploiting mass-splitting tunings, the authors demonstrate that the HL-LHC will decisively test the favored IDM parameter space—especially the gauge-interaction regime inaccessible to direct detection. These results exemplify the necessity of multi-channel approaches in the search for DM explanations of astrophysical anomalies and highlight the complementary role of high-energy colliders in the broader DM phenomenology landscape (2605.07303).