[Paper Review] Kaluza-Klein Dark Matter: Direct Detection vis-a-vis LHC (IDM 2008 proceeding)

S. Arrenberg, L. Baudis+3 more|arXiv (Cornell University)|Nov 26, 2008

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper investigates Kaluza-Klein dark matter in universal extra dimension (UED) models, showing that direct detection experiments and the LHC are highly complementary in probing the parameter space of lightest Kaluza-Klein partner (LKP) candidates. It demonstrates that combining collider constraints, direct detection sensitivities, and cosmological relic density limits can cover nearly all of the relevant parameter space—especially for the 5D UED $γ_1$ LKP—highlighting the critical role of coannihilations and mass splittings in determining detectability.

ABSTRACT

We explore the phenomenology of Kaluza-Klein (KK) dark matter in very general models with universal extra dimensions (UEDs), emphasizing the complementarity between high-energy colliders and dark matter direct detection experiments. In models with relatively small mass splittings between the dark matter candidate and the rest of the (colored) spectrum, the collider sensitivity is diminished, but direct detection rates are enhanced. UEDs provide a natural framework for such mass degeneracies. We consider both 5-dimensional and 6-dimensional non-minimal UED models, and discuss the detection prospects for various KK dark matter candidates: the KK photon γ_1 (5D) the KK Z-boson Z_1 (5D) and the spinless KK photon γ_H (6D). We combine collider limits such as electroweak precision data and expected LHC reach, with cosmological constraints from WMAP and the sensitivity of current or planned direct detection experiments. Allowing for general mass splittings, we show that neither colliders, nor direct detection experiments by themselves can explore all of the relevant KK dark matter parameter space. Nevertheless, they probe different parameter space regions and the combination of the two types of constraints can be quite powerful. For example, in the case of γ_1 in 5D UEDs the relevant parameter space will be almost completely covered by the combined LHC and direct detection sensitivities expected in the near future.

Motivation & Objective

• To explore the phenomenology of Kaluza-Klein dark matter in general UED models with universal extra dimensions.
• To assess the complementarity between high-energy collider searches (LHC) and direct detection experiments in probing the parameter space of Kaluza-Klein dark matter candidates.
• To investigate the impact of mass splittings between the LKP and heavier KK states on both direct detection rates and collider sensitivity.
• To evaluate the role of coannihilations in relic density calculations for LKP candidates such as $\gamma_1$, $Z_1$, and $\gamma_H$.
• To determine the combined sensitivity of LHC, direct detection experiments, and cosmological constraints in constraining the full parameter space of UED dark matter.

Proposed method

• Modeling 5D and 6D UED scenarios with general mass splittings between the LKP and other KK states, including quarks, gauge bosons, and Higgs bosons.
• Computing effective relic densities using Boltzmann equations that include coannihilation contributions from all relevant KK particles.
• Calculating spin-independent LKP-nucleon scattering cross sections via tree-level exchange diagrams involving KK quarks and Higgs bosons.
• Applying constraints from electroweak precision data, WMAP 2σ relic density limits, and current/future direct detection experiments (CDMS, XENON10, ton-scale detectors).
• Projecting LHC reach using the $4\ell + \cancel{E}_T$ channel with 100 fb⁻¹ luminosity to determine sensitivity to large mass splittings.
• Mapping parameter space in $\Delta$ vs. $m_{LKP}$ and $m_h$ vs. $m_{LKP}$ planes to compare exclusion limits from different probes.

Experimental results

Research questions

• RQ1 How do mass splittings between the LKP and heavier KK states affect direct detection cross sections and collider signatures in UED models?
• RQ2 To what extent can the LHC and direct detection experiments individually and jointly constrain the parameter space of Kaluza-Klein dark matter?
• RQ3 What is the impact of coannihilations on the relic density of the LKP, and how does this affect cosmological constraints?
• RQ4 How do Higgs mass variations influence direct detection sensitivity, and is the Higgs exchange contribution relevant in the allowed parameter space?
• RQ5 Can the combination of collider, direct detection, and cosmological constraints fully cover the viable parameter space for $\gamma_1$, $Z_1$, and $\gamma_H$ LKP candidates?

Key findings

• For the 5D UED $\gamma_1$ LKP, the combination of LHC and direct detection sensitivities is expected to cover nearly the entire viable parameter space.
• Direct detection experiments are most sensitive to small mass splittings ($\Delta \lesssim 50\%$), with current experiments already excluding small $\Delta$ values and future ton-scale detectors covering most of the remaining parameter space.
• The LHC is more sensitive to large mass splittings, with 100 fb⁻¹ luminosity expected to cover the yellow-shaded region in the $\Delta$ vs. $m_{LKP}$ plane for $\gamma_1$.
• Coannihilations significantly reduce relic density predictions for $\gamma_1$ and $Z_1$, with the effect being non-monotonic and dependent on the specific mass spectrum.
• Higgs exchange contributes to direct detection cross sections but plays a secondary role, as its influence is mostly confined to already excluded regions of parameter space.
• Cosmological constraints from WMAP limit the LKP mass to below ~500 GeV for $\gamma_1$ in minimal UED, with even tighter bounds if the LKP contributes only a fraction of the total dark matter.