Revisiting Bino-Slepton Coannihilation Dark Matter in Light of Recent Experimental Results

Abstract

Despite being a simple and well-motivated thermal relic scenario, coannihilation dark matter (DM) has remained largely unexplored experimentally due to the difficulty of probing its nearly degenerate mass spectrum. Recent LHC searches, however, have significantly improved the sensitivity to such compressed spectra, motivating a reassessment of the viable parameter space. We revisit the bino-slepton coannihilation scenario in supersymmetric (SUSY) models, incorporating the latest experimental results. We first focus on the minimal scenario, in which only the bino-like neutralino and left- or right-handed sleptons are light ($O(100)$ GeV), with all other SUSY particles decoupled. We find that the dark matter mass is constrained to be in the range of about 170-420 GeV (130-430 GeV) for left-handed (right-handed) slepton coannihilation, with lower bounds set by recent LHC searches. We then investigate scenarios with light higgsino, where direct detection experiments impose strong constraints on the higgsino mass. We also discuss the implications of these constraints for the muon $g-2$ in the so-called BHR, BHL, and BLR scenarios with coannihilation DM, and find that the combined LHC and LZ limits constrain the SUSY contribution to $|Δa_μ^{\rm SUSY}|\lesssim 1.2\times10^{-9}$.

Figures (6)

• Figure 1: The relic abundance contours and the LHC constraints in the $(m_{\tilde{\ell}}, \Delta m)$ plane in the minimal slepton coannihilation scenario for the BL (left) and BR (right) models. The upper panels correspond to the cases where the bino coannihilates with the selectron, and the lower panels to those with the smuon. Here, $\Delta m = m_{\tilde{\ell}}-m_{\tilde{\chi}^0_1}$ and $m_{\tilde{\ell}}$ denotes the lighter selectron or smuon mass. The solid lines show the contours of $\Omega_{\tilde{\chi}^0_1}h^2 = \Omega_{\rm DM}h^2 = 0.120$. The orange-shaded regions represent the exclusion limits from the CMS soft-lepton search CMS:2025ttk. In the BL scenario (left panels), the dot-dashed line indicates the boundary where the LSP changes from the neutralino to the slepton; the region below the line corresponds to a slepton LSP. The green and purple lines correspond to $\tan\beta = 3$ and $40$, respectively.
• Figure 2: The same as Fig. \ref{['fig:BR_BL']}, but for the flavor-universal slepton masses ($M_{L_1}=M_{L_2}=M_{L_3}$ and $M_{R_1}=M_{R_2}=M_{R_3}$). The solid and dot-dashed lines denote the relic-abundance contours and the LSP boundaries, respectively, while the orange-shaded regions indicate the LHC exclusions. Note that the x-axis shows the lighter selectron/smuon mass.
• Figure 3: Experimental constraints for the BHR model in the $(M_1,|\mu|)$ plane with $\tan\beta=10,40$ (upper and lower panels) and both signs of $\mu$ (left panels: $\mu<0$, right panels: $\mu>0$). At each point in the plane, the right-handed slepton soft mass $M_R$ is chosen to reproduce the observed relic abundance $\Omega h^{2}=0.120$ with a bino-like neutralino as the DM, and the red dashed contours indicate the corresponding physical right-handed slepton masses $m_{\tilde{\ell}_R}$ ($\ell=e,\mu$) in GeV. The navy shaded region is excluded by the LZ experiment ($90\%$ CL) LZ:2024zvo, while the orange shaded region is excluded by the CMS slepton search CMS:2025ttk. The greyed-out region corresponds to the parameter space where no solution reproducing the observed relic abundance exists. The solid (dashed) black contours represent positive (negative) values of the SUSY contribution to the muon $g-2$, $\Delta a_\mu^{\rm SUSY}$, in units of $10^{-10}$.
• Figure 4: Same as Fig. \ref{['fig:BHR']}, but for the BHL model with light left-handed sleptons. The red dashed contours indicate the values of $m_{\tilde{\ell}_L}$ ($\ell=e,\mu$) required to reproduce the observed relic abundance, while all other conventions are the same as in Fig. \ref{['fig:BHR']}.
• Figure 5: Same as Figs. \ref{['fig:BHR']} and \ref{['fig:BHL']}, but for $\tan\beta = 3$ and $\mu<0$. The left (right) panel corresponds to the BHL (BHR) model.