Viability of Sub-TeV Higgsino Dark Matter with Slepton Coannihilation

TL;DR

This work reassesses higgsino-dominated dark matter in the MSSM when slepton coannihilation is active, showing that sub-TeV Higgsino masses (around $\sim 400$–$600$ GeV) can yield the correct relic density, unlike the pure-higgsino target near $1.1~\mathrm{TeV}$. A comprehensive parameter scan incorporating relic-density constraints and the latest LZ direct-detection limits reveals a strong dependence on the relative signs of $M_1$ and $M_2$: same-sign scenarios are largely excluded by LZ2024, while opposite-sign regions remain viable due to destructive interference in the neutralino-Higgs coupling. The surviving parameter space features highly compressed spectra with near-degenerate higgsino states and sleptons, making collider detection challenging but potentially accessible at HL-LHC or through indirect searches. Overall, the paper highlights a subtle interplay between coannihilation dynamics and direct-detection interference that preserves viable sub-TeV higgsino dark matter under current constraints and guides future experimental probes.

Abstract

The higgsino-like neutralino is a compelling dark matter candidate motivated by both cosmology and naturalness considerations. While a pure higgsino typically requires a mass of around $1.1~\mathrm{TeV}$ to satisfy the observed thermal relic abundance, the presence of light sleptons can significantly alter this requirement. In this work, we revisit higgsino dark matter within the Minimal Supersymmetric Standard Model (MSSM), focusing on scenarios with slepton coannihilation. We find that efficient coannihilation allows the higgsino mass to be as light as $\sim 400~\mathrm{GeV}$ while satisfying relic density constraints. We explicitly contrast the impact of recent direct detection updates: the LZ-2022 limits raise this lower bound to approximately $450~\mathrm{GeV}$, while the stringent LZ-2024 constraints further shift the viable mass floor to $\sim 500~\mathrm{GeV}$. Crucially, we demonstrate that the direct detection sensitivity is strongly dependent on the relative signs of the gaugino mass parameters $M_1$ and $M_2$. We find that scenarios with $M_1, M_2 > 0$ are fully excluded by LZ-2024. Conversely, configurations with opposite signs ($M_1/M_2 < 0$) remain broadly viable, as destructive interference in the neutralino-Higgs coupling efficiently suppresses the spin-independent cross section. Finally, we delineate the remaining viable parameter space for both the opposite-sign cases and the specific configurations with negative $M_1$ and $M_2$.

Figures (4)

• Figure 1: The predicted relic density as a function of DM mass for four successive stages of constraints: (Top-left) after applying our baseline cuts; (Top-right) adding the LZ2022 SI constraint; (Bottom-left) further imposing the stronger LZ2024 SI constraint; (Bottom-right) additionally imposing the relic-density requirement. Note that the botom right panel is actually a zoomed-in version of the blue band for bottom-left panel.
• Figure 2: The mass splittings between $\tilde{\chi}_2^0$ and $\tilde{\chi}_1^0$ (left), between $\tilde{\chi}_1^\pm$ and $\tilde{\chi}_1^0$ (middle), and between the lightest slepton $\tilde{l}_1$ and $\tilde{\chi}_1^0$ (right) as functions of the LSP mass for the final sample that satisfies all constraints, including LZ2024 SI and SD limits as well as the relic density requirement. For each case, the left subplot shows the mass splitting versus the LSP mass, while the right shows the corresponding histogram of relative mass splittings.
• Figure 3: The SI(left pannel) and neutron SD(right pannel) cross section as a function of DM mass for samples that pass all constraints except LZ2024 SI limit. The shaded area above the green line is excluded by LZ2024 SI limit. Besides,we also plot the neutrino fog region below the dashed bright purple line for the left pannel. The orange and blue points represent the cases where $M_1$ and $M_2$ have the same sign and opposite sign respectively.
• Figure 4: Distribution of the surviving parameter points in the $M_1$-$M_2$ plane, visualized using hexagonal binning. The colored bins represent points that satisfy all constraints, including the latest LZ2024 results. The color scale indicates the mean value of $\mu$ (left panel) and $\tan\beta$ (right panel) within each bin. Grey bins denote points that were allowed by previous constraints (including LZ2022) but are now excluded by the LZ2024 results.