Viability of Sub-TeV Higgsino Dark Matter with Nearly Mass-Degenerate Sleptons
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 the presence of nearly mass-degenerate sleptons in the thermal bath can allow the higgsino mass to be as light as $\sim 400$ 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 $M_2$.