Chasing higgsino dark matter at colliders in the neutrino fog era

TL;DR

Higgsino-dominated neutralino dark matter remains a compelling possibility, with a thermal relic mass near $m_{\tilde{N}_1} \approx 1.1$ TeV; however, direct-detection limits are driving higgsino purity to the point where neutrino backgrounds (the neutrino fog) limit discovery potential. By translating these purity bounds into lower limits on gaugino masses under two well-motivated frameworks—gaugino mass unification and anomaly-mediated SUSY breaking—the paper shows that current and near-future direct-detection experiments push bino/wino/gluino masses into multi-TeV scales, diminishing direct collider prospects for pure higgsinos. Consequently, the authors advocate complementary collider strategies that rely on heavier superpartners decaying to higgsinos (notably stop and wino production) to probe the higgsino DM sector well into the neutrino fog era, with HL-LHC potential reaching stops up to ~1.7 TeV and winos up to ~1.4 TeV in favorable channels. The work emphasizes that future facilities (e.g., muon colliders or FCC) and indirect detection/EDM searches will be crucial to fully test this minimal DM scenario beyond the neutrino-floor limitations of direct detection.

Abstract

Higgsinos can be the lightest supersymmetric particles, allowing for either a full or partial dark matter interpretation, with the correct thermal freeze-out abundance obtained for masses near 1.1 TeV. Dark matter direct detection experimental results, now rapidly approaching the neutrino fog, imposes increasingly stringent requirements on higgsino purity. We begin by summarizing the purity constraints implied by the current strong limits from the LUX-ZEPLIN experiment in 2024, presenting them as lower bounds on gaugino masses in scenarios with decoupled sfermions and heavy Higgs bosons. We further quantify how these constraints will evolve as direct detection approaches various neutrino fog discovery and exclusion definitions and future exclusion projections. Finally, given that nearly pure higgsinos remain notoriously challenging to probe directly at colliders, we explore complementary signatures in which higgsinos are produced from the decays of heavier superpartners, where additional leptons and jets can be used for triggering. In particular, we advocate for searches of stop and wino pairs decaying directly to higgsinos as a promising means to probe higgsino dark matter well into the neutrino fog era.

Figures (8)

• Figure 3.1: Comparison of different neutrino fog discovery and exclusion definitions for the spin-independent nucleon (SI, left panel) and spin-dependent neutron (SDn, right panel) cross-sections. The vertical axes show the cross-sections scaled by (1 TeV$/m_{\chi}$), so that present and future limits are close to horizontal lines. Results are shown as a function of the WIMP mass $m_{\chi}$ in the range relevant for thermal dark matter higgsinos. The upper shaded region in each figure is the current LZ2024 90% CL exclusion. In the left panel, the other lines are, from top to bottom: the $5\sigma$ discovery and $3\sigma$ excess neutrino fog levels, the projected PandaX-xT and XLZD 90% CL limits with 200 tonne-years, the 90% CL exclusion neutrino fog, and the XLZD 90% CL limit with 1000 tonne-years. The right panel shows the $5\sigma$ discovery and $3\sigma$ excess neutrino fog lines, the projected PandaX-xT 90% CL limit with 200 tonne-years, and the 90% CL exclusion neutrino fog level.
• Figure 3.2: Minimum values of the bino mass $m_{\tilde{N}_3} \approx M_1$ (top) and the gluino pole mass $m_{\tilde{g}}$ (bottom) allowed by dark matter direction detection bounds in models where gaugino masses are assumed to unify at $M_{\rm GUT} = 2\times 10^{16}$ GeV. The left panels assume higgsino-like LSP with mass near 1.1 TeV so that $\Omega_\text{LSP} h^2 = 0.12$, while the right panels fix the higgsino-like LSP mass to be $m_{\tilde{N}_1} = 200$ GeV with $\Omega_\text{LSP} h^2$ fixed by the thermal relic abundance. All squark and slepton and heavy Higgs boson masses are taken to be equal to the gluino mass. The shaded region in each panel is the current LZ2024 exclusion. The other lines are various neutrino fog definitions and future exclusion projections as labeled. For the bolder portion of each curve with $\mu<0$ and $\tan\beta \lesssim 2$ in the right panels, the minimum gaugino masses are determined by the SDn cross-section in each case.
• Figure 3.3: Minimum wino-like chargino mass $m_{\tilde{C}_2}$ allowed by dark matter direction detection bounds, in AMSB models. The left panel assumes higgsino-like LSP with mass near 1.1 TeV so that $\Omega_\text{LSP} h^2 = 0.12$, while the right panel fixes the higgsino-like LSP mass to be $m_{\tilde{N}_1} = 200$ GeV, which turns out to imply $\Omega_\text{LSP} h^2 \approx 0.004$ to $0.007$. All squark and slepton and heavy Higgs boson masses are taken equal to the gluino mass. In each panel, the shaded region is the result for the current LZ2024 bounds, and the other lines assume various neutrino fog definitions and future exclusion projections as labeled. For the bolder portion of each curve with $\mu<0$ and $\tan\beta \lesssim 2$ in the right panel, the minimum wino mass is determined by the SDn cross-section.
• Figure 4.1: The decay length $c\tau$ in cm (left panel) and branching ratios (right panel) for charged higgsinos as a function of the mass difference $\Delta M_+ = m_{\tilde{C}_1} - m_{\tilde{N}_1}$.
• Figure 4.2: Higgsino-like LSP mass $m_{\tilde{N}_1}$ as a function of the stop mass $m_{\tilde{t}_1}$ that reproduces various values of $\Omega_\text{LSP} h^2$, as labeled, for $\tan \beta = 1.6$ and $\mu < 0$. The gaugino mass parameters, and all other scalar masses (except $M_h$), are set to 10 TeV. In order to show a rough estimate of the Run 2 LHC sensitivity with 139 fb$^{-1}$ at $\sqrt{s}=13.6$ TeV in the absence of reported searches targeting this specific scenario, we show as the light blue shaded region the reported exclusions for sbottoms from ref. ATLAS:2021yij.