Current status and prospects of light bino-higgsino dark matter in natural SUSY

This paper investigates the viability of light bino-higgsino dark matter within natural supersymmetry under recent experimental constraints, finding that while a small portion of the parameter space survives with a sub-dominant relic density, it will be fully tested by future High-Luminosity LHC searches.

The Gist

Imagine the universe is a giant, complex machine, and for a long time, scientists have been trying to figure out what makes up the "invisible glue" holding it together. This invisible glue is called Dark Matter. We know it's there because of how galaxies spin, but we've never actually seen a single particle of it.

This paper is like a detective story where the authors are trying to find a specific type of suspect: a "light bino-higgsino" dark matter particle. This suspect lives in a theory called Natural Supersymmetry (SUSY), which is a popular idea that suggests every known particle has a heavier, hidden "twin."

Here is the breakdown of their investigation, using simple analogies:

1. The Suspect Profile: The "Light" Twin

In the world of these hidden twins, there are different types. The authors are looking at a specific duo:

• The Higgsino: A twin related to the Higgs boson (the particle that gives things mass).
• The Bino: A twin related to the force that carries electricity and magnetism.

In a "Natural" universe, the Higgsino twin shouldn't be too heavy; otherwise, the universe would feel "unnatural" or require too much fine-tuning to work. The authors set a rule: the Higgsino must be relatively light (between 100 and 350 GeV). They then let the Bino twin vary in weight, from very light (10 GeV) up to the same heavy limit.

2. The Great Filter: The "LZ" and "LHC" Police

The authors ran a massive computer simulation to see which combinations of these twins could survive in our universe. They had to pass two very strict tests:

• The "LZ" Test (Direct Detection): Imagine the LZ experiment is a giant, ultra-sensitive net trying to catch these particles as they drift through Earth. If the particle bumps into an atom in the detector, it makes a splash. The latest results from the LZ experiment (from 2025 in this paper's timeline) are like a net with holes so small that almost any "splash" would be caught.
  • The Result: Most of the suspects were caught and eliminated. The ones that survived are so quiet they barely make a splash at all.
• The "LHC" Test (Collider Search): This is like a high-speed car crash experiment at the Large Hadron Collider (LHC). Scientists smash particles together to see if these twins pop out. The current 13 TeV LHC has already caught some of the suspects. The future 14 TeV HL-LHC (High Luminosity) will be an even bigger, faster crash test that will catch the rest.

3. The Shocking Discovery: The "Sub-Plot" Villain

Here is the biggest twist in the story. Usually, scientists hope to find a dark matter particle that makes up 100% of the invisible glue in the universe.

However, this paper found that the surviving suspects (the light bino-higgsinos) are terrible at being the only dark matter.

• The Analogy: Imagine you are looking for a person who fills a whole swimming pool. You find a person, but they only fill a single teaspoon of water.
• The Reality: The paper concludes that if this specific particle exists, it can only make up about 2% of the total dark matter in the universe. The other 98% must be something else entirely (the authors suggest it might be "axions," another type of invisible particle).

4. Why Did They Survive? The "Z-Resonance" Escape

How did these particles survive the strict LZ net if they are so light?

• The Analogy: Think of the "Z-resonance" as a specific speed bump on a road. If a car hits the speed bump at exactly the right speed, it bounces perfectly and doesn't crash.
• The Reality: The surviving particles are tuned to a very specific mass (about half the mass of the Z boson). This allows them to annihilate (destroy each other) very efficiently in the early universe, leaving very few of them behind today. Because there are so few left, they don't bump into the LZ detector often enough to get caught.

5. The Final Verdict

• Current Status: The "light bino-higgsino" scenario is not dead, but it is severely squeezed. It can no longer be the main explanation for dark matter. It is now a "side character" in the story of the universe.
• Future Outlook: The paper predicts that the next generation of the LHC (the HL-LHC) will likely catch the last few survivors. If they don't find them there, this specific theory will be completely ruled out.
• The "Blind Spot" is Gone: In the past, scientists thought there might be a "blind spot" where the particles hid perfectly from detectors. This paper shows that the 2025 LZ results are so sensitive that even those hiding spots are now exposed.

In summary: The authors looked for a specific, light dark matter particle. They found that while a few might still exist, they are too rare to be the main dark matter we see in the sky. They are likely just a small, hidden fraction of the total, and the next big particle collider will probably find them or prove they don't exist at all.

Technical Summary

Technical Summary: Current Status and Prospects of Light Bino-Higgsino Dark Matter in Natural SUSY

Problem Statement
The paper addresses the viability of light bino–higgsino neutralino dark matter (DM) within the framework of Natural Supersymmetry (SUSY). While Natural SUSY predicts light higgsinos (mass parameter $|\mu| \lesssim 350$ GeV) to minimize electroweak fine-tuning ($\Delta_{EW} < 30$), a pure higgsino LSP typically yields a thermal relic density far below the observed Planck value ($\Omega h^2 \approx 0.12$) due to rapid annihilation. The study investigates whether a mixed bino–higgsino scenario, achieved by relaxing gaugino mass unification, can satisfy current experimental constraints while remaining consistent with naturalness. The central challenge is determining if such a scenario can survive stringent limits from the 2025 LUX-ZEPLIN (LZ) direct detection results, LHC electroweakino searches, and Higgs physics, and whether it can account for the total DM abundance or only a sub-dominant fraction.

Methodology
The authors perform a systematic scan of the Minimal Supersymmetric Standard Model (MSSM) parameter space using a Markov Chain Monte Carlo (MCMC) approach based on the Metropolis-Hastings algorithm. The scan focuses on the electroweakino sector with the following parameter ranges:

• Higgsino mass parameter: $100 \le |\mu| \le 350$ GeV (constrained by $\Delta_{EW} < 30$).
• Bino mass parameter: $10 \le M_1 \le 350$ GeV.
• Trilinear coupling: $|A_t| \le 4000$ GeV.
• $\tan \beta$: $5 \le \tan \beta \le 50$.
• All other superpartners (squarks, sleptons, gluinos) are decoupled at 3 TeV to ensure consistency with the 125 GeV Higgs mass and evade colored sparticle constraints.

The analysis employs a total $\chi^2$ likelihood function ($\chi^2_{tot}$) incorporating:

1. Relic Density: A one-sided penalty where $\chi^2 = 0$ if the predicted density is below the Planck central value ($0.1186 \pm 0.002$), and a Gaussian penalty if it exceeds it. This treats the neutralino as a potential sub-dominant component.
2. Direct Detection (DD): Constraints from LZ (2025) results, rescaling the theoretical spin-independent (SI) cross-section by the DM fractional abundance $f = \Omega_{\chi^0_1}h^2 / 0.118$.
3. Flavor Physics: Constraints from rare $B$ decays ($B \to X_s\gamma$, $B_s \to \mu^+\mu^-$, $B^+ \to \tau^+\nu_\tau$).
4. Higgs Physics: Constraints on SM-like and extra Higgs bosons using HiggsSignals.
5. Collider Searches: Exclusion limits from 13 TeV LHC searches (electroweakinos with leptons + MET) and projections for the 14 TeV High-Luminosity (HL) LHC with 3000 fb$^{-1}$.

A parameter point is considered viable if $\chi^2_{tot} < 6$.

Key Contributions and Results

• Survival of a Sub-Dominant Z-Resonance: The study finds that while large portions of the parameter space are excluded, a narrow region remains viable. This surviving region is strictly localized to the Z-resonance ($m_{\tilde{\chi}^0_1} \approx m_Z/2$).
• Exclusion of Co-annihilation and H-Resonance:
  • The compressed spectrum region ($M_1 \sim \mu$), where bino–higgsino co-annihilation typically enhances relic density, is completely ruled out by the 2025 LZ SI limits due to large neutralino mixing.
  • The "theoretical blind spots" (where $M_1/\mu < 0$ induces cancellation in the Higgs coupling) that previously allowed the H-resonance region to evade detection are now excluded by the overwhelming stringency of the LZ limits.
• Relic Density Suppression: In the surviving Z-resonance region, the neutralino relic density is significantly suppressed. The fractional abundance $f$ is constrained to $5 \times 10^{-5} \lesssim f \lesssim 0.02$. Consequently, the light bino–higgsino neutralino can constitute at most ~2% of the total DM abundance.
• Mass Constraints: The requirement of $\chi^2_{tot} < 6$ forces the bino mass $M_1$ to be near the Z-resonance ($\sim 45$ GeV), while the higgsino mass $|\mu|$ is pushed to $|\mu| \gtrsim 170$ GeV (raised to $\sim 200$ GeV by current LHC data). This creates a large mass splitting ($\Delta m \gtrsim 125$ GeV) between the LSP and the next-to-lightest supersymmetric particles (NLSPs), rendering co-annihilation ineffective.
• Collider Prospects: Current 13 TeV LHC searches have already excluded parts of the parameter space. The authors project that the future 14 TeV HL-LHC with 3000 fb$^{-1}$ luminosity will be capable of probing the remaining viable region within the defined naturalness constraints.
• Muon $g-2$: The electroweakino contribution to the muon anomalous magnetic moment in the surviving parameter space is of order $O(10^{-11})$, which is consistent with the latest Fermilab measurements and the reduced discrepancy with Standard Model predictions.

Significance and Claims
The paper claims that within the strict confines of Natural SUSY ($\Delta_{EW} < 30$), the light bino–higgsino scenario is no longer a standalone dark matter candidate. Instead, it must be interpreted as a sub-dominant relic in a multi-component DM universe, likely requiring an axion or another mechanism to account for the bulk of the DM abundance.

The authors emphasize that the survival of the Z-resonance region does not rely on tuned coupling cancellations (blind spots) but rather on the deep suppression of the fractional abundance ($f$). Because resonant annihilation at the Z-pole is highly efficient, the resulting low relic density naturally suppresses the effective scattering rate ($f \times \sigma_{SI}$), allowing these points to evade current LZ sensitivities. The study corroborates broader phenomenological MSSM (pMSSM) scans regarding the restriction to the Z-resonance but provides a more rigorous quantification of the relic density suppression and explicitly rules out the co-annihilation and H-resonance mechanisms under current experimental limits. The work concludes that the remaining parameter space is highly constrained and will be fully testable by the HL-LHC.