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arxiv: 2604.04687 · v1 · submitted 2026-04-06 · ✦ hep-ph

Recognition: no theorem link

Natural SUSY with mixed axion/axino dark matter

Howard Baer , Vernon Barger , Kairui Zhang
Authors on Pith no claims yet

Pith reviewed 2026-05-10 19:08 UTC · model grok-4.3

classification ✦ hep-ph
keywords natural supersymmetryaxino dark matterPeccei-Quinn symmetrymixed dark matterDFSZ modelKSVZ modellightest supersymmetric particle
0
0 comments X

The pith

Natural SUSY permits the axino to act as the lightest supersymmetric particle, producing viable mixed axion-axino dark matter that matches the observed abundance.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines the viability of axino dark matter in natural supersymmetric models that also incorporate the Peccei-Quinn symmetry. It maps regions of the Peccei-Quinn scale versus axino mass plane where the axino serves as the lightest supersymmetric particle, yielding a mixture of axions and axinos whose total density agrees with cosmological measurements. This holds for both the DFSZ and KSVZ realizations of the axion. A reader would care because the standard higgsino-like WIMP candidate for dark matter faces tightening constraints from direct searches, so an alternative within the same natural SUSY framework becomes relevant.

Core claim

In natural supersymmetry extended by Peccei-Quinn symmetry, the axino can be the lightest supersymmetric particle and therefore produce mixed axion plus axino dark matter. Viable solutions appear in both the SUSY DFSZ and KSVZ models for axino masses near 100 keV. When the Peccei-Quinn scale sits near 10^11 GeV the dark matter is predominantly warm axinos, while at scales around 3 times 10^12 GeV it is predominantly cold axions with only a tiny axino fraction; both cases reproduce the measured dark matter abundance.

What carries the argument

The axino as lightest supersymmetric particle, whose mass and the Peccei-Quinn scale together fix the relative contributions of warm axino and cold axion dark matter components.

If this is right

  • For axino masses around 100 keV and Peccei-Quinn scales near 10^11 GeV, the dark matter is mainly warm axinos while still matching the total observed density.
  • At higher Peccei-Quinn scales near 3 times 10^12 GeV the same axino mass yields mainly cold axion dark matter with a negligible axino fraction.
  • Both outcomes remain consistent with the measured dark matter abundance in the SUSY DFSZ and KSVZ models.
  • The construction evades the direct-detection limits that apply to higgsino WIMPs.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Warm axino dark matter at the lower Peccei-Quinn scale could leave imprints on small-scale structure formation that differ from pure cold dark matter.
  • Axion searches sensitive to the 10^11 to 10^12 GeV range would directly test the allowed windows.
  • The scenario preserves the naturalness motivation for supersymmetry while shifting the dark matter candidate away from the WIMP paradigm.

Load-bearing premise

The near-exclusion of higgsino-like WIMPs by multi-ton direct detection experiments is taken as given, and keV-scale axino masses are assumed to evade further cosmological or particle-physics constraints.

What would settle it

A positive detection of higgsino dark matter in a multi-ton noble-liquid detector, or cosmological data that rule out a keV-scale axino component, would eliminate the viable regions identified in the parameter space.

Figures

Figures reproduced from arXiv: 2604.04687 by Howard Baer, Kairui Zhang, Vernon Barger.

Figure 1
Figure 1. Figure 1: Branching fraction of lightest neutralino ˜χ [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Decay temperature TD(˜χ 0 1 ) in the fa vs. ma˜ plane. We also show the rough bound for the ˜χ 0 1 to be BBN-safe, i.e. that it decays before the onset of BBN. In white are shown contours of τ (˜χ 0 1 ) ranging from 10−12 − − 1 s. start with minimal abundance at high temperatures, and then develop an abundance from sparticle decays to axinos and axino pair production in the early universe: ΩT P a˜ h 2 . 4.… view at source ↗
Figure 3
Figure 3. Figure 3: Mixed axion-axino relic abundance vs. fa for ma˜ = 100 keV in the benchmark SUSY DFSZ model with m3/2 = 10 TeV and θi = 1. Another view of the mixed aa˜ abundance is shown in [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Mixed axion-axino relic abundance vs. ma˜ for various values of PQ scale fa in the SUSY DFSZ benchmark model with m3/2 = 10 TeV, TR = 106 GeV and θi = 1. axion abundance can be dialed down with lower values of θi). For θi = 1 and ma˜ ≳ 1 MeV, then the mixed aa˜ dark matter is always over-abundant. The red contours show ratios of the portion of axion abundance to the total abundance, and so the upper portio… view at source ↗
Figure 5
Figure 5. Figure 5: Color-coded mixed axion-axino relic abundance in the [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Mixed axion-axino relic abundance vs. fa for ma˜ = 100 keV in the SUSY KSVZ model [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Mixed axion-axino relic abundance vs. ma˜ for various values of PQ scale fa in the SUSY KSVZ model using thermal axino production ala Brandenberg & Steffen, Ref. [47]. 11 [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
read the original abstract

While supersymmetric models provide a solution to the big hierarchy problem, natural SUSY is also allowed by the little hierarchy problem. In supersymmetric models which include the Peccei-Quinn (PQ) solution to the strong CP problem, one expects the presence of an axion-axino-saxion supermultiplet with a micro-eV-scale axion and a saxion with mass of order the soft breaking scale. The axino mass is much more model-dependent, and may occur in the range of keV-TeV: over 9 orders of magnitude. This leads to the possibility of the axino as lightest SUSY particle (LSP) and the presence of mixed axion plus axino dark matter. The case of natural SUSY with higgsino-like WIMPs as LSP seems (nearly) excluded by multi-ton noble liquid WIMP detector limits, even in the case where the LSP has a depleted abundance compared to axions. We examine the case where the axino is LSP leading to mixed axion-axino dark matter in a natural SUSY context. We map out regions of PQ scale f_a vs. axino mass m_{\ta} parameter space where such a scenario remains viable in both the SUSY DFSZ and KSVZ axion models. For axino mass ~100 keV, we find solutions in accord with the measured dark matter abundance with mainly warm axino dark matter for f_a~ 10^{11} GeV and also solutions with mainly axion cold DM and a tiny axino contribution for higher f_a~ 3\times 10^{12} GeV.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript examines natural SUSY models incorporating the Peccei-Quinn solution to the strong CP problem, focusing on scenarios where the axino is the LSP and yields mixed axion-axino dark matter. It maps viable regions of PQ scale f_a versus axino mass m_a~ parameter space in both SUSY DFSZ and KSVZ axion models, identifying choices that reproduce the observed dark matter abundance, including a mainly warm axino DM solution at m_a~ ~100 keV and f_a ~10^11 GeV as well as mainly axion cold DM solutions at higher f_a ~3x10^12 GeV.

Significance. If the relic density calculations hold and all relevant constraints are satisfied, the work offers a concrete way to realize viable dark matter in natural SUSY by sidestepping direct-detection limits on higgsino-like WIMPs. The explicit delineation of f_a-m_a~ space for both axion models is a useful phenomenological contribution that could guide future model-building and searches. The inclusion of warm axino components, however, requires verification against structure-formation data to realize this potential.

major comments (2)
  1. [Results on f_a ~10^11 GeV solutions] The viability claim for the mainly warm axino DM region at m_a~ ~100 keV and f_a ~10^11 GeV rests only on matching the total Omega_DM; no assessment is provided of the axino free-streaming length or its compatibility with Lyman-alpha forest, Milky Way satellite, or reionization bounds that typically restrict or exclude keV-scale warm DM fractions capable of erasing small-scale power.
  2. [Parameter space mapping for DFSZ and KSVZ models] The mapping of viable regions lacks explicit formulas, production mechanisms (thermal vs. non-thermal), or dependence on reheating temperature and saxion decays used to compute the axino relic density as a function of f_a and m_a~; without these details the assertion that solutions are 'in accord with the measured dark matter abundance' cannot be verified.
minor comments (2)
  1. Notation inconsistency: the axino mass is written as m_a~ in the abstract and m_~a in the text; adopt a single consistent symbol such as m_ã throughout.
  2. [Introduction] The statement that the axino mass 'may occur in the range of keV-TeV: over 9 orders of magnitude' would benefit from a brief reference to the model-dependent origin of this range.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We address each major point below and will revise the manuscript to incorporate clarifications and additional discussion where needed.

read point-by-point responses

  1. Referee: [Results on f_a ~10^11 GeV solutions] The viability claim for the mainly warm axino DM region at m_a~ ~100 keV and f_a ~10^11 GeV rests only on matching the total Omega_DM; no assessment is provided of the axino free-streaming length or its compatibility with Lyman-alpha forest, Milky Way satellite, or reionization bounds that typically restrict or exclude keV-scale warm DM fractions capable of erasing small-scale power.

    Authors: We agree that matching the total relic density alone is insufficient to fully establish viability for the warm axino component. The manuscript's primary focus is the computation of the mixed axion-axino relic density in natural SUSY. In the revised version we will add an estimate of the axino free-streaming length at m_a~ ~100 keV together with a brief discussion of its implications for Lyman-alpha forest, satellite, and reionization constraints, qualifying the parameter-space claims accordingly. revision: yes

  2. Referee: [Parameter space mapping for DFSZ and KSVZ models] The mapping of viable regions lacks explicit formulas, production mechanisms (thermal vs. non-thermal), or dependence on reheating temperature and saxion decays used to compute the axino relic density as a function of f_a and m_a~; without these details the assertion that solutions are 'in accord with the measured dark matter abundance' cannot be verified.

    Authors: The manuscript already outlines the relevant production channels (thermal axino production and non-thermal contributions from saxion decays) and their dependence on the reheating temperature for both DFSZ and KSVZ cases. To improve transparency and verifiability we will insert the explicit analytic expressions for the axino yield as a function of f_a and m_a~ in the revised manuscript, making the origin of the viable regions fully traceable. revision: yes

Circularity Check

0 steps flagged

No circularity: parameter scan uses external relic density formulas

full rationale

The paper scans f_a vs. m_a~ space in natural SUSY with axino LSP, computing axion and axino relic densities via standard Boltzmann equations and thermal production rates (DFSZ/KSVZ models) to identify regions matching observed Omega_DM. This is a forward calculation from input parameters to output abundance, not a fit or self-definition. No load-bearing self-citations, uniqueness theorems, or ansatze are invoked to force the result; viability regions are determined by external cosmology inputs. The derivation chain is self-contained against benchmarks and does not reduce to its own inputs by construction.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard supersymmetric and axion model assumptions without introducing new postulated entities or ad-hoc parameters beyond the scanned quantities.

free parameters (2)
  • PQ scale f_a
    Scanned over orders of magnitude to locate viable dark matter solutions
  • axino mass m_a~
    Scanned across keV-TeV range to identify regions matching observed abundance
axioms (2)
  • domain assumption Natural SUSY solves the little hierarchy problem
    Invoked in the opening sentence as the framework under study
  • domain assumption Peccei-Quinn symmetry introduces axion-axino-saxion supermultiplet with micro-eV axion
    Standard assumption stated in the abstract for SUSY plus PQ models

pith-pipeline@v0.9.0 · 5600 in / 1549 out tokens · 76829 ms · 2026-05-10T19:08:16.892648+00:00 · methodology

discussion (0)

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Reference graph

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