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arxiv: 2606.20826 · v1 · pith:BHS4QICTnew · submitted 2026-06-18 · ✦ hep-ph · hep-ex

Counting axions with IAXO

Benjam\'in Grinstein , Carlos Mir\'o , Pablo Qu\'ilez Lasanta This is my paper

Pith reviewed 2026-06-26 16:19 UTC · model grok-4.3

classification ✦ hep-ph hep-ex
keywords axionshelioscopeIAXOflavor oscillationsspectral signaturesmultiple axionsCAST boundsN-axion systems
0
0 comments X

The pith

IAXO can discriminate a two-axion signal from a single-axion hypothesis by resolving spectral signatures from flavor oscillations.

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

The paper examines whether a detection in the planned IAXO helioscope could reveal if the signal arises from one axion or from two or more. Many Standard Model extensions predict several axions that couple to photons, and their combined effect in a detector can resemble a single axion with shifted parameters. The authors recast existing CAST limits into the two-axion parameter space, derive IAXO sensitivity projections, and identify where a signal might appear. They then show that energy-dependent features produced by axion flavor oscillations differ between the quasi-degenerate and hierarchical mass regimes in ways that IAXO's expected resolution can detect. The same discrimination logic applies to systems containing any number of axions.

Core claim

If more than one axion couples to photons, their combined signal in helioscope experiments may mimic that of a single axion with different parameters. Spectral signatures of axion flavor oscillations in the quasi-degenerate and hierarchical mass regimes allow IAXO to discriminate a two-axion signal from the single-axion hypothesis given the expected energy resolutions of the detector. These results extend to a broad class of N-axion systems.

What carries the argument

Spectral signatures arising from axion flavor oscillations, examined separately in the quasi-degenerate and hierarchical mass regimes.

If this is right

  • IAXO can observe signals in identified regions of the two-axion parameter space.
  • Discrimination between one- and two-axion signals is possible in the quasi-degenerate mass regime.
  • Discrimination between one- and two-axion signals is possible in the hierarchical mass regime.
  • The discrimination method extends to systems with any number of axions.

Where Pith is reading between the lines

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

  • Current CAST data already constrain portions of the two-axion space, so any future IAXO signal would start from a narrowed set of possibilities.
  • Detector designs that improve energy resolution beyond current projections would enlarge the mass ranges where counting axions becomes feasible.
  • The oscillation signatures provide a direct test of whether the detected particles obey the flavor-mixing dynamics assumed in the two-axion analysis.

Load-bearing premise

The energy resolution of the IAXO detector is sufficient to resolve the spectral signatures arising from axion flavor oscillations in the relevant mass regimes.

What would settle it

A measured energy spectrum from an IAXO axion signal that matches the shape expected for a single axion but lacks the deviations predicted by two-axion oscillations in the quasi-degenerate or hierarchical regimes, or conversely shows those deviations when a single axion is assumed.

Figures

Figures reproduced from arXiv: 2606.20826 by Benjam\'in Grinstein, Carlos Mir\'o, Pablo Qu\'ilez Lasanta.

Figure 1
Figure 1. Figure 1: We also extend our analysis to the case of su￾pernova (SN) axions and explore whether it would allow IAXO to probe a complementary region of the param￾eter space if a nearby SN explosion occurs during the experiment’s lifetime, as indicated in [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7 [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8 [PITH_FULL_IMAGE:figures/full_fig_p018_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9 [PITH_FULL_IMAGE:figures/full_fig_p019_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10 [PITH_FULL_IMAGE:figures/full_fig_p020_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11 [PITH_FULL_IMAGE:figures/full_fig_p021_11.png] view at source ↗
read the original abstract

The existence of multiple axion species is a generic prediction of a number of extensions of the Standard Model. If more than one axion couples to photons, their combined signal in helioscope experiments may mimic that of a single axion with different parameters. This raises a fundamental question: if a next-generation helioscope such as IAXO detected a signal, would we be able to disentangle whether it originated from one or multiple axions? To answer this question, we first recast current CAST bounds and derive IAXO/IAXO+ projections in the two-axion parameter space, identifying the regions where a signal could be observed. Then, we analyze the spectral signatures of axion flavor oscillations in both the quasi-degenerate and hierarchical mass regimes, and point out where IAXO can discriminate a two-axion signal from the single-axion hypothesis given the expected energy resolutions of the detector. Finally, we show that these results extend to a broad class of $N$-axion systems.

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

0 major / 2 minor

Summary. The paper claims that a next-generation helioscope like IAXO can discriminate a two-axion signal from the single-axion hypothesis by analyzing spectral signatures arising from axion flavor oscillations. It first recasts existing CAST bounds and derives IAXO/IAXO+ sensitivity projections in the two-axion parameter space. It then examines the quasi-degenerate and hierarchical mass regimes, folds the oscillation-induced spectral features with the expected detector energy resolution, and identifies regions where discrimination is feasible. The analysis is extended to a broad class of N-axion systems.

Significance. If the central claim holds, the result is significant for the interpretation of any future axion signal in helioscope experiments. By explicitly computing the folded spectra and mapping the parameter regions where the two-axion hypothesis produces distinguishable features, the work supplies a concrete, falsifiable test rather than a generic statement about resolution. The generalization to N-axion systems and the recasting of existing limits add practical value for experimental planning.

minor comments (2)
  1. The abstract and introduction refer to 'expected energy resolutions' without a dedicated subsection summarizing the numerical values adopted for IAXO (e.g., FWHM at 1 keV and 10 keV). Adding a short table or paragraph with these benchmark numbers would improve reproducibility.
  2. In the discussion of the hierarchical regime, the transition between oscillation-dominated and resolution-limited regimes is described qualitatively; a quantitative criterion (e.g., an inequality involving Δm^{2} and the resolution width) would make the boundary between the two regimes sharper.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript and for recommending minor revision. No major comments were listed in the report.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper recasts existing CAST bounds into the two-axion parameter space and then performs an explicit forward calculation: it derives the expected spectral signatures from axion flavor oscillations in the quasi-degenerate and hierarchical regimes, convolves those spectra with the stated IAXO energy resolution, and identifies the mass and coupling regions in which the resulting binned event distributions differ enough to allow discrimination. This chain is self-contained; each step is a direct computation from the oscillation probability formulas and the detector response function rather than a fit to the target observable or a self-referential definition. No load-bearing premise reduces to a prior result by the same authors, and no parameter is fitted to a subset of the data and then relabeled as a prediction. The derivation therefore stands on its own equations and external detector specifications.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Based solely on the abstract, the work relies on standard axion-photon coupling without introducing new free parameters or entities.

axioms (1)
  • domain assumption Axions interact with photons through the standard two-photon coupling term.
    Implicit in all helioscope signal calculations.

pith-pipeline@v0.9.1-grok · 5707 in / 1152 out tokens · 31522 ms · 2026-06-26T16:19:14.500589+00:00 · methodology

discussion (0)

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Reference graph

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    Hierarchical regime When one axion is much lighter than the other,m 1 ≪ m2, we have ∆m2 21 ≃m 2 2, and the first sinc factor satisfies sinc2(m2 1L/4ω)≃1. Therefore, Eq. (6) reduces to Paγ →γ ≃ gaγBL 2 2" c4 φ +s 4 φ sinc2 m2 2L 4ω + 1 2 s2 2φ sinc m2 2L 4ω cos m2 2LES 2ω # .(8) In this regime, there are in principle two ranges where the two-axion spectrum...

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