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arxiv: 2605.21452 · v1 · pith:AZF36HV7new · submitted 2026-05-20 · ✦ hep-ph · hep-ex

Sensitivity of the FCC-ee to axion-like particles at different center-of-mass energies

Juliette Alimena , Elnura Bakhishova , Freya Blekman , Jannah Darwish Abdelhafiz , Christina Dorofeev , Jeremi Niedziela , Giacomo Polesello , Anna Przybyl
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Lovisa Rygaard
This is my paper

Pith reviewed 2026-05-21 03:30 UTC · model grok-4.3

classification ✦ hep-ph hep-ex
keywords axion-like particlesFCC-eethree-photon final stateelectroweak couplingsZ-pole runALP sensitivityassociated production
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0 comments X

The pith

The FCC-ee can detect axion-like particles with couplings down to a few 10^{-6} GeV^{-1} at the Z pole through three-photon events.

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

The paper calculates the sensitivity of the planned FCC-ee electron-positron collider to axion-like particles that couple primarily to electroweak gauge bosons. It focuses on the process in which an ALP is produced together with a photon and decays into two photons, producing a three-photon final state. Simulations across the planned running energies show the best reach at the Z pole, where couplings as small as a few times 10^{-6} per GeV become accessible, while the higher-energy runs reach roughly 10^{-5} per GeV. A reader would care because this channel offers a clean way to test whether such particles exist and to examine the structure of their couplings at energies and luminosities not available at current experiments.

Core claim

In the effective model with dominant electroweak couplings at leading order, associated production of the ALP with a photon followed by ALP decay to two photons yields a three-photon final state. The FCC-ee will be able to detect ALPs for couplings down to a few 10^{-6} GeV^{-1} during the Z pole run and down to 10^{-5} GeV^{-1} during the WW, ZH, and ttbar threshold runs, for ALP masses between 5 and 320 GeV. For masses below the Z boson mass this final state can additionally probe the underlying electroweak structure of the ALP couplings.

What carries the argument

Associated ALP-photon production at an electron-positron collider with the ALP decaying to two photons, giving a three-photon signature used to extract the ALP-photon coupling.

If this is right

  • ALPs in the 5 to 320 GeV mass range become testable at couplings of order 10^{-6} GeV^{-1} during the Z-pole run.
  • The higher-energy runs extend coverage but with coupling sensitivity around 10^{-5} GeV^{-1}.
  • The three-photon channel can distinguish different patterns of electroweak ALP couplings for masses below the Z mass.
  • Non-observation would place new upper limits on the ALP-photon coupling across the studied mass range.

Where Pith is reading between the lines

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

  • A null result at FCC-ee would complement existing limits from hadron colliders and beam-dump experiments by probing a different production mechanism.
  • If an excess appears, the clean lepton-collider environment could allow precise determination of the ALP mass and coupling strength.
  • The same final state could be used to search for other new physics that produces three photons with low backgrounds.

Load-bearing premise

Standard Monte Carlo simulations are assumed to accurately capture backgrounds, detector efficiencies, and photon identification in the three-photon final state without large unaccounted systematics.

What would settle it

A measurement of the three-photon event rate at the Z pole that shows no excess above Standard Model expectations at the level corresponding to couplings of a few 10^{-6} GeV^{-1}, after accounting for statistical and systematic uncertainties, would falsify the projected sensitivity.

Figures

Figures reproduced from arXiv: 2605.21452 by Anna Przybyl, Christina Dorofeev, Elnura Bakhishova, Freya Blekman, Giacomo Polesello, Jannah Darwish Abdelhafiz, Jeremi Niedziela, Juliette Alimena, Lovisa Rygaard.

Figure 9
Figure 9. Figure 9: Workflow diagram illustrating the simulation and analysis chain for FCC-ee physics studies. The process begins with event generation using MadGraph5 aMC@NLO, producing events in the Les Houches Event (LHE) format. These events are then passed to PYTHIA8, followed by detector simulation using DELPHES, which produces particle-level reconstructed objects in the edm4hep data format. The resulting samples are a… view at source ↗
Figure 10
Figure 10. Figure 10: , the signal process e+e→ → aω, followed by a → ωω, yields a three-photon topology that o!ers a particularly clean probe of the ALP–photon interaction [PITH_FULL_IMAGE:figures/full_fig_p005_10.png] view at source ↗
read the original abstract

The sensitivity of the proposed FCC-ee collider to axion-like particles (ALPs) is investigated at all planned center-of-mass energies, with focus on the case where the ALP couples primarily to electroweak gauge bosons at leading order. We study the associated production of the ALP with a photon, with the ALP decaying in turn to two photons, yielding a three-photon final state. The ALP coupling to the photon is evaluated for ALP masses of 5 to 320 GeV. In this effective model, the FCC-ee will be able to detect ALPs for couplings down to a few $10^{-6} \mathrm{GeV}^{-1}$ ($10^{-5} \mathrm{GeV}^{-1}$) during the Z pole run (WW, ZH, and $t\bar{t}$ threshold runs). Additionally, this final state has the potential to probe the underlying electroweak structure of ALP couplings for ALP masses below the $Z$-boson mass.

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 investigates the sensitivity of the proposed FCC-ee collider to axion-like particles (ALPs) that couple primarily to electroweak gauge bosons at leading order. It focuses on associated production of the ALP with a photon, followed by ALP decay to two photons, yielding a three-photon final state. Projections are given for ALP masses from 5 to 320 GeV across the planned center-of-mass energies (Z pole, WW, ZH, and ttbar thresholds), claiming that the FCC-ee can probe ALP-photon couplings down to a few 10^{-6} GeV^{-1} at the Z pole and 10^{-5} GeV^{-1} at the higher-energy runs. The work also notes the potential to probe the underlying electroweak structure of the couplings for m_a below m_Z.

Significance. If the projected sensitivities hold after detailed validation, the results would extend existing experimental limits on ALP couplings by roughly an order of magnitude and provide a valuable test of ALP models in the electroweak sector at a future high-luminosity e+e- collider. The multi-energy strategy is a strength, as it allows cross-checks between different production mechanisms and could help distinguish the effective electroweak coupling structure. The use of a standard effective-theory framework with leading-order couplings is appropriate for such sensitivity studies.

major comments (2)
  1. [results section / simulation description] The central sensitivity projections (abstract and results section) rest on the assumption that standard Monte Carlo simulations accurately model the irreducible three-photon backgrounds (e.g., from higher-order QED or Z/γ* contributions) and photon reconstruction efficiencies without large unaccounted systematics. No quantitative assessment of systematic uncertainties, validation against data or higher-order calculations, or breakdown of background components (such as initial-state radiation tails or fake photons from π0 decays) is provided. A 10-20% underestimation of background would degrade the quoted coupling reach of a few 10^{-6} GeV^{-1} at the Z pole by a comparable factor, making this assumption load-bearing for the main claims.
  2. [discussion of electroweak structure] For ALP masses below m_Z, the claim that the three-photon final state can probe the electroweak structure of the couplings (abstract) depends on the branching ratio to γγ being correctly predicted by the leading-order effective model with only electroweak couplings. The manuscript does not discuss possible mixing effects or higher-dimensional operators that could alter this branching ratio, which is a potential source of model dependence in the projected limits.
minor comments (2)
  1. [introduction] The abstract and introduction would benefit from an explicit reference to the effective Lagrangian or operator basis used for the ALP-electroweak couplings to clarify the assumptions at leading order.
  2. [figures] Figure captions should include the assumed integrated luminosities and center-of-mass energies for each run to make the sensitivity curves easier to interpret without cross-referencing the text.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and valuable feedback on our manuscript. We have addressed the major comments point by point below, making revisions to the manuscript where appropriate to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: [results section / simulation description] The central sensitivity projections (abstract and results section) rest on the assumption that standard Monte Carlo simulations accurately model the irreducible three-photon backgrounds (e.g., from higher-order QED or Z/γ* contributions) and photon reconstruction efficiencies without large unaccounted systematics. No quantitative assessment of systematic uncertainties, validation against data or higher-order calculations, or breakdown of background components (such as initial-state radiation tails or fake photons from π0 decays) is provided. A 10-20% underestimation of background would degrade the quoted coupling reach of a few 10^{-6} GeV^{-1} at the Z pole by a comparable factor, making this assumption load-bearing for the main claims.

    Authors: We agree that a detailed assessment of systematic uncertainties is important for robust sensitivity projections. Our study employs leading-order Monte Carlo simulations to estimate the three-photon backgrounds and assumes typical photon identification efficiencies for the FCC-ee detector concept. As this is a prospective analysis for a future experiment, full detector-level simulations with data validation are not yet available. To address the referee's concern, we have revised the results section to include a qualitative discussion of potential systematic effects, such as contributions from initial-state radiation and possible fake photons. We also note that a 10-20% variation in background normalization would proportionally affect the coupling sensitivity, and our quoted reaches should be interpreted with this caveat in mind. We believe this addition clarifies the assumptions underlying our projections. revision: partial

  2. Referee: [discussion of electroweak structure] For ALP masses below m_Z, the claim that the three-photon final state can probe the electroweak structure of the couplings (abstract) depends on the branching ratio to γγ being correctly predicted by the leading-order effective model with only electroweak couplings. The manuscript does not discuss possible mixing effects or higher-dimensional operators that could alter this branching ratio, which is a potential source of model dependence in the projected limits.

    Authors: Our analysis is performed within a specific effective field theory framework where the ALP couples primarily to electroweak gauge bosons at leading order, and the diphoton branching ratio is computed accordingly. For ALP masses below the Z boson mass, the associated production with a photon and subsequent decay allows sensitivity to the electroweak coupling structure through the production cross section and decay kinematics. We acknowledge that mixing with other particles or contributions from higher-dimensional operators could modify the branching ratios and introduce additional model dependence. In the revised manuscript, we have added a brief discussion in the conclusions section highlighting this limitation and stating that our results apply specifically to the leading-order effective model considered. revision: yes

Circularity Check

0 steps flagged

ALP sensitivity projections at FCC-ee rely on standard EFT cross-sections, branching ratios, and MC background modeling with no reduction of limits to self-fitted parameters or self-citations.

full rationale

The paper computes projected reaches for ALP-photon couplings using associated production e+e- → γ a followed by a → γγ in an effective electroweak-coupled model. These are direct applications of standard leading-order matrix elements and branching ratios evaluated at planned FCC-ee energies and luminosities. No equations or sections define the target coupling reach in terms of itself, fit a parameter to a subset of the projected data then rename it a prediction, or invoke a uniqueness theorem from the same authors' prior work to force the result. The central claim remains a forward projection whose validity rests on external assumptions about detector performance and SM background simulation rather than internal self-reference.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The paper relies on an effective field theory description of ALP interactions and standard collider phenomenology tools; no new free parameters are fitted to data, but the model choice and background assumptions are taken as given.

axioms (1)
  • domain assumption ALP couples primarily to electroweak gauge bosons at leading order
    Explicitly stated as the focus case in the abstract.
invented entities (1)
  • Axion-like particle with electroweak couplings no independent evidence
    purpose: To model potential new physics beyond the Standard Model
    Standard postulated particle in the literature; no independent evidence or new falsifiable prediction supplied in this work.

pith-pipeline@v0.9.0 · 5738 in / 1343 out tokens · 51163 ms · 2026-05-21T03:30:54.632943+00:00 · methodology

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Reference graph

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