Conceptual Design Report of the SUPAX Experiment

Dhruv Chouhan; Kristof Schmieden; Matthias Schott; Tim Schneemann

REVIEW 2 major objections 5 minor 25 references

Conceptual Design Report of the SUPAX Experiment

T0 review · 2 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read A prototype haloscope using a superconducting cavity excludes previously unexplored dark photons around 35 µeV with kinetic mixing down to χ = 5×10⁻¹⁴, and the full SUPAX design projects sensitivity to the QCD axion band.

desk verdict A credible haloscope design report with a genuinely new dark-photon exclusion, but the limit's confidence level is compromised by a data-dependent re-scan threshold. read the letter →

arxiv 2505.07541 v1 pith:AER2BPJ7 submitted 2025-05-12 hep-ex astro-ph.IM

Dhruv Chouhan , Tim Schneemann , Kristof Schmieden , Matthias Schott This is my paper · ORCID

classification hep-exastro-ph.IM

keywords axion-likeparticlesdarkphotonshaloscopesuperconductingcavitieskineticmixingtunablemicrowavecavityJosephsonparametricamplifierQCDaxion

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The SUPAX project sets out to search for axion-like particles in the mass range 8 to 30 µeV by converting them into photons in tunable superconducting cavities inside a 12 T magnetic field. The paper's prototype, a superconducting cavity cooled to 2 K, has already produced the project's first physics result: a scan of 300 kHz around 8.471 GHz finds no dark-photon signal and excludes kinetic mixing parameters χ above 5×10⁻¹⁴ at masses near 35 µeV, a part of parameter space not reached by previous experiments. The design report argues that the full experiment, with three simultaneously scanned cavities and a projected quality factor of about 4.5×10⁵, would reach axion-photon couplings below 2×10⁻¹⁵ GeV⁻¹ and enter the QCD axion band. If correct, this would turn a compact haloscope into a competitive probe of both axion dark matter and dark photons with one instrument.

What carries the argument

The load-bearing object is the tunable superconducting cavity: a copper body shaped as a tetracontagon (a 40-sided polygon), coated with superconducting tape, whose $\mathrm{TM}_{010}$ mode is read out through a weakly coupled port. Coarse tuning comes from a piezo-actuated off-center dielectric rod; fine tuning comes from changing helium gas pressure inside the cryostat, which shifts the dielectric constant and moves the resonance frequency by $-27.45\,\mathrm{kHz/mbar}$. The signal-search chain, following the analysis procedure of reference [23], removes the cavity response with a Savitzky-Golay filter, rescales spectra by the expected dark-photon conversion power, combines frequency bins into a grand unified spectrum, and converts a null result into a 95% CL limit using a re-scan threshold set to the largest observed fluctuation.

What would settle it

The limit could be checked by injecting a calibrated, narrow-band tone through the weakly coupled cavity port at a level corresponding to $\chi \approx 5\times10^{-14}$ and verifying that the full analysis chain recovers it at the expected signal-to-noise ratio; a second check is to recompute the limit with the re-scan threshold fixed before looking at the data (for instance, at 5σ) rather than at the largest observed 4.1σ fluctuation.

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Extended reading notes

Core claim

The paper claims that a haloscope using superconducting cavities can simultaneously reach previously unexplored dark-photon parameter space in prototype form and project sensitivity to the QCD axion band in the final design. The prototype, a copper cavity coated with NbN operated in a 14 T field at 2 K, achieved a quality factor of $Q \approx 3\times10^5$ and scanned a 300 kHz band around 8.471 GHz by varying helium pressure. From a null result, the analysis sets 95% CL upper limits on the kinetic mixing parameter in the range $\chi$ between $1\times10^{-14}$ and $5\times10^{-14}$ for dark photon masses near 35 µeV, assuming random polarization. The full experiment is designed around three combined cylindrical cavities with tetracontagon cross-sections, ReBCO-coated superconducting surfaces, and simultaneous multi-frequency scanning between 2 and 7.2 GHz.

Load-bearing premise

The result depends on the rule used to decide when a bump is a candidate: that rule was set to the largest fluctuation seen in the same data that produced the limit, and the smoothing step is assumed not to remove a real signal.

Editorial extensions

If this is right

If the full experiment performs as designed, SUPAX will scan axion masses from 8 to 30 µeV over roughly 80 weeks of data taking starting in 2026, with sensitivity reaching the QCD axion band.
The verified pressure-tuning mechanism, with a measured response of −27.45 kHz/mbar, gives a 550 kHz scan range per 20 mbar pressure change, which allows the mechanical tuning to use coarse steps.
The prototype's dark-photon limit already establishes the readout chain (cavity, cryogenic amplifier, spectrum analyzer, and analysis software) as ready for the final experiment's axion search.
Because the dark-photon search needs no magnetic field, the same cavity technology can run as a standalone dark-photon experiment in the gaps between axion scans.
With superconducting coatings, the design target $Q_0 \approx 4.5\times10^5$ would improve the scanning speed by roughly an order of magnitude over the normal-conducting copper configuration.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The quoted limit would be more reliable if the re-scan threshold were fixed before the data were examined; a blind analysis would test whether the 4.1σ threshold contributes to the result.
The pressure-tuning mechanism is generic and could be adopted by other cryogenic cavity searches, since helium gas tuning avoids moving parts in the high-field region.
A simple extension of the prototype scan beyond 300 kHz, using the same linear pressure response, could cover a substantially wider dark-photon mass window at similar sensitivity before the full experiment turns on.
The quality factor of superconducting cavities at these frequencies may be limited by surface defects and coating inhomogeneities; direct comparisons of NbN and ReBCO coatings in the same cavity geometry would test whether the design $Q_0$ is achievable.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Colleague,

Here's my read on SUPAX. The main thing you should know: it's a serious conceptual design for a multi-cavity haloscope, and the prototype produces a dark photon exclusion at ~35 µeV that looks genuinely new. The soft spot is the statistics behind that limit—the re-scan threshold is set from the same data that defines the exclusion, so the 95% CL label is not as clean as it should be.

What's actually new: the three-cavity-in-one-body design with off-center dielectric tuning and helium pressure fine-tuning is a sensible combination I haven't seen in exactly this form. The prototype measurement at 8.471 GHz using a NbN-coated cavity is real work—they measure Q ≈ 3×10^5, scan 300 kHz, and get a limit χ > 5×10^-14 in a window that appears unexplored. The technical sections on the cryostat, magnet, and readout are detailed enough to be useful to someone planning a haloscope. The projected sensitivity for the full experiment follows standard formulas and reaches the QCD axion band if the ReBCO cavities and JPA readout perform as claimed; those are explicitly labeled as targets, not yet demonstrated, so I don't hold that against them.

The soft spot is the analysis in Section 5.3. They state that no re-scans were performed, then set the threshold Θ to the largest observed fluctuation at 4.1σ in the same dataset used to compute the limit. That makes Θ a random variable from the noise realization under test. The quoted 95% CL is therefore not a pre-specified coverage; it's a post-hoc description of what this particular noise run would have allowed. The numerical bias may be moderate—a 1σ move in the maximum changes R_T by about 1, roughly 18% on the limit—but there is no coverage check. This is fixable: use a fixed threshold chosen before the scan, apply a trials factor for the number of independent frequency bins, or at least calibrate with Monte Carlo noise-only simulations. As written, the central claim is not statistically certified. Minor issues: Section 2.3 says the readout is implemented "two times in parallel" while there are three cavities, and the introduction misnumbers the sections. Small stuff, but symptomatic of a rushed draft.

Who gets value from this: experimentalists working on haloscope design and cavity R&D, and anyone tracking dark photon limits in the 30–40 µeV range. The sensitivity projections are useful as a benchmark even if the experiment never reaches them.

My recommendation: send it to peer review, but with a request that the dark photon limit be re-derived using a statistically sound procedure. The design content deserves a referee; the current limit claim needs more work.

Best, [You]

Referee Report

2 major / 5 minor

Summary. The paper is a conceptual design report for SUPax, a haloscope experiment combining a 12 T magnet, tunable superconducting cavities, and quantum-limited readout to search for axion-like particles in the 8–30 µeV mass range. It also reports on a prototype dark-photon search performed with an 8.47 GHz cavity at 2 K, from which the authors derive upper limits on the kinetic mixing parameter around mass 35 µeV, with a headline value of about 5×10^-14, and project future axion sensitivity reaching the QCD axion band. The manuscript includes technical design details, prototype hardware characterization, data-acquisition and analysis descriptions, and a comparison to existing limits.

Significance. If the dark-photon exclusion is statistically sound, the prototype result is a useful proof of principle: it covers a previously unexplored mass/coupling region, and the full design provides a concrete, falsifiable projection for a next-generation haloscope. The paper also gives useful detail on multi-cavity tuning, superconducting coatings, and an end-to-end analysis chain. However, the central quantitative claim depends on a data-dependent re-scan threshold, so the 95% CL statement is not currently certified. The projected sensitivities rest on unverified assumptions about quantum-limited readout and high-Q cavities, which is acceptable for a design report only if clearly labeled.

major comments (2)

[Section 5.3, Eq. (5)] The 95% CL dark-photon exclusion shown in Fig. 13 is not statistically justified because the re-scan threshold is taken from the same dataset used to derive the limit. After stating that no re-scans were performed, the authors set Θ to the largest observed fluctuation (4.1σ) and define R_T = Θ + Φ^{-1}(0.95) = 5.7; since Θ is a random draw from the noise realization, the quoted confidence level does not correspond to the coverage of the resulting interval. The paper should either use a pre-specified threshold, incorporate a trials factor for the scanned frequency range, and calibrate coverage with Monte Carlo or injected-signal studies, or present Fig. 13 as an expected/median sensitivity rather than an observed 95% CL exclusion. The numerical bias may be moderate, but no calibration check is provided.
[Section 5.3] The analysis is not self-contained: the 'dark photon conversion power profile' used to produce the rescaled spectra is never defined, and Eq. (5) introduces quantities |χ0| and R̃_g^l without giving the expected signal SNR as a function of the kinetic mixing parameter and cavity parameters such as volume, quality factor, mode overlap, noise temperature, and polarization averaging. As a result, the mapping from the measured spectra to the limits in Fig. 13 cannot be reproduced or checked. Please include the full signal model and all numerical inputs.

minor comments (5)

[Section 2.2/2.3] The text says each cavity has a dedicated readout chain and that the system is implemented two times in parallel, while Section 2.2 describes three cavities and Fig. 2 indicates C1–C3; the number of cavities and readout chains should be reconciled.
[Section 2.2 and Section 6] The design description mentions 'three triple-cavities' in Section 2.2 and 'both cavities' in the Conclusion; please clarify the intended number of cavities and scan configurations.
[Section 5.3 / Fig. 13] The text quotes a scanned frequency band of 150 kHz around f0, but the horizontal range shown in Fig. 13 spans about 120 kHz; please reconcile the stated and plotted scan range.
[Fig. 12] The statement that the normalized-power distribution 'follows nicely a gaussian' would be more informative with a fit result, a χ²/dof value, and a statement of the number of independent frequency bins, since adjacent bins in the grand unified spectrum are not independent.
[Section 3, Fig. 3] The projected sensitivity curves rest on assumptions such as quantum-limited readout and ReBCO-coated cavities with Q0 ≈ 4.5×10^5; these are appropriate as design goals, but the text should state explicitly which curves are demonstrated by the prototype and which are assumed for the final experiment.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The analysis introduces no new free parameters beyond the data-dependent threshold. The sensitivity projections rely on standard axion/dark photon models and assumed device performance. No new particles or forces are postulated.

free parameters (1)

Re-scan threshold Θ = 4.1σ
Set to the largest observed fluctuation in the grand unified spectrum to define the target SNR for the exclusion limit.

assumptions (3)

domain assumption Haloscope signal power formula (Eq. 1) assumes resonant axion-photon conversion and a Maxwell-Boltzmann dark matter velocity distribution.
The sensitivity projections use this standard formula without derivation.
domain assumption The system noise temperature model with added noise NA ≥ 0.5 (standard quantum limit) is used for sensitivity projections.
Assumes ideal quantum-limited readout, which is not demonstrated in the prototype.
domain assumption Dark photon kinetic mixing model and local dark matter density ρ_a = 0.45 GeV/cm³ are used for the limit interpretation.
Standard assumptions in the field; the density value is an input, not a fit.

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Cite this review

Pith. "Pith review of Conceptual Design Report of the SUPAX Experiment." pith.science (2026). https://pith.science/paper/AER2BPJ7

@misc{pith2026250507541,
  author       = {Pith},
  title        = {Pith review of: Conceptual Design Report of the SUPAX Experiment},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AER2BPJ7}},
  note         = {Machine review of arXiv:2505.07541}
}

abstract

The SUPerconduction AXion search experiment (Supax) is a haloscope designed to probe axion-like particles (ALPs) as candidates for dark matter and solutions to the strong CP problem. ALPs are predicted to couple to photons, allowing their detection through resonant conversion in electromagnetic cavities placed within strong magnetic fields. \Supax employs a 12 T magnetic field and tunable superconducting cavities with resonance frequencies ranging from 2\,GHz to 7.2\,GHz, enabling the exploration of axion masses between 8\,$\mu$eV and 30\,$\mu$eV. The tuning mechanism, based on piezo motors and gas-pressure regulation, allows for simultaneous scanning of up to three frequencies, significantly improving search efficiency. This paper presents the technical design of the Supax experiment, preliminary R\&D efforts, and results from prototype experiments. In particular, we exclude dark photons with masses around $35\,\mu$eV with a kinetic mixing parameter $\chi > 5\cdot 10^{-14}$, i.e. a region of parameter space which has not been previously explored.

Figures

Figures reproduced from arXiv: 2505.07541 by the authors.

**Figure 2.** Schematic view of the cryogenic RF layout. The receiver signal path is shown in red. The calibration signal path is shown in black. The TM010 mode exhibits the largest sensitivity to the axion field and is chosen as readout mode. The tripplecavity setup offers several advantages. The ability to scan multiple resonance frequencies simultaneously while optimally using the available magnetized volume significantly imp… view at source ↗

**Figure 3.** Projected sensitivity of the Supax experiment for a measurement time of one year is indicated in the shaded areas for low (blue and purple) and high frequency (orange) frequency ranges using normal conducting copper cavities (dotted line) and superconducting cavities (dashed line). The scan of the purple area would take an additional half year to close the gap to the HAYSTAC measurement. 4.1 Experimental Setup The c… view at source ↗

**Figure 4.** Picture of the Supax experiment during initial tests. In The Background the cryostat is visible, to it’s left a VNA and to its right a monitoeing display [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗

Figures from the paper (6 more)

**Figure 7.** Figure 7: Schematic of the vacuum system used to control the pressure in the probe volume of the cryostat. This system is also used to evacuate the cryostat and flush it with helium gas before the cool-down. . adjustable rate. The vacuum system is shown in [PITH_FULL_IMAGE:fig…

**Figure 8.** Figure 8: Images of the cavity under test. Left: the electropolished copper cavity. Right: after depositing the NbN superconductor. 4.4 Heated load To measure the system’s noise temperature, a heated load is installed at the third port of the circulator. This setup enables the …

**Figure 10.** Figure 10: Shown is the amount of collected data in seconds per frequency interval. A minimum of 600 seconds is required per frequency bins to be considered in the analysis. Data files of arbitrary length can be stored until disk space runs out. In addition to the frequency doma…

**Figure 11.** Figure 11: Plots showing the different analysis steps exemplary for frequency bin 2. Upper left: Measured power spectrum integrated over 30 min. Upper right: Power spectrum divided by the SG-Filter result and shifted by -1, called SNR. Lower left: Rescaled power SNR spectrum. Lo…

**Figure 12.** Figure 12: Distribution of the normalized SNR entries in the grand unified spectrum. As can be seen the distribution follows nicely a gaussian distribution, as expected. 180000 200000 220000 240000 260000 280000 300000 Frequency [Hz] +8.471e9 1 2 3 4 5 6 7 8 9 Limit o n kin e ti…

**Figure 14.** Figure 14: Comparison of the measured limits in comparison to other results taken from [25]. 6 Conclusion The SUPerconduction AXion search experiment represents a significant advancement in the search for axionlike particles (ALPs) and dark photons, two well-motivated candidate…

Discussion (0). Continue with ORCID to comment.

Reference graph

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Reviewed August 15, 2026 · model on record in the stance chip above.

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