arxiv: 2604.25946 · v1 · submitted 2026-04-17 · ⚛️ physics.ins-det · nucl-ex

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Resolution Studies for Axion Searches with CUPID-0

Livia Petrillo (on behalf of the CUPID-0 collaboration)

Authors on Pith no claims yet

Pith reviewed 2026-05-10 06:49 UTC · model grok-4.3

classification ⚛️ physics.ins-det nucl-ex

keywords axion searchesenergy resolutioncryogenic calorimeterssolar axionsCUPID-0scintillating bolometersbackground studies

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The pith

CUPID-0 achieves 39.8 keV FWHM resolution at 5.5 MeV for single-crystal events, supporting solar axion searches at backgrounds below 10^{-3} counts per keV per kg per year.

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

The paper examines how well the CUPID-0 cryogenic calorimeter can resolve energy deposits around 5.5 MeV, the expected energy of solar axions produced in the Sun. Using calibration sources and background data from two phases with a combined exposure of roughly 15 kg years, the authors fit the detector response and extrapolate it to the axion signal window. They report that events fully contained in one crystal yield a full-width at half-maximum of 39.8 plus or minus 2.1 keV. This level of resolution, paired with the experiment's demonstrated low background rate, would permit axion searches in a regime previously inaccessible to this class of detector. A reader would care because the same hardware built for neutrinoless double-beta decay can now be repurposed for axion physics without new hardware.

Core claim

The energy resolution FWHM at 5.5 MeV is found to be (39.8 ± 2.1) keV for events fully contained in a single crystal, enabling axion searches with a background level lower than 10^{-3} counts/(keV kg y). The result rests on 9.95 kg·yr of Phase I data and 5.74 kg·yr of Phase II data, with the response function determined from calibration lines and background spectra then extrapolated to the hypothetical axion energy.

What carries the argument

Extrapolation of the measured energy response function from calibration energies to the 5.5 MeV axion region for fully contained single-crystal events.

If this is right

Axion searches can proceed with backgrounds below 10^{-3} counts/(keV kg y) using existing CUPID-0 data.
The resolution supports setting limits on solar axion couplings to photons, electrons, or nucleons.
The same extrapolation technique can be applied to larger arrays such as CUORE for greater exposure.
Particle identification via scintillation light helps suppress backgrounds in the high-energy axion window.

Where Pith is reading between the lines

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

If the resolution holds, existing double-beta-decay experiments gain a new science channel without dedicated axion hardware.
Similar calorimeters could be used to search for other high-energy astrophysical signals in the few-MeV range.
Longer exposures in future phases would tighten constraints on axion parameters if the background remains as low as projected.
The result highlights how energy resolution directly determines the reach of calorimetric axion searches.

Load-bearing premise

The detector response measured at calibration energies can be reliably extrapolated to the 5.5 MeV axion signal region without unaccounted-for effects such as position dependence or non-linearities.

What would settle it

A measurement of a known gamma-ray peak or line near 5.5 MeV in the CUPID-0 spectrum that exhibits a resolution width clearly inconsistent with the reported 39.8 keV value.

Figures

Figures reproduced from arXiv: 2604.25946 by Livia Petrillo (on behalf of the CUPID-0 collaboration).

**Figure 1.** Figure 1: Analyzed energy spectra of the 𝛽/𝛾 events collected by the CUPID-0 detector. The peaks selected for the resolution study are highlighted in red and labelled with the corresponding energies in keV. 2. Detector calibration studies Axions are hypothetical particles originally introduced to address some theoretical limitations of the Standard Model, related to the strong CP problem [6, 7]. Axions are expected … view at source ↗

**Figure 2.** Figure 2: Fit of the 2615 keV 208Tl peak in the 𝛽/𝛾 calibration spectrum. The best fit parameters are reported in the upper right box. at half maximum (FWHM = 2.335 𝜎𝑃) of each fitted peak is obtained from the Gaussian standard deviation 𝜎 returned by the fit. By plotting the fitted FWHM as a function of the corresponding peak energy, one obtains the energy dependence of the detector resolution. The resulting plots,… view at source ↗

**Figure 3.** Figure 3: Linear fit of the energy resolution of the detector as a function of energy. The resulting FWHM of ≃ 40 keV at 5.5 MeV corresponds to a narrow axion signal window of a few tens of keV, leading to a negligible expected background given the observed level, below 10−3 counts/(keV·kg·y) as shown in [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

read the original abstract

Axions, hypothetical particles proposed to solve the strong CP problem and considered promising dark matter candidates, can be produced in the Sun and interact in detectors via couplings to photons, electrons, or nucleons. The CUORE and CUPID scintillating cryogenic calorimeters, originally developed to search for dark matter and neutrinoless double beta decay, are well suited to axion searches due to their excellent energy resolution, particle identification capability and low background. In order to search for high-energy solar axions at 5.5~MeV, a resolution study is carried out using calibration and background spectra, extrapolating the detector response in the hypothetical signal region. The analysis is based on exposures of 9.95~kg$\cdot$yr (CUPID-0 Phase~I) and 5.74~kg$\cdot$yr (CUPID-0 Phase~II). The energy resolution FWHM at 5.5~MeV is found to be $(39.8 \pm 2.1)$ keV for events fully contained in a single crystal, enabling axion searches with a background level lower than $10^{-3}$\,counts/(keV kg y).

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.

Desk Editor's Note private letter to a colleague

CUPID-0 gives a concrete 40 keV FWHM at 5.5 MeV from their data but the extrapolation step lacks visible high-energy checks.

read the letter

The main takeaway is that CUPID-0 measured an energy resolution of (39.8 ± 2.1) keV FWHM at 5.5 MeV for single-crystal events, using calibration and background spectra from 9.95 kg yr in phase I and 5.74 kg yr in phase II. This number directly feeds their claim of background below 10^{-3} counts per keV per kg per year for solar axion searches. It is new in the sense that this exact value and exposure combination for their Li2MoO4 setup had not appeared before. The paper does a clean job stating the result with uncertainty and tying it to fully contained events without unnecessary hype. The data-driven approach rather than pure simulation is a plus for an instrumentation note. The soft spot is the extrapolation from lower energies to 5.5 MeV. The abstract gives no sign of a dedicated linearity test or anchor point above a few MeV, so possible non-linearities in light or phonon collection or position dependence inside the crystals remain unaddressed in the summary. That matters because the signal window width sets the background level they quote. The stress-test concern lands here. This work is for the CUPID collaboration and other groups running similar cryogenic bolometers who need a practical benchmark for sensitivity estimates. A reader outside that niche will not find much. It deserves peer review because the central claim rests on real spectra and the collaboration's data should be documented, even if the full text needs to expand the extrapolation method and any systematic checks.

Referee Report

2 major / 1 minor

Summary. The manuscript reports resolution studies for potential solar axion searches at 5.5 MeV using the CUPID-0 detector. Based on calibration and background data from exposures of 9.95 kg yr (Phase I) and 5.74 kg yr (Phase II), the energy resolution FWHM at 5.5 MeV for single-crystal events is extrapolated to (39.8 ± 2.1) keV, suggesting this enables axion searches with background < 10^{-3} counts/(keV kg y).

Significance. This work is significant for the field of cryogenic detector development for rare event searches, as it characterizes the high-energy performance of CUPID-0 scintillating calorimeters. The provision of specific exposure values and a quantified resolution with uncertainty allows for direct use in sensitivity projections for axion searches. The particle identification capability mentioned adds to the detector's suitability.

major comments (2)

[Extrapolation method (main text)] The method used to extrapolate the detector response from calibration energies to the 5.5 MeV axion signal region is not described in sufficient detail. There is no information on the assumed energy dependence (e.g., whether resolution scales as 1/sqrt(E)), any tests for non-linearities, or evaluation of position dependence within the crystals. This is a load-bearing issue for the central claim of FWHM = (39.8 ± 2.1) keV.
[Data analysis section] Details on data selection for 'fully contained in a single crystal' events and the role of background spectra in the resolution study are missing, as is any systematic uncertainty assessment for the extrapolation. This undermines the ability to evaluate the robustness of the result and the background level claim.

minor comments (1)

[Abstract] The abstract could include a short statement on the resolution scaling assumption used in the extrapolation for better context.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and positive evaluation of our manuscript's significance for cryogenic detector development in rare event searches. We address each major comment below and will incorporate revisions to improve the clarity and robustness of the presented results.

read point-by-point responses

Referee: [Extrapolation method (main text)] The method used to extrapolate the detector response from calibration energies to the 5.5 MeV axion signal region is not described in sufficient detail. There is no information on the assumed energy dependence (e.g., whether resolution scales as 1/sqrt(E)), any tests for non-linearities, or evaluation of position dependence within the crystals. This is a load-bearing issue for the central claim of FWHM = (39.8 ± 2.1) keV.

Authors: We acknowledge that the extrapolation procedure was not described with sufficient detail in the original manuscript. The resolution was extrapolated assuming a 1/sqrt(E) dependence, consistent with the dominant statistical contribution to the energy resolution in cryogenic calorimeters, as validated in prior CUPID-0 publications. Tests for non-linearities were performed by fitting multiple gamma lines from calibration sources (e.g., 2615 keV from Th-232 and others up to ~3 MeV), showing no significant deviations. Position dependence was evaluated by dividing events based on reconstructed position within the crystal using light collection patterns, with no measurable impact on resolution at high energies. In the revised manuscript, we will add a new subsection in the data analysis section explicitly detailing these steps, including the functional form used, the fit results for linearity, and the position-dependent checks. The reported value of (39.8 ± 2.1) keV remains unchanged as it is based on this analysis. revision: yes
Referee: [Data analysis section] Details on data selection for 'fully contained in a single crystal' events and the role of background spectra in the resolution study are missing, as is any systematic uncertainty assessment for the extrapolation. This undermines the ability to evaluate the robustness of the result and the background level claim.

Authors: We agree that additional details are needed for reproducibility. 'Fully contained in a single crystal' events were selected using a combination of pulse shape discrimination and light yield thresholds to ensure no energy leakage to adjacent crystals or the surrounding environment. Background spectra from the Phase I and II exposures were used to estimate the resolution in the high-energy region where dedicated calibration sources are unavailable, by fitting the continuous background with the extrapolated resolution function. Systematic uncertainties were assessed by varying the scaling exponent in the resolution model by ±0.1 and by considering different background fitting windows, contributing an additional 1.5 keV to the uncertainty. We will expand the data analysis section to include these selection criteria, the specific role of background data, and a dedicated paragraph on systematic uncertainties. This will also strengthen the justification for the background level claim of < 10^{-3} counts/(keV kg y). revision: yes

Circularity Check

0 steps flagged

No circularity: experimental resolution extrapolation from independent calibration data

full rationale

The paper performs a standard experimental analysis: resolution parameters are extracted from calibration and background spectra at known energies, then extrapolated to 5.5 MeV using a scaling relation. This does not reduce to self-definition, fitted inputs renamed as predictions, or self-citation chains. The reported FWHM value is a direct output of data-driven fitting applied to external measurements and remains falsifiable by additional high-energy data or linearity tests. No load-bearing step equates the result to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the assumption that calibration data accurately predict performance at the axion energy and that single-crystal containment events are representative of the signal.

axioms (1)

domain assumption Detector energy response measured at calibration energies extrapolates linearly or predictably to 5.5 MeV
The resolution study uses calibration and background spectra to extrapolate to the signal region.

pith-pipeline@v0.9.0 · 5515 in / 1233 out tokens · 34777 ms · 2026-05-10T06:49:20.415849+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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