⚛️ phenomenology

First constraints on QCD axion dark matter using James Webb Space Telescope observations

This paper presents the first constraints on QCD axion dark matter in the 0.1–4 eV mass range by analyzing James Webb Space Telescope blank-sky observations, thereby establishing a new method for repurposing astrophysical missions to constrain axion-photon coupling.

Original authors: Elena Pinetti

Published 2026-09-21

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CC BY 4.0

Original authors: Elena Pinetti

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, astronomers have known that the visible stars and galaxies in our universe make up only a small fraction of what is actually there. The rest is dark matter, an invisible substance that holds galaxies together through its gravity but refuses to reveal itself through light. While we can see its effects, we have never directly detected the particles that make it up. One of the most promising candidates for this missing mass is a hypothetical particle called the axion. Originally proposed to solve a different puzzle in physics regarding the behavior of atomic nuclei, the axion is predicted to be extremely light and to interact very weakly with normal matter. If these particles exist and make up the dark matter halo surrounding our galaxy, they might occasionally decay into two photons, which are particles of light. Because the axions are so light, this decay would produce light in the infrared part of the spectrum, a type of radiation that is invisible to the human eye but detectable by powerful space telescopes.

A researcher has now used the James Webb Space Telescope to search for this faint glow of decaying dark matter. Instead of pointing the telescope at a specific galaxy or star cluster, they turned their attention to empty patches of sky. These "blank-sky" observations are routinely taken by the telescope to measure the background glow of the universe, which is then subtracted from images of real targets to make them clearer. The researcher realized that this vast archive of empty-sky data, which had been sitting unused for dark matter searches, was actually a perfect laboratory for their work. By analyzing over 15,000 measurements from the telescope's Mid-Infrared Instrument and more than 1,000 from its Near-Infrared Spectrograph, they looked for a specific, sharp spike in the light that would signal the presence of axions.

The search covered a wide range of possible masses for the axion, from 0.1 electron volts up to 4 electron volts. In the world of particle physics, these are incredibly light weights, but they are heavy enough that their decay would produce infrared light. The researcher had to be extremely careful because the background of the universe is not perfectly smooth; it contains dust, stray light, and other cosmic noise that can mimic a signal. To find the axion signal hidden within this noise, they developed a method to model the background as a smooth curve, allowing them to spot any sharp deviations that stood out. They treated the data with rigorous statistical checks to ensure that any signal they found was real and not just a fluctuation in the cosmic dust or an instrumental artifact.

The results of this analysis are a significant step forward in the hunt for dark matter. The researcher found no evidence of the sharp light spikes that would indicate axions are decaying. Because they found nothing, they were able to set strict limits on the properties of these particles. Specifically, they ruled out the possibility that QCD axions, a specific and well-motivated type of axion, exist with masses between 0.5 and 4 electron volts if they are to make up all the dark matter in our galaxy. For the lighter axions, down to 0.1 electron volts, they established the strongest limits ever set using infrared telescopes. This means that if axions exist in this mass range, they must interact with light even more weakly than previously thought, or they do not make up the entirety of the dark matter.

This work demonstrates a new way of using space telescopes. Rather than just looking at the most interesting objects in the sky, the researcher showed that the routine, empty-sky data collected for calibration purposes is a powerful tool for fundamental physics. By repurposing this existing data, they were able to explore a region of the universe that had not been thoroughly tested before. The study confirms that the James Webb Space Telescope is capable of probing the nature of dark matter in ways that were previously impossible, opening a new window into the invisible universe. While the axion remains a mystery, this research has successfully narrowed the search, telling scientists exactly where not to look next and guiding the future of the hunt for the invisible substance that shapes our cosmos.

Technical Summary: First Constraints on QCD Axion Dark Matter using James Webb Space Telescope Observations

Problem Statement
Unraveling the nature of dark matter (DM) remains a fundamental challenge in modern physics. While cosmological observations confirm its existence, its microscopic properties are unknown. Among Beyond-the-Standard Model (BSM) candidates, the QCD axion is a prominent solution to the strong charge-parity (CP) problem, potentially constituting the DM halo. In the mass range of 0.1–4 eV, axions are predicted to decay into two photons, producing a monochromatic emission line in the infrared and optical spectrum. Previous searches in this mass range have primarily relied on targeted observations of dwarf galaxies, globular clusters, and galaxy clusters. However, these observations cover limited sky areas. The James Webb Space Telescope (JWST) routinely acquires "blank-sky" observations for background subtraction in astronomical surveys, creating a vast, untapped dataset that could be repurposed to search for the diffuse axion decay signal across the entire Milky Way halo.

Methodology
This study utilizes publicly available Level 3 science products from two JWST spectrographs: the Near-Infrared Spectrograph (NIRSpec, 0.6–5 µm) and the Mid-Infrared Instrument (MIRI, 4.9–28.8 µm). The analysis employs 1,077 NIRSpec and 15,524 MIRI blank-sky exposures (sky flats) originally collected for calibration purposes.

The core of the methodology involves searching for a spectral line corresponding to the axion decay $a \to \gamma\gamma$ within the Milky Way halo. The expected signal flux is modeled based on the axion-photon coupling ( $g_{a\gamma\gamma}$ ), the axion mass ( $m_a$ ), and the DM column density (D-factor) calculated using a standard Navarro-Frenk-White (NFW) profile. The signal is modeled as a Gaussian distribution, broadened by the DM velocity dispersion (assumed to be a Maxwell-Boltzmann distribution with $\sigma_v = 160$ km/s) and the instrumental line spread function.

To isolate the potential axion signal from the overwhelming astrophysical background (zodiacal light, detector thermal emission, stray light, and interstellar medium emission), the author employs a flexible, non-parametric approach:

Background Modeling: Instead of assuming a fixed parametric form for the background, the local background spectrum is modeled as a smooth polynomial (order $n=4$ ) fitted over a narrow wavelength window. The width of this window is determined by the coherence length of background fluctuations, calculated after filtering out known line features.
Line Identification and Removal: To prevent astrophysical absorption lines from mimicking or masking signals, the analysis identifies and excludes observations containing absorption features with significance greater than $2\sigma$ . Emission-like features are retained to avoid discarding potential axion signals.
Statistical Analysis: A $\chi^2$ goodness-of-fit test is performed, profiling over nuisance parameters representing the polynomial background coefficients.
Error Calibration: Recognizing that JWST pipeline uncertainties often underestimate the true scatter, the author implements a data-driven error calibration. This involves inflating uncertainties to match local residual scatter, rescaling based on control regions away from the signal wavelength, and accounting for correlated residuals across observations using a "spurious signal" nuisance parameter.

Key Contributions

First JWST Constraints: This work presents the first constraints on QCD axion dark matter derived specifically from JWST observations.
Utilization of Blank-Sky Data: It demonstrates the viability of repurposing routine calibration sky flats for particle physics research, effectively utilizing the entire surveyed area of JWST rather than limited target fields.
MIRI Spectrograph Application: It is the first time MIRI spectroscopic data have been used to probe axion dark matter, extending sensitivity to lower masses than previously possible with ground- or space-based telescopes.
Robust Statistical Framework: The paper introduces a rigorous statistical pipeline for handling complex, non-parametric backgrounds and calibrating instrumental uncertainties in the context of axion line searches.

Results
The analysis derives 95% confidence level upper limits on the axion-photon coupling $g_{a\gamma\gamma}$ as a function of axion mass $m_a$ :

Mass Range: The study covers the mass range of 0.1 eV to 4 eV.
- MIRI constrains the range 0.1–0.5 eV.
- NIRSpec constrains the range 0.5–4 eV.
Exclusion Limits: The results rule out the QCD axion parameter space (where axions constitute all DM and solve the strong CP problem) down to masses of 0.5 eV.
Improvement: The constraints improve upon existing bounds for masses between 0.5 and 0.8 eV.
Systematics: The author tested various systematics, including different DM density profiles (Einasto, Burkert), polynomial orders, and the impact of masking the Galactic plane ( $|b| < 30^\circ$ ). While the Galactic plane masking weakens MIRI constraints by up to ~55%, the results remain robust, with the bounds not dominated by a small subset of targets.

Significance and Claims
The paper claims that this analysis underscores the potential of blank-sky observations as a powerful tool for constraining dark matter models. By leveraging the continuous accumulation of JWST data, this strategy allows for the consistent updating and enhancement of axion searches without requiring dedicated telescope time. The work demonstrates that astrophysical missions can be effectively repurposed for particle physics research, providing the first constraints on QCD axion dark matter down to 0.1 eV using infrared telescope data. The author notes that while some high-significance line features were found, many are consistent with known atomic and molecular lines, leaving their origin for future investigation.

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