Mono-Energetic Axions: Production & Detection

• Mono-energetic axions are pseudoscalar particles produced via non-thermal processes, nuclear de-excitations, and collider events, resulting in sharply defined energy signatures.
• Their distinct mono-energetic spectral lines enable precise probing of dark radiation, rare nuclear processes, and resonance features in particle interactions.
• Detection strategies leverage advanced spectral line searches, inverse Primakoff conversion, and angular distribution analyses to constrain axion properties.

Mono-energetic axions are pseudoscalar particles that, through their production mechanisms or resonance structure, acquire sharply defined energy distributions rather than broad thermal spectra. This phenomenon arises in several physical contexts, including cosmological decays, nuclear transitions, and specific collider processes. Mono-energetic axions play a central role in probing non-thermal dark radiation, rare nuclear processes, and particle searches due to their distinctive experimental signatures and unique cosmological consequences.

1. Production Mechanisms: Non-thermal Origins and Nuclear Excitations

Mono-energetic axion populations can be produced by diverse mechanisms with sharply defined kinematics. Cosmologically, the decay of heavy moduli fields $\Phi$ through two-body channels ($\Phi \to a + a$) produces axions with a fixed energy, $E_a = m_\Phi/2$, at the time of decay (Conlon et al., 2013). This non-thermal process ensures that axions are initially mono-energetic before redshift effects distribute their energy over cosmic time, resulting in a relic population with energy $E_a(t) = (m_\Phi / 2) \cdot (R_0 / R(t))$ where $R$ is the expansion scale factor.

In stellar and laboratory environments, mono-energetic axions are generated via nuclear de-excitations, particularly from M1-type transitions like the $^{57}$Fe line at 14.4 keV. The occupation probability of an excited state, $\omega^*$, and the branching ratio for axion versus photon emission, $\Gamma_a/\Gamma_\gamma$, determine the production rate per unit mass,

$$\dot{N}_a = \mathcal{N} \, \omega^* \, \left[\frac{1}{\tau_m(1+\alpha)}\right] \frac{\Gamma_a}{\Gamma_\gamma}$$

where $\mathcal{N}$ is the isotope density, $\tau_m$ the mean lifetime, and $\alpha$ the internal conversion coefficient (III et al., 2017, Ning et al., 3 Sep 2025, Massarczyk et al., 2021). Because the energy levels are fixed, axion emission results in a highly monochromatic spectral line.

In particle colliders, two-body annihilation such as $\mu^+ \mu^- \to \gamma a$ produces a single photon with a sharply defined energy, $E_\gamma \approx \sqrt{s}/2$, when the axion is light and escapes undetected. This gives rise to mono-chromatic photon signals (Casarsa et al., 2021).

2. Axion Interactions: Enhanced Kinematics and Resonance Features

Mono-energetic axions participate in interactions with Standard Model particles in ways forbidden or highly suppressed for thermal populations. Due to their large initial energies, processes like $a+\gamma \to q+\bar{q}$ during Big Bang Nucleosynthesis (BBN) can occur at center-of-mass energies $E_\text{CoM}^2=2E_aE_\gamma(1-\cos\theta)$, often far exceeding the photon temperature $T_\gamma$ (Conlon et al., 2013).

Nuclear transitions yield branching ratios between axions and photons given by

$$\frac{\Gamma_a}{\Gamma_\gamma} = \left( \frac{k_a}{k_\gamma} \right)^3 \frac{1}{2\pi \alpha_\text{EM} (1+\delta^2)} \left\{ \frac{g_{app}((\beta+1)/2) + g_{ann}((\beta-1)/2)}{(\mu_0-0.5)\beta+\mu_1-\eta} \right\}^2$$

where $g_{app}$ and $g_{ann}$ are axion-proton and axion-neutron couplings, and $\delta$, $\beta$, $\eta$ encapsulate nuclear structure (Ning et al., 3 Sep 2025).

For collider production, portal operators such as $$\mathcal{L}_{\text{photon-ALP}} = (1/\Lambda)aF^{\alpha\beta}\tilde{F}_{\alpha\beta}$$ grow with center-of-mass energy, leading to constant cross sections at large $\sqrt{s}$ (Casarsa et al., 2021). These energy-dependent operators facilitate the creation of mono-energetic axions and distinguish spin-0 (axion-like) signals from spin-1 (dark photon) signals via angular distributions.

3. Cosmological and Astrophysical Implications

Axions produced from modulus decay linger as a cosmic background ("Cosmic Axion Background") with a flux $\Phi_a \sim 10^6~ \text{cm}^{-2}~\text{s}^{-1}$ and present-day energy $E_a \sim \mathcal{O}(100)$ eV for $m_\Phi \sim 10^6-10^7$ GeV (Conlon et al., 2013). This population is distinct from thermal relics due to its non-thermal spectrum.

During BBN, high-energy axion-induced scatterings inject electromagnetic and hadronic energy, impacting primordial element abundances. Observational bounds on $^4$He production constrain the axion decay constant $f_a$, reheating temperature $T_\text{reheat}$, and the modulus branching fraction $B_a$.

In galaxies, nuclear de-excitation in hot stellar environments – especially $^{57}$Fe at 14.4 keV – produces large populations of mono-energetic ultralight axions. These axions can convert into X-rays in galactic magnetic fields, with coherent conversion maximized for axion masses $m_a \lesssim 10^{-10}$ eV and field coherence lengths $L \sim$ kpc (Ning et al., 3 Sep 2025). This enables searches for narrow X-ray lines in external galaxies (M87, M82), with current constraints setting $$|g_{ann} \times g_{a\gamma\gamma}| \lesssim 1.1 \times 10^{-22}$$ GeV$^{-1}$ at 95% C.L.

4. Experimental Detection Strategies

Mono-energetic axion signals are well-suited for spectral line searches due to their sharp energy distributions. Solar axions produced via $^{57}$Fe transitions are detectable through inverse coherent Bragg-Primakoff conversion in TeO$_2$ bolometers, enabled by excellent energy resolution and coherent final-state enhancement (III et al., 2017).

In nuclear reactors, hundreds of magnetic dipole transitions populate a spectrum of correlated mono-energetic axions, allowing detection via Primakoff or Compton-like conversion in nearby low-background detectors (Massarczyk et al., 2021). The short source–detector distances (∼10 m) allow heavier axions (up to ∼1 MeV) to be probed.

Collider searches focus on mono-chromatic photon signals in muon colliders, exploiting portal operator enhancements at high energy. Discrimination between axion-like and dark photon signals is achieved via differences in angular distributions, with spin separation possible at $\mathcal{O}(500)$ events (Casarsa et al., 2021).

Kaon decay-at-rest experiments (e.g., JSNS$^2$) target mono-energetic axions ($K^+ \to \pi^+ a$) in the mass range $40~\text{MeV} \lesssim m_a \lesssim 350~\text{MeV}$ via subsequent $a \to \gamma\gamma$ decays, providing world-leading sensitivity for hadronically coupled axions (Ema et al., 2023).

Astrophysical telescopes (NuSTAR and future X-ray/gamma-ray missions like Athena, AMEGO, e-ASTROGAM) search for mono-energetic lines from decaying axion dark matter ($a \to \gamma\gamma$), sensitive to lifetimes up to $10^{27}– 10^{28}$ s in the keV–MeV band (Foster et al., 2022).

5. Theoretical Models and Composite/Resonant Axions

Mono-energetic axions are not exclusively fundamental particles: emergent/composite axions can arise as bound states or resonances in hidden sectors coupled to the SM by high-scale messenger exchange (Anastasopoulos et al., 2018). The "emergent axion" is the low-energy avatar of the instanton density correlator in a confining large-$N$ hidden theory. If the spectrum contains an isolated light pole,

$G_{hh}(p) \simeq F_0^2/(p^2 + m_0^2)$

the emergent axion behaves essentially mono-energetically.

Model-dependent scenarios can further enhance mono-energetic features. Photophilic hadronic axion models based on heavy magnetic monopoles yield couplings $g_{a\gamma\gamma}$ orders of magnitude larger than conventional KSVZ/DFSZ models, producing narrower and brighter lines in photon-based experiments and resolving astrophysical anomalies (e.g., TeV-ray transparency, enhanced globular cluster cooling) (Sokolov et al., 2021).

Axion mass and couplings are dictated by the underlying nonperturbative dynamics (e.g., in axiverse theories: $m_a \sim \Lambda_D^2/f_a$ for dark sector confinement scale $\Lambda_D$ and large $f_a$). These UV completions naturally produce heavy axions with GUT-scale $f_a$ and masses from keV to PeV, decaying into mono-energetic photons suitable for next-generation cosmic searches (Foster et al., 2022).

6. Cosmogenic and Detector-based Mono-energetic Axions

Cosmogenic sources (dark matter decays $X \to a a$, supernovae, topological defect radiation) yield highly boosted axions that remain relativistic and mono-energetic today. Their essential signature in neutrino detectors (DUNE, JUNO, HK, SK, IceCube) is a mono-energetic single photon generated through inverse Primakoff absorption, $$\mathcal{L} = -(1/4)g_{a\gamma\gamma} a F^{\mu\nu} \tilde{F}_{\mu\nu}$$ (Cui et al., 2022). This expands sensitivity beyond traditional cold axion dark matter approaches, with competitive reach to stellar cooling bounds ($g_{a\gamma\gamma} \lesssim 10^{-10}$ GeV$^{-1}$).

In laboratory settings, mono-energetic axion detection benefits from the correlation between production fluxes, narrow spectral features, and low-background event selection that enhances sensitivity, especially in rare decay or conversion experiments.

7. Constraints, Complementarity, and Future Prospects

Current and projected experiments constrain axion coupling parameter space through searches for mono-energetic signals in environments ranging from hot stellar interiors to colliders and cosmic telescopes. The most stringent bounds on the product $|g_{ann} \times g_{a\gamma\gamma}|$ for ultralight axions reach $1.1 \times 10^{-22}$ GeV$^{-1}$ in the absence of detected 14.4 keV X-ray lines from M87 and M82 (Ning et al., 3 Sep 2025).

Mono-energetic axion searches complement thermal relic and broad-band astrophysical probes. Their distinct spectral signatures facilitate resonance and angular analyses, enable discrimination against vector candidates, and open new avenues for testing strong CP-violation models, baryogenesis scenarios, and axiverse cosmology.

Future directions involve larger target volumes, improved energy resolution, advanced magnetic field modeling for conversion dynamics, and expanded analysis of multi-line features in nuclear-rich environments. A plausible implication is that mono-energetic axion phenomenology will continue to bridge particle, nuclear, and cosmological physics, refining constraints on axion properties and guiding experimental strategies in the search for new fundamental particles.