Electrophilic Axion-Like Particles

• Electrophilic ALPs are light pseudoscalars defined by significant electron couplings (gₐₑ) that govern their production and decay across diverse experimental environments.
• Production via solar ABC channels, reactor Compton processes, and electron bremsstrahlung distinguishes their phenomenology from standard photophilic axion models.
• Integrated astrophysical, collider, and multimessenger analyses tightly constrain the independent electron and electroweak anomaly-induced operator spaces in these models.

Electrophilic axion-like particles are light pseudoscalars whose phenomenology is controlled by sizable couplings to electrons, usually denoted $g_{ae}$ or $g_{aee}$, rather than by the QCD axion mass–coupling relation. In the electron-coupled case, the relevant operators are a derivative axial interaction or an equivalent Yukawa-like pseudoscalar coupling, while some collider analyses extend the adjective “electrophilic” to ALPs coupled predominantly to electrically charged electroweak gauge bosons and photons. Across these usages, the distinctive feature is that electron interactions, electroweak anomaly couplings, or both determine production, decay, and experimental reach (Graham et al., 2016, Lu, 2022, Aiko et al., 2023).

1. Effective description and operator structure

For electron-coupled ALPs, the standard parameterizations are

$$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$

or, after integration by parts and use of the electron equations of motion,

$$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$

In the notation used for solar and reactor phenomenology, the coupling is often written $g_{aee}$ (Graham et al., 2016, Sierra et al., 2020). A 2025 supernova analysis explicitly demonstrates that the pseudoscalar and derivative forms are equivalent in a relativistic plasma, and that the equivalence is not spoiled by the medium; the only additional structure is the axial anomaly induced by the chiral rotation (Fiorillo et al., 19 Mar 2025).

A broader EFT language appears in collider studies of ALP–lepton interactions. The most general lepton current used in electroweak-violating analyses is

$$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$

which, after using the anomalous divergence, generates three structures: an $a\ell\ell$ coupling, anomaly-induced $aVV'$ couplings, and a genuine four-point $W$-$\ell$-$g_{aee}$0-$g_{aee}$1 contact interaction (Lu, 2022). In the electroweak-violating benchmark,

$g_{aee}$2

the contact term

$g_{aee}$3

is present, whereas it vanishes in the electroweak-preserving case $g_{aee}$4, $g_{aee}$5 (Lu, 2022).

A distinct but related usage appears in electroweak-gauge EFTs where the ALP couples to $g_{aee}$6 and $g_{aee}$7. After electroweak symmetry breaking this yields correlated couplings $g_{aee}$8, $g_{aee}$9, $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$0, and $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$1. In that literature, “electrophilic” can mean coupling to electrically charged states such as photons and $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$2, even without direct fermionic couplings (Yue et al., 2021, Aiko et al., 2023).

2. Production and decay channels

Electron couplings qualitatively change both production and detection. In the Sun, non-hadronic axion models with tree-level electron coupling $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$3 receive additional production from axion recombination, bremsstrahlung, and Compton-like processes, the “ABC” channels. The resulting solar ALP flux can be $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$4 larger than the standard Primakoff flux and peaks at $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$5 keV, rather than in the usual Primakoff 1–10 keV range (Graham et al., 2016). This is the canonical stellar signature of electrophilic ALPs.

In relativistic plasmas and supernova cores, the dominant electron-driven channels are electron bremsstrahlung, semi-Compton production $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$6, and pair annihilation $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$7. A 2025 reassessment incorporates the latter two channels explicitly and provides analytic bremsstrahlung emissivities, together with uncertainty estimates from thermal masses, Coulomb interactions, and the Landau-Pomeranchuk-Migdal effect (Fiorillo et al., 19 Mar 2025).

Reactor environments access the sub-10 MeV regime. There, ALPs are produced through Compton-like scattering $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$8, Primakoff conversion, and nuclear magnetic transitions, and can be detected via inverse Compton-like scattering, axio-electric absorption, or decay into $$\mathcal{L}_{ae}=g_{ae}(\partial_\mu a)\,\bar e\gamma^\mu\gamma_5 e$$9 or $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$0 (Sierra et al., 2020). The reactor photon spectrum

$$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$1

makes MeV-scale electrophilic ALPs particularly natural targets (Sierra et al., 2020).

The dominant decay modes depend on mass and operator content. For a direct electron coupling,

$$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$2

while a photonic coupling gives

$$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$3

In the electroweak-violating collider setup, the UV interaction is electron-philic, but anomaly-induced diphoton decays dominate once $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$4 exceeds about $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$5 GeV; above that scale, heavier $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$6ALPs are effectively photophilic in their visible decay phenomenology (Lu, 2022).

3. Stellar, supernova, and multimessenger constraints

Stellar cooling is the oldest constraint on electron-coupled ALPs. In the solar context, CAST has performed dedicated searches for ABC solar axions, and IAXO is explicitly designed to test ABC solar axions for non-excluded values of $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$7, including regions motivated by white-dwarf cooling anomalies (Graham et al., 2016). At the same time, the same review emphasizes that astrophysical limits on $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$8 are already “quite restrictive” and disfavor much of the parameter space where the ABC flux would be very large (Graham et al., 2016).

For MeV-scale electrophilic ALPs, supernovae provide a different regime. A model-independent multimessenger program parameterizes the injected electron and positron yield per core-collapse supernova by a single number $$\mathcal{L}_{ae}\simeq i g_{ae}\,a\,\bar e\gamma_5 e.$$9, then constrains it using Voyager-1, the 511 keV line, diffuse X-rays, diffuse MeV gamma rays, and in-flight annihilation. In the ALP case, the paper fits

$g_{aee}$0

for a benchmark $g_{aee}$1 MeV scenario in which production is controlled by nucleon couplings while decay is controlled by $g_{aee}$2 (Luque et al., 2023). The strongest robust continuum limit in that analysis is $g_{aee}$3 from combined XMM/MOS rings, while the most aggressive limits from the 511 keV longitude profile reach $g_{aee}$4 with substantial morphology systematics (Luque et al., 2023).

A 2024 review of the same program identifies in-flight annihilation as the strongest and most robust probe, with benchmark limits as strong as $g_{aee}$5, and states that these multimessenger bounds are much stronger than those obtained from SN cooling in the relevant electrophilic-FIP regime (Luque et al., 2024). A plausible implication is that MeV-scale electrophilic ALPs can no longer be assessed solely through the traditional energy-loss argument; the Galactic propagation and decay products of the emitted $g_{aee}$6 become central observables.

The 2025 supernova reanalysis sharpens the small- and large-coupling boundaries. At small $g_{aee}$7, the dominant constraints come from the previously neglected decay $g_{aee}$8, except in a fireball-formation region where SN 1987A X-ray observations offer the best probe; at large couplings, the bounds are dominated by energy deposition, with a new prescription for the trapping regime (Fiorillo et al., 19 Mar 2025).

4. Laboratory searches: reactors, beam dumps, and colliders

Laboratory coverage of electrophilic ALPs is fragmented because different experiments probe different operator combinations. Reactor experiments are the most direct electron-based probes below 10 MeV. Using Compton-like production in the reactor core and detection via inverse Compton-like scattering, axio-electric absorption, or $g_{aee}$9, reactor facilities can test $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$0 and $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$1, extending beyond TEXONO and Borexino limits (Sierra et al., 2020).

Electron beam dumps remain especially relevant in the 10–30 MeV range, but reinterpretation is coupling-structure dependent. The SLAC 141 case shows that limits obtained for pseudoscalars decaying to $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$2 cannot be naively re-used as limits on photon-coupled ALPs, and more generally that translating between electron-coupled and photon-coupled scenarios is non-trivial (Döbrich, 2017). For genuinely electrophilic ALPs, the relevant production channel is $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$3, followed by $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$4 if kinematically open (Döbrich, 2017).

High-energy colliders introduce an important twist. In the electroweak-violating electron-ALP EFT, the $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$5-$$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$6-$$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$7-$$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$8 contact term produces energy-enhanced $$J^{\mu}_{\text{PQ},\ell} = \frac{c^V_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\ell + \frac{c^A_{\ell}}{2\Lambda}\,\overline{\ell}\gamma^{\mu}\gamma_5\ell + \frac{c_{\nu}}{2\Lambda}\,\overline{\nu_{\ell}}\gamma^{\mu} P_L \nu_{\ell},$$9-channel processes,

$a\ell\ell$0

with visible final states dominated by $a\ell\ell$1 or by photon-jets once the ALP is sufficiently boosted (Lu, 2022). Relative to electroweak-preserving couplings, the cross sections are larger by more than four orders of magnitude in $a\ell\ell$2 and by about nine orders of magnitude in $a\ell\ell$3 for the benchmark choices discussed in the paper (Lu, 2022). In this sense, a UV-electrophilic ALP can manifest experimentally as a diphoton resonance plus missing energy.

A different collider program targets gauge-electrophilic ALPs. At CLIC, $a\ell\ell$4 fusion,

$a\ell\ell$5

probes masses up to about $a\ell\ell$6 TeV and reaches $a\ell\ell$7 at $a\ell\ell$8 in the 3 TeV stage under the simplifying choice $a\ell\ell$9 (Yue et al., 2021). This is not an electron-Yukawa probe, but it is part of the broader charged-state phenomenology sometimes grouped under the same label.

Precision Higgs data add a further indirect handle. In the gauge-only electroweak EFT, one-loop ALP effects in $aVV'$0 cancel exactly, while $aVV'$1 remains highly sensitive. Current Higgs data exclude $aVV'$2–$aVV'$3 or $aVV'$4–$aVV'$5, depending on $aVV'$6, with future colliders projected to probe down to roughly $aVV'$7–$aVV'$8 (Aiko et al., 2023). This is a strong reminder that electrophilic need not mean tree-level $aVV'$9; in some models the dominant precision probes are entirely bosonic.

5. Cosmology, dark sectors, and UV settings

String compactifications provide one route to electron-coupled ALPs. In type IIB LARGE Volume Scenario models, open-string axions localized on visible-sector branes couple directly to visible fermions, including electrons, with the expected derivative form

$W$0

For sequestered visible-sector singularities, the decay constant can lie near $W$1 GeV, making open-string axions natural QCD-axion or ALP candidates, though not automatically dominantly electrophilic (Cicoli, 2013). This suggests that genuinely electrophilic charge assignments are easiest to engineer for open-string rather than bulk closed-string axions.

Electroweak baryogenesis driven by an ALP provides another setting where electron couplings are induced rather than fundamental. In that framework, the ALP couples anomalously to $W$2, mixes with the Higgs, and inherits scalar couplings to charged leptons proportional to Yukawas. The viable window is $W$3–$W$4 GeV, with $W$5 MeV–GeV, and the associated electron EDM remains far below current bounds because the baryogenesis-relevant CP violation is controlled by the field excursion $W$6, not by a large low-energy $W$7 (Jeong et al., 2018). This is not a strongly electrophilic model, but it clarifies that Higgs mixing can generate modest lepton couplings without making them the dominant phenomenological portal.

A fully explicit electrophilic portal appears in the SIMP dark-matter model of 2026. There the ALP couples exclusively to electrons,

$W$8

and to dark pions through the dark-fermion mass matrix (Fiorentino et al., 2 Feb 2026). For $W$9, the leading dark-sector interaction is the quartic $\ell$0 coupling, allowing viable portal masses $\ell$1 MeV and $\ell$2 GeV. For nonzero dark $\ell$3, a linear $\ell$4 coupling appears, enabling off-shell $\ell$5 thermalization and opening a heavy-ALP regime with $\ell$6 and even $\ell$7 (Fiorentino et al., 2 Feb 2026). The 17 MeV region associated with the reported $\ell$8 anomaly lies inside the allowed portal window for representative $\ell$9 values (Fiorentino et al., 2 Feb 2026).

6. Conceptual issues, ambiguities, and outlook

Several recurrent misconceptions shape the literature. First, electrophilic ALPs are not generically photophilic. Electron couplings can dominate production in stars, reactors, and supernovae even when photon couplings are loop-induced or absent at tree level (Graham et al., 2016, Sierra et al., 2020). Conversely, once $g_{aee}$00 is large enough, the visible decay of an electron-coupled ALP can still be dominated by $g_{aee}$01, as happens in the electroweak-violating collider framework above about $g_{aee}$02 GeV (Lu, 2022).

Second, the term itself is not uniform across subfields. In most astrophysical and low-energy contexts it means significant $g_{aee}$03 coupling, but some collider and Higgs studies apply it to ALPs coupled predominantly to photons and electroweak gauge bosons because those are electrically charged states (Yue et al., 2021, Aiko et al., 2023). The distinction is not semantic: it changes which channels vanish at tree level, which rates are loop-induced, and which experiments dominate.

Third, the mainstream axion program remains primarily photophilic. Microwave haloscopes, light-shining-through-walls setups, and standard helioscopes mostly probe $g_{aee}$04, while reactor searches, axio-electric detectors, reanalyses of low-threshold electron recoils, and multimessenger supernova studies are the natural probes of suppressed-$g_{aee}$05, enhanced-$g_{aee}$06 scenarios (Graham et al., 2016, Luque et al., 2024). CASPEr-style searches for electron spin were already identified as conceptually possible, but not sensitive enough to improve on existing limits in the 2016 review (Graham et al., 2016).

The present research picture therefore has a clear structure. Electrophilic ALPs occupy a parameter space in which $g_{aee}$07, $g_{aee}$08, $g_{aee}$09, $g_{aee}$10, and, in some settings, $g_{aee}$11 are effectively independent. Solar ABC production, reactor Compton-like production, supernova multimessenger constraints, and electroweak-violating collider channels are the defining probes of this sector. Higgs precision data and high-energy $g_{aee}$12 fusion become decisive when the same ALP also carries sizeable electroweak anomaly couplings. This suggests that future progress will come less from any single “best” experiment than from global analyses that consistently combine electron, photon, electroweak, and nucleon operators across stellar, cosmological, fixed-target, reactor, and collider environments.