Luca Di Luzio

The meV mass range has emerged as a focal point in axion physics, where advances in theory, cosmology, astrophysics, and experimental techniques converge. Axions in this mass range are theoretically well motivated, can arise in ultraviolet-complete models, and can have significant cosmological impacts as dark matter or dark radiation. In parallel, their efficient production in stellar and supernova environments provides powerful astrophysical probes. Here, we provide a comprehensive overview of meV axions across these domains, highlighting both established results and open questions. We discuss the theoretical underpinnings of meV axions, their cosmological and astrophysical signatures, and the diverse experimental strategies — ranging from helioscopes and haloscopes to quasiparticle systems and large-volume Cherenkov detectors — that aim to explore this regime. The convergence of these approaches emphasizes the pivotal role of the meV mass range for axion discovery in the coming years, identifying meV axions as a key probe for testing beyond-Standard-Model physics. This review document is the direct outcome of the discussions at the dedicated workshop “The meV Mass Axion Frontier: Challenges and Opportunities”, held at Laboratori Nazionali di Frascati (IT) on 27–28 October 2025, and organized by the EU funded COST Action “Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments” (CA21106, https://www.cost.eu/actions/CA21106). Its aim is to provide an overview of current efforts in meV axion research, their motivations, and the research goals that animate the community involved in this search.

A bstract: We study light spin-1 gauge bosons coupled to electroweak-anomalous currents. For generic charge assignments, anomaly cancellation requires new fermions (anomalons) that are chiral under the new abelian symmetry and carry electroweak charges. If their masses arise from the breaking of the new gauge symmetry, integrating them out generates Wess-Zumino interactions fixed by mixed-anomaly matching, providing the infrared description of the theory. We classify minimal anomalon spectra, derive the corresponding effective interactions, and combine experimental constraints with finite-naturalness considerations to bound the UV completion scale. Motivated by recent NA62 and Belle II results, we then develop a unified phenomenological framework for the missing-energy signatures of these anomalous light vectors, focusing on scenarios where the new vector decays predominantly into neutrinos so that the leading probes are rare processes with invisible final states. As applications, we survey current and projected searches across flavour and electroweak observables, including K → πE _miss , B → K ^(*) E _miss , and Z → γE _miss , and discuss their interplay with direct searches for anomalons.

We report first results from RadioAxion, an underground experiment searching for axion dark matter through periodic modulations of radioisotope decays. We monitor the α decay of Am 241 via its 59.5 keV γ line using a NaI detector installed at the Gran Sasso Laboratory, where cosmic-ray-induced systematics are strongly suppressed. We present the measured spectra and the corresponding time-series analysis. No evidence for a periodic modulation is observed. From these data, we derive constraints on the axion decay constant in the axion mass range from 10 − 21 to 10 − 9 eV .

A bstract: Murayama [arXiv:2601.02465] proposed a GeV-scale axion theory where the up-quark mass term is generated dynamically by the QCD chiral condensate, spontaneously breaking a Peccei-Quinn symmetry. It predicts a too large mass splitting between neutral and charged pions. Trying to solve this problem we explore extensions. Despite some partial improvements, we identify a structural obstruction: the new Peccei-Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, leading to an enhanced higher-order operator. As a consequence, pion scatterings too are distorted. We also examine the limit where the axion becomes light, finding that it is excluded by fifth-force constraints.

We propose to investigate the production of a light scalar boson ϕ in low-energy proton-nucleus interactions using the 3.5 MV accelerator of the Bellotti Ion Beam Facility, located in the underground Gran Sasso National Laboratory. Nuclear reactions induced by a few-MeV proton beam on suitable target materials can act as a controlled source of ϕ particles. Owing to the deep-underground location, the facility benefits from substantial cosmic-ray shielding, enabling searches for rare processes with minimal background. The produced ϕ particles will be sought with large-volume, low-background detectors already operating or currently under construction at the Gran Sasso National Laboratory. This approach combines a tunable accelerator-based production mechanism with the exceptional sensitivity of underground rare-event searches, offering a novel avenue to probe light scalar bosons beyond the Standard Model.

We consider a class of theories with a U ( 1 ) X gauge symmetry associated with leptonic chiral currents. The low-energy effective field theory includes a light spin-1 boson coupled to the electroweak gauge sector via a Wess-Zumino term, which ensures anomaly cancellation in the infrared. As a concrete application, we show that a light vector boson with mass m X ≃ 2.1 GeV , coupled to a τ -lepton chiral current, can readily account for the recent 3 σ excess observed in B → K ( * ) E miss at Belle II, while remaining consistent with existing constraints from Z → γ E miss and direct searches for anomalon fields responsible for anomaly cancellation in the ultraviolet. After classifying phenomenologically viable models, we explore in greater detail two concrete realizations which give rise to distinctive phenomenological signatures, potentially accessible at future experiments at the high-energy and intensity frontiers.

We extend the catalogue of “phenomenologically preferred” hadronic axion models to include heavy fermion representations associated with higher-dimensional decay operators. The latter have recently been shown to self-consistently trigger a period of early matter domination, making the underlying axion models cosmologically viable. After identifying all possible representations up to decay operator dimension d ≤ 9, we update the hadronic axion band for the axion-photon coupling. The central regions of the axion band are similar to those found previously and approximately independent of the axion decay constant f_a , suggesting that they are robust predictions and targets for future axion searches. Moreover, we find that d = 6 and d = 7 operators can lead to two new viable “model islands” around f_a ∼ 10¹² GeV and f_a ∼ 10¹⁴ GeV, i.e., beyond the standard post-inflationary mass region.