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Crossing into the ma>fa region for leptophilic ALPs

Marta F. Zamoro*, Álvaro Lozano-Onrubia†, Luca Merlo‡, and Samuel Rosende Herrero§

  • *Contact author: marta.zamoro@uam.es
  • †Contact author: alvaro.lozano.onrubia@csic.es
  • ‡Contact author: luca.merlo@uam.es
  • §Contact author: samuel.rosende@uam.es
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Phys. Rev. D 114, 095002 – Published 2 November, 2026

DOI: https://doi.org/10.1103/xf8c-823x

Abstract

Axion-like particles (ALPs) are typically identified as pseudoscalars whose couplings are shift-symmetry invariant with the exception of their couplings to gauge bosons and their mass term. Additionally, the ALP mass ma is usually assumed to be (much) smaller than the ALP decay constant fa. The latter condition is conservative, at best, and excludes part of the ALP parameter space that is presently viable. We revisit the criterion of the ma≪fa and perform an analysis focusing on leptophilic ALPs. In particular, we explore regions of the parameter space still uninvestigated, where ma>fa, thus providing a phenomenological study of the ALP-lepton couplings complementary to the existing literature. We point out that a leptophilic ALP may explain the −3.8σ tension in the anomalous magnetic dipole moment of the electron for the cesium determination in a large region of the ma×fa parameter space, testable in the near future through studies on μ→e conversion in nuclei.

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Phys. Rev. D 114, 095002– Published 2 November, 2026
  • Received 12 June 2026
  • Accepted 26 August 2026
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