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  • Letter
  • Open Access

Good qualities of the weak axion

Giacomo Cacciapaglia*

Francesco Sannino†

Jessica Turner‡

  • *Contact author: cacciapa@lpthe.jussieu.fr
  • †Contact author: sannino@qtc.sdu.dk
  • ‡Contact author: jessica.turner@durham.ac.uk
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Phys. Rev. D 113, L051705 – Published 20 March, 2026

DOI: https://doi.org/10.1103/w8wc-6ryc

Abstract

The presence of a topological susceptibility in the electroweak sector of the Standard Model motivates the existence of a good quality weak axion aW, associated with the spontaneous breaking of B+L. Its anomalous couplings and tiny mass, generated from electroweak instantons, render aW photophobic. We find that the strongest bound on the associated decay constant fW stems from a loop-induced coupling to electrons, leading to fW≳1000  TeV from stellar cooling. Spontaneous breaking of the Abelian B+L symmetry induces proton decay via higher-dimensional operators controlled by a new physics scale Λ. Existing Super-Kamiokande limits on these decay channels constrain the new physics scale to be Λ≳1012  GeV. The characteristic channel p→e+aW and other possible operators mediating interactions with the Standard Model fields yield signals which are not detectable within the allowed parameter space. Future proton decay searches at the next generation of neutrino experiments offer the most promising avenues to test the good qualities of the weak axion paradigm.

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Phys. Rev. D 113, L051705– Published 20 March, 2026
  • Received 6 November 2025
  • Accepted 23 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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