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Vectorial axion-down-strange coupling from $K^{+} \to π^{+} ν \bar{ν}$ data

Diego Guadagnoli ^1,*, Axel Iohner ^1,†, Cristina Lazzeroni ^2,‡, Diego Martínez Santos ^3,§, Joel C. Swallow ^4,∥, and Claudio Toni ^1,¶

¹LAPTh, Université Savoie Mont-Blanc et CNRS, 74941 Annecy, France
²School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
³Universidade de A Coruña, Ferrol Industrial Campus, Dr. Vázquez Cabrera, s/n 15403, A Coruña, Spain
⁴CERN, Esplanade des Particules 1, 1211 Meyrin, Switzerland

^*Contact author: diego.guadagnoli@lapth.cnrs.fr
^†Contact author: axel.iohner@lapth.cnrs.fr
^‡Contact author: cristina.lazzeroni@cern.ch
^§Contact author: diego.martinez.santos@cern.ch
^∥Contact author: joel.swallow@cern.ch
^¶Contact author: claudio.toni@lapth.cnrs.fr

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Phys. Rev. Research 8, 033262 – Published 2 September, 2026

DOI: https://doi.org/10.1103/r2rg-mc5s

Abstract

We reinterpret publicly available $K^{+} \to π^{+} ν \bar{ν}$ data collected by NA62 from 2016 to 2024 to constrain the fundamental vectorial coupling of the QCD axion to down and strange quarks. Using a fully reproducible likelihood analysis and a complete renormalization-group evolution of the axion couplings from the Peccei-Quinn (PQ) scale to the kaon scale, we translate the experimental limit into bounds on both the low-energy flavor-violating coupling and the fundamental UV parameters. In the generic regime where strong contributions dominate the decay amplitude, we obtain $| {(F_{V})}_{sd} (μ_{K}) | > 1.6 \times 10^{12} GeV$ . The coexistence of parametrically suppressed weak contributions implies a second, conceptually distinct constraint: The fact that weak-amplitude dominance arises from highly tuned UV coupling configurations translates into a conservative general lower limit on the PQ scale, $f_{a} > 4.9 \times 10^{4} GeV$ . These results provide the strongest accelerator-based constraints on axion-induced $d \leftrightarrow s$ transitions and establish a robust lower bound on $f_{a}$ , complementary to astrophysical limits.

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References (60)

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Phys. Rev. Research 8, 033262– Published 2 September, 2026

Vol. 8, Iss. 3 — September - November 2026

Received 31 March 2025
Revised 6 March 2026
Accepted 6 July 2026

DOI: https://doi.org/10.1103/r2rg-mc5s

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.

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Vectorial axion-down-strange coupling from $K^{+} \to π^{+} ν \bar{ν}$ data

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Vectorial axion-down-strange coupling from K+→π+νν¯ data

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References (60)

Vectorial axion-down-strange coupling from $K^{+} \to π^{+} ν \bar{ν}$ data