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Probing axion and flavored new physics with the NA64μ experiment

Haotian Li1, Zuowei Liu1,*, and Ningqiang Song2,†

  • *Contact author: zuoweiliu@nju.edu.cn
  • †Contact author: songnq@itp.ac.cn
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Phys. Rev. D 112, 115013 – Published 3 December, 2025

DOI: https://doi.org/10.1103/27kn-ly9l

Abstract

High-energy muon beam dump experiments are powerful probes of new physics models beyond the Standard Model, particularly those involving flavor-dependent interactions. We demonstrate the potential of muon beam dump by placing strong constraints on three new physics models utilizing data from the recent NA64μ experiment: (1) axions coupling to photons, (2) axions coupling to muons, and (3) a dark sector mediated by a massless U(1)Lμ−Lτ gauge boson. The new particles can be identified from the significant missing energy as the invisible channel, or the distinct energy deposition signature as the visible channel. We find that the current NA64μ data do not yet probe new parameter region on axion-photon coupling, while excluding new parameter space for the axion-muon coupling gaμμ≳4×10−3  GeV−1 and the axion mass ma≲0.2  GeV. For the dark sector, the current data provide stringent constraints that surpass existing ones by nearly one order of magnitude. The data from the 2023 NA64μ run, once available, will be capable of excluding new axion-photon coupling parameter space and probing the flavor structure of new physics, with more sensitivity advancement expected in near future runs.

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Phys. Rev. D 112, 115013– Published 3 December, 2025
  • Received 6 February 2025
  • Revised 22 July 2025
  • Accepted 14 November 2025
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