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  • Open Access

New physics particles mixing with mesons: Production in the fragmentation chain

Yehor Kyselov1,*, Stephen Mrenna2,3,†, and Maksym Ovchynnikov4,‡

  • *Contact author: kiselev883@gmail.com
  • †Contact author: mrenna@fnal.gov
  • ‡Contact author: maksym.ovchynnikov@cern.ch
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Phys. Rev. D 112, 055033 – Published 24 September, 2025

DOI: https://doi.org/10.1103/6pvj-gknv

Abstract

A class of extensions to the Standard Model adds hypothetical long-lived particles (LLPs) that have mass- or kinetic-mixing with neutral mesons, such as pions or rho mesons. The mixing can contribute significantly to the production of LLPs at proton accelerator experiments, and no consistent description of these production modes exists in the literature. In this paper, we develop a framework for studying different LLPs—dark photons, vector mediators coupled to the baryon current, and axionlike particles with different coupling patterns. In particular, we implement the production mechanisms in pythia8, study how the overall flux and kinematic distributions depend on the LLP’s mass, and compare various subprocesses where the mixing contributes—proton bremsstrahlung, meson decay, and production in the fragmentation chain. We find that our new description of LLP production predicts an integrated flux that differs from current approaches by one to two orders of magnitude, and highlight the unavoidable theoretical uncertainties coming from poor knowledge of the properties of heavy mesons.

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Phys. Rev. D 112, 055033– Published 24 September, 2025
  • Received 16 April 2025
  • Accepted 25 August 2025
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