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Constraints on kinetic mixing of dark photons from dilepton spectra

A. W. Romero Jorge1,2,3,*, E. Bratkovskaya2,3,4,†, T. Song4, and L. Sagunski2

  • 1Frankfurt Institute for Advanced Studies (FIAS), Ruth Moufang Str. 1, 60438 Frankfurt, Germany
  • 2Institut für Theoretische Physik, Johann Wolfgang Goethe University, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany
  • 3Helmholtz Research Academy Hessen for FAIR (HFHF), GSI Helmholtz Center for Heavy Ion Physics Campus Frankfurt, 60438 Frankfurt, Germany
  • 4GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany

  • *Contact author: jorge@itp.uni-frankfurt.de
  • †Contact author: E.Bratkovskaya@gsi.de
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Phys. Rev. C 112, 054905 – Published 18 November, 2025

DOI: https://doi.org/10.1103/qzzg-h1xw

Abstract

Dark photons, the hypothetical gauge bosons associated with an additional U(1)′ symmetry, can couple to standard model particles through a small kinetic mixing parameter ɛ with the ordinary photon. This mechanism provides a portal between the dark sector and visible matter. In this study, we present a procedure to derive theoretical upper bounds on the kinetic mixing parameter ɛ2(MU) by analyzing dilepton spectra from heavy-ion collisions across a broad energy range, from SIS to LHC energies. Our analysis is based on the microscopic parton-hadron-string dynamics (PHSD) transport approach, which successfully reproduces the measured dilepton spectra in p+p, p+A, and A+A collisions across the same energy range. Besides the dilepton channels resulting from interactions and decays of standard model particles (such as mesons and baryons), the PHSD has been extended to include the decay of hypothetical dark photons into dileptons, U→e+e−. The production of these dark photons occurs via Dalitz decays of π0, η, ω, η′, and Δ resonances; direct decays of ρ, ω, and ϕ; the kaon mode K+→π+U; and thermal qq¯ annihilation in the quark-gluon plasma. Our results show that high-precision measurements of dilepton spectra in heavy-ion collisions provide a sensitive and competitive probe of dark photons in the MeV to multi-GeV mass range. Furthermore, we quantify the experimental accuracy required to constrain the remaining viable parameter space of kinetic mixing in dark photon scenarios.

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Phys. Rev. C 112, 054905– Published 18 November, 2025
  • Received 18 July 2025
  • Accepted 30 September 2025
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