Constraints on Kinetic Mixing of Dark Photons from Dilepton Spectra

TL;DR

This paper develops a method to set upper bounds on dark photon kinetic mixing parameters by analyzing dilepton spectra from heavy-ion collisions using an extended transport model, providing a sensitive probe across a wide mass range.

Contribution

It introduces a procedure to derive theoretical upper bounds on dark photon kinetic mixing from dilepton spectra, extending the PHSD model to include dark photon decays and analyzing data across multiple energies.

Findings

High-precision dilepton measurements can constrain dark photon parameters.

The extended PHSD model successfully reproduces dilepton spectra including dark photon decays.

Quantified experimental accuracy needed to explore remaining dark photon parameter space.

Abstract

Dark photons, the hypothetical gauge bosons associated with an additional $U(1)^{\prime}$ symmetry, can couple to Standard Model particles through a small kinetic mixing parameter $\varepsilon$ 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 $\varepsilon^2(M_U)$ 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, $U \to e^+ e^-$. The production of these dark photons occurs via Dalitz decays of $\pi^0$, $\eta$, $\omega$, $\eta^{\prime}$, and $\Delta$ resonances; direct decays of $\rho$, $\omega$, and $\phi$; the kaon mode $K^+ \to \pi^+ U$; and thermal $q\bar q$ 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.