Exploring Dark Photon Production and Kinetic Mixing Constraints in Heavy-Ion Collisions

Vector U-bosons, often referred to as “dark photons,” are potential candidates for mediating dark matter interactions. In this study, we outline a procedure to derive theoretical constraints on the upper bound of the kinetic mixing parameter ${ϵ}^2\left(M_U\right)$ using dilepton data from heavy-ion from SIS to RHIC energies. The analysis is based on the microscopic Parton–Hadron–String Dynamics (PHSD) transport model, which successfully reproduces the measured dilepton spectra in $p+p$, $p+A$, and $A+A$ collisions. Besides the dilepton channels resulting from interactions and decays of Standard Model particles (such as mesons and baryons), we extend the PHSD approach to include the decay of hypothetical U-bosons into dileptons, $U\to e^{+}{e}^{-}$. The production of these U-bosons occurs via Dalitz decays of pions, η-mesons, ω-mesons, Delta resonances, as well as from the decays of vector mesons and $K^{+}$ mesons. This analysis provides an upper limit on ${ϵ}^2\left(M_U\right)$ and offers insights into the accuracy required for future experimental searches for dark photons through dilepton experiments.