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The vector $U$-bosons, or so called 'dark photons', are one of the possiblecandidates for the dark matter mediators. They are supposed to interact withthe standard matter via a 'vector portal' due to the $U(1)-U(1)^\prime$symmetry group mixing which might make them visible in particle and heavy-ionexperiments. While there is no confirmed observation of dark photons, thedetailed analysis of different experimental data allows to estimate the upperlimit for the kinetic mixing parameter $\epsilon^2$ depending on the mass $M_U$of $U$-bosons which is also unknown. In this study we present theoreticalconstraints on the upper limit of $\epsilon^2(M_U)$ in the mass range $M_U \le0.6$ GeV from the comparison of the calculated dilepton spectra with theexperimental data from the HADES Collaboration at SIS18 energies where the darkphotons are not observed. Our analysis is based on the microscopicParton-Hadron-String Dynamics (PHSD) transport approach which reproduces wellthe measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions.Additionally to the different dilepton channels originating from interactionsand decays of ordinary matter particles (mesons and baryons), we incorporatethe decay of hypothetical $U$-bosons to dileptons, $U\to e^+e^-$, where the$U$-bosons themselves are produced by the Dalitz decay of pions $\pi^0\to\gamma U$, $\eta$-mesons $\eta \to \gamma U$ and Delta resonances $\Delta \to NU$. Our analysis can help to estimate the requested accuracy for futureexperimental searches of 'light' dark photons by dilepton experiments.

physical review. d/physical review. d.

Constraints on the kinetic mixing parameter $\epsilon^2$ for the light  dark photons from dilepton production in heavy-ion collisions in the few-GeV  energy range