Speaker
Description
The vector $U$-bosons, or so-called 'dark photons', are potential candidates for dark matter mediators. They are expected to interact with standard matter via a 'vector portal' due to the $U(1)-U(1)^\prime$ symmetry group mixing, which may make them detectable in particle and heavy-ion experiments. Although there has been no confirmed observation of dark photons, detailed analyses of various experimental data have provided upper limits for the kinetic mixing parameter $\epsilon^2$, depending on the mass $M_U$ of $U$-bosons, which is also currently unknown. In this study, we present a procedure to define theoretical constraints on the upper limit of $\epsilon^2(M_U)$ from heavy-ion as well as $p+p$ and $A+A$ dilepton data from SIS to LHC energies, where dark photons have not been observed. Our analysis is grounded in the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach, which accurately reproduces the measured dilepton spectra in $p+p$, $p+A$, and $A+A$ collisions. In addition to considering the various dilepton channels originating from interactions and decays of ordinary matter particles (mesons and baryons), we incorporate the decay of hypothetical $U$-bosons to dileptons, $U \to e^+e^-$. 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 ($\Delta \to N U$). We also include the Dalitz decay of the $\omega$, direct decays of vector mesons such as the $\rho$, $\omega$, and $\phi$, as well as decays from $K^+$ mesons. Our analysis can provide insight into the required accuracy for future experimental searches for 'light' dark photons in dilepton experiments.
