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

Understand

The vector $U$-bosons, or so called 'dark photons', are one of the possible candidates for the dark matter mediators.

• They are supposed to interact with the 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-ion experiments.
• While there is no confirmed observation of dark photons, the detailed analysis of different experimental data allows to estimate the upper limit 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 theoretical constraints on the upper limit of $\epsilon^2(M_U)$ in the mass range $M_U \le 0.6$ GeV from the comparison of the calculated dilepton spectra with the experimental data from the HADES Collaboration at SIS18 energies where the dark photons are not observed.