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

TL;DR

This paper sets theoretical upper limits on the kinetic mixing parameter of light dark photons in heavy-ion collisions by comparing dilepton spectra from experiments with detailed transport model calculations.

Contribution

It introduces a method to constrain dark photon parameters using the PHSD transport approach and existing dilepton data, focusing on the mass range up to 0.6 GeV.

Findings

Established upper limits on $\epsilon^2$ for dark photons in the 0-0.6 GeV mass range.

Demonstrated the compatibility of the PHSD model with dilepton spectra from HADES data.

Provided guidance for future experimental searches of light dark photons.

Abstract

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. Our analysis is based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which reproduces well the measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions. Additionally to the different dilepton channels originating from interactions and decays of ordinary matter particles (mesons and baryons), we incorporate the decay of hypothetical $U$-bosons to dileptons, $U\to e^+e^-$, 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 $\Delta \to N U$. Our analysis can help to estimate the requested accuracy for future experimental searches of 'light' dark photons by dilepton experiments.