Search for dark photons in heavy‐ion collisions

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)−Ufalse(1false)′$$ U(1)-U{(1)}^{\prime } $$ 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 us to estimate the upper limit for the kinetic mixing parameter ϵ2$$ {\epsilon}^2 $$ depending on the mass MU$$ {M}_U $$ of U‐bosons which is also unknown. We have introduced a procedure to define theoretical constraints on the upper limit of ϵ2()MU$$ {\epsilon}^2\left({M}_U\right) $$ from heavy‐ion (as well as p + p and p+A$$ p+A $$) dilepton data. Our analysis is based on the microscopic Parton‐Hadron‐String Dynamics transport approach where we incorporated the decay of hypothetical U‐bosons to dileptons, U→e+e−$$ U\to {e}^{+}{e}^{-} $$, where the U‐bosons themselves are produced by the Dalitz decay of pions π0→γU$$ {\pi}^0\to \gamma U $$, η‐mesons η→γU$$ \eta \to \gamma U $$, and Delta resonances normalΔ→italicNU$$ \Delta \to NU $$. The extension of our procedure to other dark matter candidates is foreseen.