New bounds from positronium decays on massless mirror dark photons

Understand

We present the results of a search for a hidden mirror sector in positronium decays with a sensitivity comparable with the bounds set by the prediction of the primordial He$^{4}$ abundance from Big Bang Nucleosynthesis.

• No excess of events compatible with decays into the dark sector is observed resulting in an upper limit for the branching ratio of this process of $4.0\times10^{-5}$ ($90\%$ C.L.).
• This is an order of magnitude more stringent than the current existing laboratory bounds and it constraints the mixing strength of ordinary photons to dark mirror photons at a level of $\varepsilon<5.8\times 10^{-8}$.