# New bounds from positronium decays on massless mirror dark photons

## TL;DR

This study searches for evidence of a hidden mirror sector through positronium decays, setting new stringent limits on the interaction strength between ordinary and dark mirror photons, thus constraining dark matter models.

## Contribution

It provides the most sensitive laboratory bounds to date on positronium decays into dark sectors, improving previous limits by an order of magnitude.

## Key findings

- No excess events observed, consistent with Standard Model predictions.
- Set an upper limit on the branching ratio of positronium decays into dark sector.
- Constrained the photon-mirror photon mixing strength to less than 5.8×10⁻⁸.

## Abstract

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 $\varepsilon<5.8\times 10^{-8}$.