Improved limit on invisible decays of positronium

@article{Badertscher2007ImprovedLO,
  title={Improved limit on invisible decays of positronium},
  author={A. Badertscher and P. Crivelli and Wulf Fetscher and Ulisse Gendotti and Sergei Gninenko and V. E. Postoev and Andr{\'e} Rubbia and V. D. Samoylenko and Daniel Sillou},
  journal={Physical Review D},
  year={2007},
  volume={75},
  pages={032004}
}
The results of a new search for positronium decays into invisible final states are reported. Convincing detection of this decay mode would be a strong evidence for new physics beyond the standard model (SM): for example, the existence of extra\char21{}dimensions, of milli-charged particles, of new light gauge bosons or of mirror particles. Mirror matter could be a relevant dark matter candidate. In this paper the setup and the results of a new experiment are presented. In a collected sample of… 

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