An Improved Limit on Invisible Decays of Positronium

Abstract

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 evid ence for new physics beyond the Standard Model (SM): for example the existence of extra--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 about $(6.31\pm 0.28)\times 10^6$ orthopositronium decay s, no evidence for invisible decays in an energy window [0,80] keV was found and an upper limit on the branching ratio of orthopositronium \invdecay could be set: $\binvdecay<4.2\times 10^{-7}$ (90% C.L.) Our results provide a limit on the photon mirror-photon mixing strength $\hat{I}\mu \le 1.55\times 10^{-7}$ (90% C.L.) and rule out particles lighter than the electron mass with a fraction $Q_x\le 3.4\times 10^{-5}$ of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium $Br(p-Ps\to invisible)\le 4.3\times 10^{-7}$ (90% C.L.) and the direct annihilation $Br(e^{+}{e}^{-}\to invisible)\le 2.1\times 10^{-8}$ (90% C.L.) could be set.

17 pages, 7 figures, added references, fixed limit on millicharged particles and changed two plots accordingly