New Bounds from Positronium Decays on Massless Mirror Dark Photons

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 ${\mathrm{He}}^4$ 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 $3.0\times {10}^{-5}$ (90% C.L.). This is an order of magnitude more stringent than the current existing laboratory bounds and it constrains the mixing strength of ordinary photons to dark mirror photons at a level of $ϵ<5.0\times {10}^{-8}$.

Figure 1

Sketch of the experimental setup and technique. ECAL granularity and vacuum pipe have been omitted for better visualization. See text for further details.

Figure 2

The ECAL surrounding the vacuum pipe and the porous silica target consists of 91 BGO crystals in a honeycomb structure, providing granularity and high Hermiticity.

Figure 3

A sketch of the newly designed cavity. The high resistance germanium layer connecting the target with the grounded pipe reduces the extreme fields near the target.

Figure 4

(a) Time distribution of the positron pulse at the target for $E_{e^{+}}=2\mathrm{keV}$. (b)–(f) Measured zero-energy pulse shape after subtraction of the flat background contribution arising from accidentals normalized to the total number of positrons on target.

Figure 5

Background estimation due to fast backscattered positrons from MC simulations (full circles) and observed fraction of zero-energy compatible events (empty circles). The small offset in implantation energy between data points is artificially added for visualization purposes.