hidden sectors with massive, extra, U (1) gauge bosons, so-called heavy photons (or dark photons or $A'$). Such theories received a lot of attention as a possible dark matter explanation for the larger than expected numbers of cosmic-ray electrons and positrons. The Heavy Photon Search (HPS) and APEX experiments at Jefferson Lab are designed to look for these heavy photons in the mass range 20-1000 MeV that couple to electrons through kinetic mixing with couplings α /α in the range 10$^{−5}$ to

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... 10$^{−10}$. The HPS will search for the $e^ + e^ −$ or $\mu^ + \mu^ −$ decay of the heavy photon, possibly with a displaced vertex, using a compact forward spectrometer, while the APEX experiment will use the spectrometers of Hall-A at Jefferson lab.