The Heavy Photon Search experiment took its first data in a 2015 engineering run using a 1.056 GeV, 50 nA electron beam provided by CEBAF at the Thomas Jefferson National Accelerator Facility, searching for a prompt, electroproduced dark photon with a mass between 19 and $81\mathrm{MeV}/{\mathrm{c}}^2$. A search for a resonance in the $e^{+}{e}^{-}$ invariant mass distribution, using 1.7 days ($1170{\mathrm{nb}}^{-1}$) of data, showed no evidence of dark photon decays above the large QED background, confirming earlier searches and demonstrating the full functionality of the experiment. Upper limits on the square of the coupling of the dark photon to the standard model photon are set at the level of $6\times {10}^{-6}$. Future runs with higher luminosity will explore new territory.

• Received 10 August 2018

DOI:https://doi.org/10.1103/PhysRevD.98.091101

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Published by the American Physical Society