The Heavy Photon Search (HPS), an experiment to search for a hidden sector
photon in fixed target electroproduction, is preparing for installation at the
Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As
the first stage of this project, the HPS Test Run apparatus was constructed and
operated in 2012 to demonstrate the experiment's technical feasibility and to
confirm that the trigger rates and occupancies are as expected. This paper
describes the HPS Test Run apparatus and readout electronics and its
performance. In this setting, a heavy photon can be identified as a narrow peak
in the e$^+$e$^-$ invariant mass spectrum, above the trident background or as a
narrow invariant mass peak with a decay vertex displaced from the production
target, so charged particle tracking and vertexing are needed for its
detection. In the HPS Test Run, charged particles are measured with a compact
forward silicon microstrip tracker inside a dipole magnet. Electromagnetic
showers are detected in a PbW0$_{4}$ crystal calorimeter situated behind the
magnet, and are used to trigger the experiment and identify electrons and
positrons. Both detectors are placed close to the beam line and split
top-bottom. This arrangement provides sensitivity to low-mass heavy photons,
allows clear passage of the unscattered beam, and avoids the spray of degraded
electrons coming from the target. The discrimination between prompt and
displaced e$^+$e$^-$ pairs requires the first layer of silicon sensors be
placed only 10~cm downstream of the target. The expected signal is small, and
the trident background huge, so the experiment requires very large statistics.
Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition
electronics and a fast trigger to exploit the essentially 100% duty cycle of
the CEBAF accelerator at JLab.
