  The Heavy Photon Search experiment (HPS) is searching for a new gauge boson,
the so-called "heavy photon." Through its kinetic mixing with the Standard
Model photon, this particle could decay into an electron-positron pair. It
would then be detectable as a narrow peak in the invariant mass spectrum of
such pairs, or, depending on its lifetime, by a decay downstream of the
production target. The HPS experiment is installed in Hall-B of Jefferson Lab.
This article presents the design and performance of one of the two detectors of
the experiment, the electromagnetic calorimeter, during the runs performed in
2015-2016. The calorimeter's main purpose is to provide a fast trigger and
reduce the copious background from electromagnetic processes through matching
with a tracking detector. The detector is a homogeneous calorimeter, made of
442 lead-tungstate (PbWO4) scintillating crystals, each read out by an
avalanche photodiode coupled to a custom trans-impedance amplifier.
