  The Heavy Photon Search (HPS) experiment is designed to search for a new
vector boson $A^\prime$ in the mass range of 20 MeV/$c^2$ to 220 MeV/$c^2$ that
kinetically mixes with the Standard Model photon with couplings $\epsilon^2
>10^{-10}$. In addition to the general importance of exploring light, weakly
coupled physics that is difficult to probe with high-energy colliders, a prime
motivation for this search is the possibility that sub-GeV thermal relics
constitute dark matter, a scenario that requires a new comparably light
mediator, where models with a hidden $U(1)$ gauge symmetry, a "dark", "hidden
sector", or "heavy" photon, are particularly attractive. HPS searches for
visible signatures of these heavy photons, taking advantage of their small
coupling to electric charge to produce them via a process analogous to
bremsstrahlung in a fixed target and detect their subsequent decay to
$\mathrm{e}^+ \mathrm{e}^-$ pairs in a compact spectrometer. In addition to
searching for $\mathrm{e}^+ \mathrm{e}^-$ resonances atop large QED
backgrounds, HPS has the ability to precisely measure decay lengths, resulting
in unique sensitivity to dark photons, as well as other long-lived new physics.
After completion of the experiment and operation of engineering runs in 2015
and 2016 at the JLab CEBAF, physics runs in 2019 and 2021 have provided
datasets that are now being analyzed to search for dark photons and other new
phenomena.
