  The Heavy Photon Search (HPS) is a new, dedicated experiment at Thomas
Jefferson National Accelerator Facility (JLab) to search for a massive vector
boson, the heavy photon (a.k.a. dark photon, \Aprimebold{}), in the mass range
20-500~MeV/c$^{2}$ and with a weak coupling to ordinary matter. An
\Aprimebold{} can be radiated from an incoming electron as it interacts with a
charged nucleus in the target, accessing a large open parameter space where the
\Aprimebold{} is relatively long-lived, leading to displaced vertices. HPS
searches for these displaced \Aprimebold{} to e$^+$e$^-$ decays using actively
cooled silicon microstrip sensors with fast readout electronics placed
immediately downstream of the target and inside a dipole magnet to instrument a
large acceptance with a relatively small detector. With typical particle
momenta of 0.5-2~GeV/c, the low material budget of 0.7\% $\mathbf{X_0}$ per
tracking layer is key to limiting the dominant multiple scattering uncertainty
and allowing efficient separation of the decay vertex from the prompt QED
trident background processes. Achieving the desired low-mass acceptance
requires placing the edge of the silicon only 0.5~mm from the electron beam.
This results in localized hit rates above 4~MHz/mm$^2$ and radiation levels
above $\mathbf{10^{14}}$ 1~MeV neutron equivalent /cm$^2$ dose. Hit timing at
the ns level is crucial to reject out-of time hits not associated with the
\Aprimebold{} decay products from the almost continuous CEBAF accelerator beam.
To avoid excessive beam-gas interactions the tracking detector is placed inside
the accelerator beam vacuum envelope and is retractable to allow safe operation
in case of beam motion. This contribution will discuss the design, construction
and first performance results from the first data-taking period in the spring
of 2015.
