Searching for heavy photons at Jefferson Lab using detached vertices

The Heavy Photon Search (HPS) experiment in Jefferson Lab Hall B will look for a new U(1) vector boson (called a "heavy photon", "dark photon", or A') in the mass range from 20 to 600 MeV/c$^{\mathrm{2}}$. This A' could potentially couple to the ordinary photon through kinetic mixing, which would create a coupling to electric charge of ${\alpha }'/\alpha =\epsilon ^{2}$. If so, then the A' can be produced through electron interactions with a heavy target through a process analogous to bremsstrahlung. The A' could then decay to $e^{+}e^{-}$. If the coupling $\epsilon $ is large, we can directly detect a peak in the $e^{+}e^{-}$ mass spectrum above the dominant QED background. If $\epsilon $is small, then we can look for $e^{+}e^{-}$ events with a detached vertex resulting from A' decays downstream of the production target. In 2015 and 2016 we measured a few beam days each of 1.1 and 2.2 GeV electron scattering from tungsten using a large acceptance forward spectrometer consisting of a silicon vertex tracker located inside a dipole magnet and a lead tungstate electromagnetic calorimeter for triggering. This talk will present the detached vertex A' search, including preliminary results at 1.1 GeV if available.