UC Santa Cruz

The Heavy Photon Search (HPS) is a new experiment at Jefferson Lab that will search for heavy U(1) vector bosons (heavy photons, dark photons or A') in the mass range of 10 MeV/c^2 to 1 GeV/c^2 that couple weakly to ordinary matter. Heavy photons in this mass range are theoretically favorable and may also mediate dark matter interactions. The heavy photon couples to electric charge through kinetic mixing with the photon, in turn, inducing an effective gauge coupling of the A' to electric charge, which is suppressed relative to the electron charge by a factor of ε ~ 10^-2 - 10^-12. Since heavy photons couple to electrons, they can be produced through a process analogous to bremsstrahlung radiation, subsequently decaying to narrow e+e- resonances which can be observed above the dominant QED trident background. For suitably small couplings, dark photons travel detectable distances before decaying, providing a second signature.

HPS will utilize this production mechanism to probe heavy photons with relative couplings of ε^2 ~ 10^-5 - 10^-10 and search for the e+e- decay of the heavy photon via two signatures: invariant mass and displaced vertex. Using Jefferson Lab’s high luminosity electron beam incident on a thin tungsten target along with a compact, large acceptance forward spectrometer consisting of a silicon vertex tracker and lead tungstate electromagnetic calorimeter, HPS will access unexplored regions in the mass-coupling phase space.

The HPS engineering run took place in spring of 2015 using a 1.056 GeV, 50 nA beam. This dissertation will present the results of a resonance search for a heavy photon in the mass range between 20 MeV/c^2 to 60 MeV/c^2 using a portion of the unblinded engineering run data which amounts to a luminosity of 74 nb^-1 (.4671 mC of charge).