Searching for long-lived dark photons with the heavy photon search experiment
Keywords: Physics.

A heavy photon (also called a dark photon or A') is a hypothetical vector boson that arises from a massive U(1) abelian gauge symmetry. Heavy photons kinetically mix with the Standard Model photon, thus they are a natural portal to hidden sectors that are favored in a variety of dark sector scenarios, particularly for dark matter at the sub-GeV mass scale. The Heavy Photon Search Experiment (HPS) is a fixed target experiment at Jefferson Laboratory dedicated to searching for heavy photons in the MeV - GeV mass range and kinetic mixing strength ~1e-5 - 1e-10. It does so through two distinct searches - a search for a narrow mass resonance and, for sufficiently small couplings, a search for secondary vertices beyond a large prompt QED background. In order to perform such searches, the HPS utilizes a compact, forward acceptance spectrometer that must be able to reconstruct particle masses and vertices with extreme precision. Heavy photons are electro-produced from a continuous electron beam incident on a thin tungsten foil, and HPS is able to reconstruct the momentum of the subsequent decays to e+e- pairs using a silicon vertex tracker (SVT). HPS currently has three data sets - engineering runs in 2015 and 2016 as well as a physics run with an upgraded detector in 2019 - all at different beam energies and currents. Presented in this dissertation are heavy photon physics and motivations, introduction to the HPS detector and reconstruction, detector upgrades and other physics models of interest, and the results from the displaced vertex search from the HPS 2016 Engineering Run which was taken with a 2.3 GeV, 200 nA continuous electron beam and collected a total luminosity of 10753 1/nb (equivalent to 5.4 days of continuous beam). The 2016 Engineering Run displaced vertex search was performed in the mass range 60 - 150 MeV and in the range of kinetic mixing strength ~1e-10 - 1e-8, and the new results, which have a sensitivity to canonical A' production of ~0.4 events over a region of mass/coupling parameter space, exclude A' production above 6.05 times the canonical cross-section at a mass of 80.2 MeV and kinetic mixing strength of 2.12e-9. Even though HPS had insufficient data to set meaningful limits on the canonical A' production, this analysis demonstrated that the displaced vertex method is viable, backgrounds can be reduced to acceptable levels, and larger data sets can yield real exclusions or discovery. In fact, the background required to perform a displaced A' search (0.5 background events per mass search bin) was achieved in the unblinded 10% portion of the data set by implementing a new set of cuts. This significant background reduction stands as a considerable improvement over the previous analysis and approaches the sensitivity needed to observe the first A' candidates. After unblinding the entire data set, the remaining background events were studied and a search for decays which are further downstream and miss part of the acceptance of the tracker was performed. Finally, the sensitivity to another model which leads to displaced vertices is explored and preliminary projections show that HPS will have sensitivity to new territory with this data set. This combined work on the displaced vertex search is informative for future data sets that will search for A's in the same way but include simple, yet critical, upgrades to the detector. Studies of the detector upgrades are discussed and the expected sensitivity to future data sets with these upgrades is shown