DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Sarah Gaiser

Public zoom link:   https://stanford.zoom.us/j/91889103281?pwd=lH5scMTPdJ6LGGzbg5vHXceWupwdAm.1

Password: Email physicstudentservices [at] stanford.edu (physicstudentservices[at]stanford[dot]edu)

Title:  

 Searching for Long-Lived Dark Photons with the HPS Experiment

Abstract: 

One of the popular theories for light (sub-GeV) Dark Matter predicts the existence of a Hidden Sector that is uncharged under the Standard Model (SM) forces, yet able to couple to the SM through new force carriers. This coupling can be realized through a theoretically well-motivated minimal extension to the SM that introduces a new light U(1) gauge boson, referred to as dark photon A’. The kinetic mixing of dark and SM photons opens a production channel for dark photons analogous to bremsstrahlung in which an electron radiates an A’. The Heavy Photon Search (HPS) experiment at the Thomas Jefferson National Accelerator Facility has been primarily designed to search for these electro-produced dark photons. In particular, HPS is sensitive to an experimental signature where long-lived dark photons decay into electron-positron pairs, a few millimeters displaced from the target. To carry out such a displaced vertex search, it is vital to understand and mitigate SM backgrounds that mimic the dark photon signatures. In this thesis, I present my contributions to the background modeling at HPS, including improved Monte Carlo simulations and proper treatment of out-of-time backgrounds. Using the insights from these efforts, I then develop a sophisticated event selection for the 2021 dataset, consisting of a preselection to assure data quality, and a tight selection to remove long-lived backgrounds. Finally, I present the projected reach and exclusion power of the full 2021 dataset, 162/pb at 3.74 GeV, considering the lessons learned from the thorough investigation of a small subset of this data.