Preparing for Dark Matter Searches at the Light Dark Matter Experiment

The constituents of dark matter are still unknown, and the viable possibilities span a very large mass range. Specific scenarios for the origin of dark matter sharpen the focus to within about an MeV to 100 TeV. Most of the stable constituents of known matter have masses in the lower range, and a thermal origin for dark matter works in a simple and predictive manner in this mass range as well. LDMX aims to decisively test a variety of dark matter scenarios (including thermal origin scenarios) in the sub-GeV mass range using a missing momentum search. It will also be able to provide strong sensitivity to any other dark sector physics that couples to electrons below the GeV scale; due to its unique detector design, LDMX effectively operates as a fully instrumented beam dump with large acceptance, and thus is ideal for searching for visible decays of exotic particles that have a fairly long lifetime. For the minimal dark photon model, the kinematics are identical to the primary missing momentum search. However, while the signal of the main search is only a soft recoil electron, this secondary search, referred to as the visibles search, has a signal of a soft recoil electron along with energy deposition near the beam energy in one of the calorimeters. This dissertation presents Monte Carlo simulation work completed to assist the search for light dark matter in both the standard missing momentum and the visibles search of LDMX, with emphasis on the visibles search. Additionally, the use of prototype LDMX detector components to help commission a new beamline at SLAC is discussed at length. LDMX plans to use this new beamline for the experiment, so ensuring the beam meets the necessary requirements is paramount to a successful dark matter search.