Physics at the MeV-Scale in Liquid Argon Time Projection Chambers Lepetic, Ivan Thomas Particle physics ArgoNeuT Fermilab LArTPC MeV-scale Millicharged particles Neutrino experiments The liquid argon time projection chamber (LArTPC) is the detection technology chosen for several Fermilab-based neutrino experiments. This technology will be used in studies of neutrino cross-sections and oscillations, neutrinos from supernovae as well as a variety of studies of beyond the Standard Model physics. This thesis explores the use of these detectors to study MeV-scale activity. MeV-scale electrons arising from Compton scatters of deexcitation photons and photons from inelastic neutron scattering in neutrino-nucleus interactions are reconstructed using novel methods presented here. This work represents the first demonstration of MeV-scale physics capabilities in a LArTPC neutrino experiment as well as the first observation of neutrino-produced photons from nuclear de-excitation and inelastic neutron scattering. A search for millicharged particles, postulated by theories of beyond the standard model physics, is also performed using data from a LArTPC and the low-energy reconstruction techniques developed. The results set world-leading bounds on the parameter space of millicharged particles. The work in this thesis demonstrates that studies of MeV-scale activity and new physics are possible with LArTPC technology and provides the foundation for future LArTPC studies of low energy neutrinos and new physics. Littlejohn, Bryce Palamara, Ornella 2020 Dissertation application/pdf islandora:1009804 http://hdl.handle.net/10560/islandora:1009804 Illinois Institute of Technology PHYS / Physics en In Copyright http://rightsstatements.org/page/InC/1.0/ Restricted Access