Monica Leys Dissertation Defense

Thesis Title: Searching for Axion-like Particles at the Electron-Ion Collider Abstract: Axion-like particles (ALPs) are well motivated candidates for new physics beyond the Standard Model and may play a role in addressing some of the open questions in particle physics, such as the Strong CP problem. The future Electron-Ion Collider (EIC) will have the capability to collide electrons with protons and various heavy ions with large luminosities in a relatively clean environment, surrounded by a large, sensitive, hermetic detector, allowing for new physics searches. In this dissertation, we explore the capability of the EIC to probe GeV-scale ALPs that are created via photon fusion and promptly decay to photons. We explore this predominantly through elastic electron-proton scattering and coherent electron-ion scattering, the latter of which benefits from a Z² coherent enhancement to the production cross section. We develop analyses which can mitigate the key backgrounds from light-by-light scattering and double bremsstrahlung, which allow the EIC to be sensitive to the ALP diphoton signal. We also explore ALP searches in the complementary channels of electron-proton deep inelastic scattering (DIS) and electron-ion DIS and incoherent scattering. These searches have the potential to extend reach to untouched ALP-photon coupling parameter space in the ALP mass range of roughly 0.1-100 GeV.