Long-Lived Axion-Like Particles at the Future Circular Electron-Positron Collider

As the leading theory in particle physics, the Standard Model (SM) defines our understanding of the subatomic world. However, observed phenomena have proven that the SM, despite its great success, is not complete yet and the search for particles Beyond the Standard Model (BSM) has surged ever since. In models that try to address the problems of the SM, BSM long-lived particles (LLPs), which have longer lifetimes than unstable SM particles, occur naturally. With a proposed circular electron-positron collider, the FCC-ee, these BSM LLPs could be discovered.This thesis focuses specifically on the production and detection of long-lived axion-like particles (ALPs), which are well-motivated BSM particles and also possible dark matter candidates. Utilizing the e+e− → Z → aγ → γγγ channel, which only depends on the ALP-photon coupling cγγ under certain assumptions, the sensitivity of the FCC-ee to detect ALPs is examined. Employing the analysis framework of the Future Circular Collider (FCC), the simulation and analysis of events for ALP production were conducted, testing coupling constants ranging from 10−6 to 1.6 and ALP masses between 0.01 GeV and 30 GeV. Finally, the sensitivity of the FCC-ee to a long-lived ALP signature is studied for various settings of the ALP parameters.