Kaluza–Klein Axions: From Theoretical Formulation to Solar Bounds through Data Analysis

This thesis investigates constraints on axion-like particles (ALPs) and Kaluza–Klein axions through the study of the solar axion basin. The work begins by reviewing the theoretical motivation for axions, namely the strong CP problem. Possible alternative solutions to this problem are discussed, together with several axion models. Existing bounds and phenomenological arguments constraining the ALP parameter space are then examined, with particular focus on the mass, axion-photon coupling plane. The framework of extra-dimensional theories is subsequently introduced, along with the emergence of axions in this context, referred to as Kaluza–Klein (KK) axions. After this theoretical overview, the analysis focuses on axion production in the Sun. The three main production channels are reviewed, and the corresponding energy spectra and fluxes are derived as functions of the axion energy. The number density of gravitationally trapped axions in the solar gravitational potential is then estimated, allowing the associated decay rate to be computed. These results are applied both to a generic ALP model and to KK axions. Finally, the data collected by LIME, a CYGNO collaboration experiment located at the Gran Sasso National Laboratories, are used to constrain ALP and KK axion models. A dedicated data analysis is performed, including a study of the efficiency of the selection cuts applied to the signal. From this analysis, a sensitivity projection is derived for both generic ALPs and Kaluza–Klein axions.