Exploring the impact of Axion-Like Particles on keV–MeV Primordial Plasma

This report focuses on the study of Axion-Like Particles and their impact in the early stages of the universe. A deep analysis is performed, taking into consideration the ALP photonic decay and its impact on the weak interconversion rates, and the ALP hadronic decay and its effect on the neutron-to-proton ratio. Constraints for ALPs parameters have been developed taking into consideration measurements from Big Bang Nucleosynthesis and Cosmic Microwave Background, specifically the abundance of Helium and Neff . It has been observed that the ALPs decay into hadrons plays a crucial role in these constraints, as it has a great impact on the neutron-to-proton ratio, even if the initial abundance of ALPs is small. Additionally, different reheating temperatures, in which the ALPs could have been formed, have been studied. It has been observed that the constraints developed from the abundance of Helium, considering the hadronic decay, are less sensitive to the reheating temperature in comparison with the constraints from Nef f .