Limits on beyond standard model messengers as ultra high energy cosmic rays

Up to date, there is no consensus regarding the origin of ultra-high energy cosmic rays (UHECR) beyond the Greisen--Zatsepin--Kuzmin (GZK) limit. In order for these UHECRs to reach the Earth, an extremely suppressed interaction between them and the cosmic microwave background (CMB) is required, which is impossible for standard model (SM) particles, except neutrinos. In this letter, we present constraints on the parameter space of models involving axion-like particles and dark photons, as candidates for UHECRs, by assuming that these particles can traverse the visible Universe without decaying. In the case of axion-like particles, the constraints are tighter than those given in previous works. We present for the first time constraints on the parameter space of dark photon models, by considering their kinetic term mixing with SM photons.