Status of particle physics solutions to the UHECR puzzle

The status of solutions to the ultra-high energy cosmic ray (UHECR) puzzle that involve particle physics beyond the standard model is reviewed. Signatures and experimental constraints are discussed for most proposals such as the Z burst model and topological defects (both allowed only as sub-dominant contributions), supermassive dark matter (no positive evidence from its key signatures galactic anisotropy and photon dominance), strongly interacting neutrinos or new primaries (no viable models known), and violation of Lorentz invariance (viable). Lorentz invariance violation should be considered seriously as an explanation for the UHECR puzzle, if there is not a considerable fraction of photon primaries at the highest energies, correlations with sources at cosmological distance can be established, and the spectrum extends well beyond the GZK (Greisen-Zatsepin-Kuzmin) cutoff. If only the two first conditions are found to be true, and the UHECR spectrum is close to the one measured in the HiRes experiment, then bottom-up scenarios are a sufficient explanation for the data