Sterile Neutrinos in Extra Dimensions as Warm Dark Matter Candidates

Many years after it has been established that most of the matter density in the universe is made up of some form of dark matter, its nature remains a mystery. The consensus is that dark matter is non-baryonic, and for a long time it was widely believed that it has to be dynamically cold to allow for structure formation at early times in the universe. However, it now seems like warm dark matter may alleviate tensions between observations and simulations based on theoretical predictions from cold dark matter driven structure formation, especially on smaller, sub-galactic scales. Sterile neutrinos arise in a variety of extensions of the Standard Model of particle physics, and can be used to generate a neutrino mass term or account for short baseline oscillation anomalies depending on their mass in the given scenario. They are also very good candidates for warm dark matter. Here, we present one such Standard Model extension, featuring a large compactified extra dimension, and study how the tower of sterile neutrino Kaluza- Klein modes arising in this setting affects the sterile neutrino properties. In particular, we focus on its inuence on the sterile neutrino abundance, if it is to match the dark matter abundance, and possible signatures both in astrophysical observations and in nuclear β-decay. We find that the extra dimensional setting only insignificantly changes both production and signatures of sterile neutrinos, and conclude that if existing, the additional Kaluza-Klein modes will most likely remain hidden for many years to come.