Early matter domination from a dark photon at colliders

In this thesis we study the detectability at colliders of long-lived particles (LLP) which, at the early stages of the Universe, dominate its energy density and decay injecting entropy in the thermal bath, thus diluting early Universe relics. Such periods of early matter domination are predicted in many models, and allow for example to obtain thermal dark matter heavier than 100 TeV. We compute the dilution factor induced by a generic LLP as a function of its decay length and mass. Then, we apply our results to the well known dark photon model. After re-deriving known dark photon properties, we demonstrate that forthcoming LLP detectors such as CODEX-b, ANUBIS and MATHUSLA (ordered from realised in the next few years to maybe realizable in the future) could test the parameter space where a dark photon, produced from heavier dark matter annihilations, induces a period of early matter domination with sizeable dilution of pre-existing relics.