Phenomenology of closed string dark photons

This Master Thesis project is focused on the study of the phenomenology of RamondRamond Photons in type IIB string compactifications. These modes are Standard Model gauge-singlets which are typically massless, and so do not develop any physical mixing with the hypercharge U(1). They are instead gravitationally coupled to closed string moduli. Moreover, a generic Calabi-Yau compactification can feature hundreds of these Dark Photons. It is well known that, after neutrinos decoupled from the thermal bath, they have remained as relativistic degrees of freedom constituting the cosmic neutrino background. While in the Standard Model this number of relativistic species is fixed as the effective number of neutrino species at Cosmic Microwave Background temperature and at Big Bang Nucleosynthesis temperature, any measured excess from the Standard Model value would constitute an evidence of extra Dark Radiation and a hint for new physics. In the context of string models of the early universe, these relativistic degrees of freedom, decoupled from the Standard Model ones, could be generated by the decay of the lightest modulus. The main purpose of this thesis is therefore to study the cosmological production of Ramond-Ramond Dark Photons from the decay of the longest-living moduli, setting bounds from the requirement to avoid Dark Radiation overproduction.