The curvaton mechanism in string inflation

In this Master Degree Thesis we study the Curvaton Mechanism to generate curvature perturbations in String Inflation. First of all, we introduce basic bosonic string theory, which works in 26 dimensions, and then extend it to its 10D supersymmetric version in the RNS formalism. In doing so, we study the spectrum of this theory and we focus in particular on Type IIB superstring theory and the compactification of its 6 extra dimensions. After a brief Mathematical introduction, we focus on the moduli fields which arise from the extra dimensions. We investigate how to stabilise them in order to get a correct and phenomenologically viable inflationary scenario. In particular, we consider 2 models where the inflaton is a Kähler modulus: Non-perturbative and Loop Blow-Up inflation. Finally, we focus on the second model and check if the saxion associated to the inflaton 4-cycle volume can behave as a curvaton field. Its isocurvature perturbations get converted into standard curvature fluctuations when the axion decays. We find the conditions under which this contribution is subdominant with respect to the one arising from inflaton fluctuations, hence guaranteeing compatibility with observational constraints from the Planck satellite.