The main open problem in High Energy Physics is the description of a model that unifies all the interactions observed in Nature. Indeed, the Standard Model of particle physics (SM) describes the electromagnetic, weak and strong force, but it cannot include a description of gravitational interactions. Moreover, the SM contains a large number of free parameters, whose values clearly suggest the existence of a more fundamental theory beyond it. The research proposal is focused on string phenomenology, i.e. on the search of a unifying model from compactifications of string theory. The main aim is the explicit construction of realistic string models on smooth backgrounds with non trivial flux both for the gauge fields and the closed string fields. Such an achieve ment would have great relevance in string model building and, more generally, in high energy physics, since it would merge the efforts in constructing a model with the correct discrete choices (number of families, gauge group etc.) and continuous parameter s (couplings, masses etc.) with the promising recent developments in understanding a small positive cosmological constant. A crucial aspect of the project is the characterization of the phenomenologically appealing mechanisms that will arise from the string models. These mechanisms will be introduced in field theory models, from a bottom-up and point of view, in a way that allows the extraction of their signatures and even in absence of a completely consistent string mode l. The proposal includes training of the candidate. He is expected to complete his scientific formation. He will spend his fellowship at LPT Orsay, an Institution that ensures research excellence both in string model building and in Particle Phenomenology and Cosmology, the latter being crucial in order to develop the more phenomenological aspects of the research project.