Solar Axion search with Micromegas detectors in the CAST Experiment with $^{3}$He as buffer gas

Axions are well motivated particles proposed in an extension of the SM as a solution to the strong CP problem. Also, there is the category of Axion-Like Particles (ALPs) which appear in extensions of the SM and share the same phenomenology of the axion. Axions and ALPs are candidates to solve the Dark Matter problem. CAST, the CERN Axion Solar Telescope is looking for solar axions since 2003. CAST exploit the helioscope technique using a decommissioned LHC dipole magnet in which solar axions could be reconverted into photons. Three of the four detectors operating at CAST are of the Micromegas type. The analysis of the data of the three Micromegas detectors during the 2011 data taking campaign at CAST is presented in this thesis, obtaining a limit on the coupling constant of g$_{a \gamma}$ < 3.90 $\times$ 10$^{-10}$ GeV$^{-1}$ at a 95$\%$ of confidence level, for axion masses from 1 to 1.17 eV. CAST Micromegas detectors exploit different strategies developed for the reduction of the background level. Moreover, different test benches have been developed in order to understand the origin of the background. The state of art in low background techniques is shown in the upgrades of the Micromegas detectors at CAST, which has led to a reduction of the background in a factor $\sim$6. It translates in an improvement of the sensitivity of CAST in a factor $\sim$2.5. Beyond CAST a new generation axion helioscope has been proposed: IAXO-the International Axion Observatory. IAXO will enhance the helioscope technique by exploiting all the singularities of CAST implemented into a large superconducting toroidal magnet, dedicated X-ray optics and ultra-low background detectors. A description of the IAXO proposal and the study of the sensitivity of IAXO are presented in this thesis. IAXO will surpass CAST in more than one order of magnitude, entering into an unexplored parameter space area.