  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.
