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Axions are hypothetical very light pseudoscalar particles, closely related to neutral pions, that arise in the context of the Peccei-Quinn solution of the CP problem of QCD, i.e. they help explain CP conservation in strong interactions. Axions are also an ideal particle candidate for the cosmic cold dark matter or, depending on their properties, could contribute a hot dark matter fraction. After introducing axions we review recent result and future prospects of the CAST experiment at CERN to search for axions emitted by the Sun and report the status of the search experiments for galactic axion dark matter. New limits on the axion mass are derived from cosmic structure formation in analogy to the well-known cosmological neutrino mass limits.
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