Abstract

Axions are a natural consequence of the Peccei-Quinn mechanism, the most compelling solution to the strong-CP problem. Similar axion-like particles (ALPs) also appear in a number of possible extensions of the Standard Model, notably in string theories. Both axions and ALPs are often invoked in models explaining inflation, dark radiation or trying to solve several astrophysical anomalies. Most relevantly, they are very well motivated candidates for the Dark Matter. If they exists, model-independently, axions and ALPs should be copiously produced at the sun’s interior. Solar axions have been searched by the CAST experiment for more than a decade at CERN, producing a series of results of reference on these particles. More recently CAST has released an improved result, that constitutes the most restrictive limit to the axion-photon coupling, achieved thanks to improvements in detectors and telescopes carried out as part of the preparatory activities of a completely new large-scale infrastructure follow-up of CAST: the International Axion Observatory (IAXO). IAXO will be a fourth generation axion helioscope. As its primary physics goal, IAXO will look for solar axions or ALPs with a signal to background ratio of about 5 orders of magnitude higher than CAST. For this IAXO envisions the construction of a new large superconducting toroidal magnet designed optimizing the axion helioscope figure of merit, extensive use of x-ray focusing optics and low background x-ray detectors. IAXO will venture deep into unexplored axion parameter space, thus having discovery potential. IAXO has also potential to host additional detection setups. Most interestingly, the large magnetic volume of IAXO could be used to detect relic axion or ALPs potentially composing the galactic halo of Dark Matter. IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade.

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