The International Axion Observatory IAXO. Letter of Intent to the CERN SPS committee

This Letter of Intent describes IAXO, the International Axion Observatory, a proposed 4th generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal to background ratio, IAXO will be about 4-5 orders of magnitude more sensitive than CAST, which means that this instrument will reach sensitivity to axion-photon couplings down to a few $\times 10^{-12}$ GeV$^{-1}$. IAXO has the potential for the discovery of axions and other ALPs, since it will deeply enter into unexplored parameter space. At the very least it will firmly exclude a large region of this space of high cosmological and astrophysical relevance. In particular it will probe a large fraction of the high mass part (1 meV to 1 eV) of the QCD axion allowed window. Additional physics cases for IAXO include the possibility of detecting solar axions produced by mechanisms mediated by the axion-electron coupling $g_{ae}$ with sensitivity $-$for the first time$-$ to values of $g_{ae}$ not previously excluded by astrophysics. IAXO follows the conceptual layout of an enhanced axion helioscope, with a purpose-built 8-coils toroidal superconducting magnet. All the eight 60-cm diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into $\sim$0.2 cm$^2$ spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives the will allow for solar tracking for $\sim$12 h each day. All the enabling technologies exists, there is no need for development. All the needed know-how is present in the proponent groups. Potential additional physics cases for IAXO to be developed in the future are the search of axionic dark radiation, relic cold dark matter axions or the realization of microwave light-shining-through wall setups, as well as the search of more specific models of weakly interacting sub-eV particles (WISPs) at the low energy frontier of particle physics. IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade.