The International Axion Observatory (IAXO) is a next generation axion helioscope aiming at a sensitivity to the axion-photon coupling of a few 10^{-12} GeV^{-1}, i.e. 1-1.5 orders of magnitude beyond sensitivities achieved by the currently most sensi
tive axion helioscope, the CERN Axion Solar Telescope (CAST). Crucial factors in improving the sensitivity for IAXO are the increase of the magnetic field volume together with the extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested at CAST. Electron-coupled axions invoked to explain the white dwarf cooling, relic axions, and a large variety of more generic axion-like particles (ALPs) along with other novel excitations at the low-energy frontier of elementary particle physics could provide additional physics motivation for IAXO.
The International Axion Observatory (IAXO) is a new generation axion helioscope aiming at a sensitivity to the axion-photon coupling of a few 10$^{12}$ GeV$^{-1}$, i.e. 1 - 1.5 orders of magnitude beyond the one currently achieved by CAST. The projec
t relies on improvements in magnetic field volume together with extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested in CAST. Additional physics cases of IAXO could include the detection of electron-coupled axions invoked to solve the white dwarfs anomaly, relic axions, and a large variety of more generic axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics. This contribution is a summary of our paper [1] to which we refer for further details.
We sketch the proposal for a PVLAS-Phase II experiment. The main physics goal is to achieve the first direct observation of non-linear effects in electromagnetism predicted by QED and the measurement of the photon-photon scattering cross section at l
ow energies (1-2 eV). Physical processes such as ALP and MCP production in a magnetic field could also be accessible if sensitive enough operation is reached. The short term experimental strategy is to compact as much as possible the dimensions of the apparatus in order to bring noise sources under control and to attain a sufficient sensitivity. We will also briefly mention future pespectives, such as a scheme to implement the resonant regeneration principle for the detection of ALPs.
Experimental bounds on induced vacuum magnetic birefringence can be used to improve present photon-photon scattering limits in the electronvolt energy range. Measurements with the PVLAS apparatus (E. Zavattini {it et al.}, Phys. Rev. D {bf77} (2008)
032006) at both $lambda = 1064$ nm and 532 nm lead to bounds on the parameter {it A$_{e}$}, describing non linear effects in QED, of $A_{e}^{(1064)} < 6.6cdot10^{-21}$ T$^{-2}$ @ 1064 nm and $A_{e}^{(532)} < 6.3cdot10^{-21}$ T$^{-2}$ @ 532 nm, respectively, at 95% confidence level, compared to the predicted value of $A_{e}=1.32cdot10^{-24}$ T$^{-2}$. The total photon-photon scattering cross section may also be expressed in terms of $A_e$, setting bounds for unpolarized light of $sigma_{gammagamma}^{(1064)} < 4.6cdot10^{-62}$ m$^{2}$ and $sigma_{gammagamma}^{(532)} < 2.7cdot10^{-60}$ m$^{2}$. Compared to the expected QED scattering cross section these results are a factor of $simeq2cdot10^{7}$ higher and represent an improvement of a factor about 500 on previous bounds based on ellipticity measurements and of a factor of about $10^{10}$ on bounds based on direct stimulated scattering measurements.
