The International Axion Observatory (IAXO): The Next Generation of Axion Helioscopes

More than 80 years after the postulation of dark matter, its nature remains one of the fundamental questions in cosmology. Axions are currently one of the leading candidates for the hypothetical, non-baryonic dark matter that is expected to account for about 25% of the energy density of the Universe. Especially in the light of the Large Hadron Collider at CERN slowly closing in on weakly-interacting massive particle (WIMP) searches, axions and axion-like particles provide a viable alternative approach to solving the dark matter problem. The fact that makes them especially appealing is that they were initially introduced to solve a long-standing problem in quantum chromodynamics rather than as an ad hoc solution for dark matter.

Helioscopes are a type of axion experiment searching for axions produced in the core of the Sun via the Primakoff effect. The International Axion Observatory (IAXO) is a next generation axion helioscope aiming at a sensitivity to the axion-photon coupling of 1 - 1.5 orders of magnitude beyond the current most sensitive axion helioscope which is the CERN Axion Solar Telescope (CAST). IAXO will be able to challenge the stringent bounds from supernova SN1987A and test the axion interpretation of anomalous white-dwarf cooling. Beyond standard axions, this new experiment will also be able to search for a large variety of ALPs and other novel excitations at the low-energy frontier of elementary particle physics. BabyIAXO is proposed as a small pilot experiment increasing the sensitivity to axion-photon couplings down to a few 10^-11 GeV^-1 and thus delivering significant physics results while demonstrating the feasibility of the full-scale IAXO experiment. Here we introduce the IAXO and BabyIAXO experiments, report on the current status of both and outline the expected IAXO science reach.