Physics potential of the International Axion Observatory (IAXO) (1904.09155v3)

Abstract: We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as $g_{a\gamma}\sim 10^{-12}$ GeV$^{-1}$, or to electrons $g_{ae}\sim$10$^{-13}$, IAXO has the potential to find the QCD axion in the 1 meV$\sim$1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

• The paper demonstrates that IAXO can detect axion-like particles with photon coupling as low as 10⁻¹² GeV⁻¹, marking a breakthrough in axion searches.
• It details how IAXO’s enhanced sensitivity addresses critical issues such as the strong CP problem and unexplained stellar cooling phenomena.
• The study illustrates IAXO’s complementarity with earlier experiments like CAST, potentially transforming approaches to probing physics beyond the Standard Model.

An Overview of “Physics potential of the International Axion Observatory (IAXO)”

The paper "Physics potential of the International Axion Observatory (IAXO)" provides a comprehensive overview of the capabilities and implications of the IAXO in the detection of axions and axion-like particles (ALPs). The project aims to advance our understanding of these particles, potentially offering solutions to multiple outstanding problems in physics.

At the core of IAXO’s mission is the pursuit of solar axions. IAXO is designed to have a sensitivity to discover ALPs with photon coupling as small as $g_{a\gamma} \sim 10^{-12}$ GeV$^{-1}$ and electron coupling down to $g_{ae} \sim 10^{-13}$. The detection of such particles would have significant ramifications, potentially including their identification as the quantum chromodynamics (QCD) axion—thereby solving the strong CP problem, contributing to the understanding of cold dark matter, and addressing unexplained stellar cooling phenomena.

Key objectives of IAXO include demystifying several astrophysical and cosmological phenomena. One is the "transparency" of the Universe to gamma-rays, where ALPs could explain why photons travel larger than expected distances without being absorbed by the cosmic microwave background (CMB). Another objective involves providing an explanation for the observed soft X-ray excess from galaxy clusters.

Importantly, the paper references string theory models that grant axions a role as Dark Matter and Dark Radiation. These roles are pivotal given the consideration of axions as potential contenders for dark energy, thereby influencing our understanding of cosmological and astrophysical processes. IAXO's detection potential may illuminate these roles further, elucidating the connection between axions and the broader challenges they might help resolve.

The paper emphasizes the complementarity between IAXO and other experimental lines, such as the CERN Axion Solar Telescope (CAST) from which IAXO will derive substantial technological groundwork. The authors convincingly argue that while CAST has laid important groundwork, IAXO's increased scale and sensitivity afford it unprecedented opportunities in probing unexplored axion parameter spaces.

In summary, the paper underscores IAXO’s capability to test a wide section of axion parameter space, with significant implications for physics beyond the Standard Model. It highlights theoretical frameworks in which IAXO's results might have strong implications, including string theory, modifications to cosmological models, and resolutions to open astrophysical questions. By bridging these domains, IAXO represents a promising endeavor with profound potential to impact our fundamental understanding of both particle physics and cosmology. Future developments in IAXO’s technology and analysis methods will shape the trajectory of axion research deeply, with potential outcomes poised to redefine prevailing theories across modern physics.