# New CAST Limit on the Axion-Photon Interaction

## TL;DR

The paper reports a new, more stringent limit on the axion-photon coupling constant from the CERN Axion Solar Telescope's final phase, utilizing improved detection techniques and vacuum conditions to enhance sensitivity for low-mass axions.

## Contribution

It presents the most restrictive laboratory limit to date on the axion-photon coupling for axion masses below 0.02 eV, achieved through technological advancements in the CAST experiment.

## Key findings

- Established a new upper limit of $g_{a o ext{γ}} < 0.66 imes 10^{-10} ext{GeV}^{-1}$ for $m_a o 0$ eV.
- Enhanced sensitivity compared to previous phases with low-background detectors and new x-ray telescope.
- Demonstrated the effectiveness of vacuum conditions and advanced detection for future axion searches.

## Abstract

During 2003--2015, the CERN Axion Solar Telescope (CAST) has searched for $a\to\gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. In its final phase of solar axion searches (2013--2015), CAST has returned to evacuated magnet pipes, which is optimal for small axion masses. The absence of a significant signal above background provides a world leading limit of $g_{a\gamma} < 0.66 \times 10^{-10} {\rm GeV}^{-1}$ (95% C.L.) on the axion-photon coupling strength for $m_a \lesssim 0.02$ eV. Compared with the first vacuum phase (2003--2004), the sensitivity was vastly increased with low-background x-ray detectors and a new x-ray telescope. These innovations also serve as pathfinders for a possible next-generation axion helioscope.