New CAST Limit on the Axion-Photon Interaction

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.

• The paper establishes a new upper limit on the axion-photon coupling constant at gₐγ < 0.66×10⁻¹⁰ GeV⁻¹ for axion masses below 0.02 eV.
• The methodology uses a dipole magnet helioscope with an advanced X-ray telescope that improves the signal-to-noise ratio by a factor of about three.
• The results tighten constraints on axion models and provide a benchmark for future dark matter experiments like the International Axion Observatory.

New CAST Limit on the Axion--Photon Interaction

The study conducted by the CERN Axion Solar Telescope (CAST) collaboration represents a significant advancement in the search for axions, hypothetical particles that are considered likely candidates for dark matter. This paper presents findings from the latest data collection phase, covering the years 2013 to 2015, during which the researchers set constraints on the coupling strength between axions and photons through the axion-photon conversion process in the presence of a magnetic field. The researchers achieved new sensitivity benchmarks for axion detection, extending the exploration of the parameter space defined by axion mass and coupling constant.

Methodology Overview

The CAST experiment is fundamentally designed around the helioscope concept, where a strong magnetic field converts solar axions into x-rays as they traverse the field. For this purpose, the CAST collaboration employed a dipole magnet (originally an LHC prototype) capable of tracking the Sun and facilitating axion-to-photon conversion. The absence of a significant x-ray signal above the background levels resulted in setting a limit on the axion-photon coupling constant at $g_{a\gamma} < 0.66 \times 10^{-10}~{\rm GeV}^{-1}$ for axion masses $m_a \lesssim 0.02$ eV, surpassing previous CAST results.

Instrumentation and Data Analysis

The experiment featured two specialized low-background detectors coupled with an X-ray telescope (XRT) tailored explicitly for this research. These innovations improved the sensitivity of solar axion searching, enhancing the signal-to-noise ratio by a factor of approximately three compared to previous experimental phases. The analysis incorporated a likelihood function to assess the background and tracking data collected during the experiment. A notable improvement over prior methodologies was the advanced background suppression techniques and refined x-ray focusing through the XRT, which concentrated the expected x-ray signal, increasing detection potential.

Results and Implications

The reported results represent the most stringent laboratory constraints on the axion-photon coupling constant, particularly intersecting with and sometimes surpassing astrophysical bounds derived from stellar evolution considerations. This research not only strengthens existing constraints but also brings clarity to the viability of certain axion models, such as the QCD axion, within the explored parameter space. The quiet background levels achieved with the newly implemented detection technologies pave the way for scaling these methods to future experiments like the proposed International Axion Observatory (IAXO).

Towards Future Axion Research

The outcomes of the CAST collaboration hold significant implications for future experimental searches of axions or axion-like particles (ALPs). The precision and sensitivity achieved in this experiment establish a benchmark for helioscopes globally while also motivating the need for larger-scale efforts capable of probing deeper into the potential axion parameter space. The methodologies and technological advancements from this study provide a foundation for upcoming projects aimed at unraveling the mysteries of dark matter and beyond-standard-model physics. Furthermore, these findings emphasize the importance of continued interdisciplinary collaboration in tackling one of physics' most challenging enigmas: the nature of dark matter.