New CAST Limit on the Axion-Photon Interaction

TL;DR

CAST searches for solar axions via the a→γ conversion in a strong magnetic field. In 2013–2015, the experiment used evacuated magnet bores, a dedicated X-ray telescope, and low-background Micromegas detectors to achieve a world-leading limit on the axion-photon coupling for m_a ≲ 0.02 eV. The analysis employed an unbinned likelihood incorporating the solar axion flux and the detector response, yielding no signal and a 95% C.L. bound of g_{aγ} < 0.66×10^-10 GeV^-1. The result strengthens constraints on QCD axions and axion-like particles, informs the design of future helioscopes like IAXO, and demonstrates the viability of dedicated XRT optics with ultra-low background detectors.

Abstract

During 2003--2015, the CERN Axion Solar Telescope (CAST) has searched for $a\toγ$ 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 $g_{aγ} < 0.66 \times 10^{-10} {\rm GeV}^{-1}$ (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.

Figures (6)

• Figure 1: Sketch of the CAST helioscope at CERN to search for solar axions. These hypothetical low-mass bosons are produced in the Sun by Primakoff scattering on charged particles and converted back to x-rays in the $B$-field of an LHC test magnet. The two straight conversion pipes have a cross section of 14.5 cm$^2$ each. The magnet can move by $\pm8^\circ$ vertically and $\pm40^\circ$ horizontally, enough to follow the Sun for about 1.5 h at dawn and dusk with opposite ends. Separate detection systems can search for axions at sunrise and sunset, respectively. The sunrise system is equipped with an x-ray telescope (XRT) to focus the signal on a small detector area, strongly increasing signal-to-noise. Our new results were achieved thanks to an XRT specifically built for CAST and improved low-noise x-ray detectors.
• Figure 2: CAST excluded region (95% C.L.) in the $m_a$--$g_{a\gamma}$--plane. Solid black line: Envelope of all CAST results from 2003--2011 data Zioutas:2004hiAndriamonje:2007ewArik:2008mqArik:2011rxArik:2013nyaArik:2015cjv. Solid red line: Exclusion from the data here presented. Diagonal yellow band: Typical QCD axion models (upper and lower bounds set according to a prescription given in Ref. DiLuzio:2016sbl). Diagonal green line: The benchmark KSVZ axion model with $E/N=0$, where $g_{a\gamma}=(E/N-1.92)\,\alpha/(2\pi f_a)$ with $f_a$ the axion decay constant.
• Figure 3: 2D hitmap of events detected in the sunrise detector in a typical in-situ calibration run (left), as well as in the background (middle) and tracking (right) data (both K and L data sets of Table \ref{['tab:datasets']}). The calibration is performed with an x-ray source placed $\sim$12 m away (at the sunset side of the magnet). The contours in the calibration run represent the 95%, 85% and 68% signal-encircling regions from ray-trace simulations, taking into account the source size and distance. In the tracking and background plots, grey full circles represent events that pass all detector cuts but that are in coincidence with the muon vetoes, and therefore rejected. Black open circles represent final counts. Closed contours indicate the 99%, 95%, 85% and 68% signal-encircling regions out of detailed ray-trace simulations of the XRT plus spatial resolution of the detector. The large circle represents the region of detector exposed to daily energy calibration.
• Figure 4: Spectra of tracking (dots) and background (solid line) data for each of the sunset datasets. The error bars correspond to the 1-$\sigma$ statistical fluctuation of each bin content following Poissonian statistics. The error bars of the background data are omitted for the sake of clarity, although they are typically $\sim$3 times smaller than the tracking data error bars.
• Figure 5: Energy spectrum of background data in the sunrise detector (both K and L datasets). The error bars correspond to the 1-$\sigma$ statistical fluctuation of each bin content following Poissonian statistics.