Probing keV mass QCD axions with the SACLA X-ray free electron laser

Abstract

Axions are hypothetical particles, proposed to account for the invariance of CP symmetry in quantum chromodynamics. While axions and axion-like-particles are well-motivated by string theory and beyond-Standard-Model extensions, they have remained elusive to experimental searches even after significant effort over many decades. Building on a recent development using an X-ray free electron laser to search for cosmologically favoured axions of mass $m_{a} \lesssim 0.01$ eV, we extend previous bounds on the ALP-photon coupling, $g_{aγγ}$, by over an order of magnitude. We exploit the Bormann effect of Laue crystals in a light-shining-through-wall experiment, with broad sensitivity to $m_a \lesssim$ 22 eV. Moreover for $m_{a} \in$ (3460, 3480) eV our sensitivity reaches down to the QCD axion coupling prediction, providing the most stringent laboratory constraints in this mass range.

Figures (4)

• Figure 1: Our experimental setup: $\sigma$ polarised X-rays enter the hutch, 1. X-rays impinge on the first Ge crystal. The crystal is mounted on motorised stages to have $\theta$ and $\xi$ rotation control, 2. A thick retractable blocker stops the X-rays, but ALPs pass through the blocker, 3. To verify alignment, transmission from the first crystal is recorded on the Transmission MPCCD, 4. For aid in alignment and to characterise the setup, a second Diffraction MPCCD is placed above the setup, 7. The ALPs reach the second Ge crystal where there is a probability, $P_{a\leftrightarrow\gamma}$, that they are reconverted to 10 keV X-rays, 5. The second Ge crystal too has $\theta$ and $\xi$ control and can be retracted to record diffraction from the first crystal on the Diffraction MPCCD. We slit the X-rays before entering and after exiting the 2.6 m flight tube, 8, with 4 jaw slits, 6, 9. X-rays are detected on the CITIUS or MPCCD detector, 10.
• Figure 2: Diffracted and transmitted efficiencies of the first Ge crystal; as expected for Laue diffraction: $I_{\text{\rm Ge}}^{S} \approx I_{\text{\rm Ge}}^{T}$. Both measurements are corrected for attenuation due to scattering in air Henke:1993eda. The fit to the rocking curve has width $33$$\upmu$rad and peak diffracted efficiency of $1.9\times10^{-3}$.
• Figure 3: A hit-map of detected photons across all of the axion search data plotted over the intensity profile of the double diffracted X-rays, in blue. A droplet algorithm is applied to the frames of CITIUS data to attribute single photon events. The region $\mathcal{R}$ encloses the photon hits.
• Figure 4: Bounds on $g_{a\gamma\gamma}$ (90% C.L.) derived from our X-ray-shining-through-walls experiment at SACLA represent a significant improvement over the previous such experiment at EuXFEL Halliday:2024lca. While a broad mass region is excluded for $m_{a} <22$ eV, for $m_{a} \in (3462,3482)$ our sensitivity reaches down to the QCD coupling (blue band) obtained from Eq. \ref{['QCDAxion']}ParticleDataGroup:2024cfk. The width of the bounds is set by the width of the effective rocking curve of the Ge crystals. Other experimental bounds AxionLimits are shown for comparison: NOMAD NOMAD:2000usb, BaBar BaBar:2017tiz, SAPPHIRES Nobuhiro:2020fubSAPPHIRES:2022bqg, ALPS Ehret:2010mh and PVLAS DellaValle:2014wea.