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The axion was originally proposed to explain the absence of CP violation in quantum chromodynamics, and more general axion-like particles (ALPs) arise naturally in many extensions of the Standard Model. While most experimental searches focus on ultra-light axions motivated by dark matter, increasing attention has recently turned to heavier axions, with masses from the sub-eV to keV scale. These models can evade several theoretical challenges, such as the axion quality problem. Many of the strongest existing bounds on heavy axions come from solar searches, which rely on model-dependent assumptions about axion production in hot, dense plasmas. An attractive alternative is direct laboratory detection, in which axions are both produced and detected under controlled conditions.
In this seminar I will describe recent direct-detection experiments performed at X-ray Free-Electron Laser (XFEL) facilities, including the European XFEL and follow-up measurements at SACLA. These experiments exploit the Primakoff effect, using the extremely large internal electric fields present in crystalline solids to enable photon-axion-photon conversion in a light-shining-through-a-wall configuration. In germanium crystals, electric fields can reach ~10¹¹ V/m, corresponding to effective magnetic fields of order kilotesla – far beyond what is achievable with conventional magnets. Using these techniques, we demonstrate sensitivity approaching QCD axion couplings for axion masses in the keV range.
Beyond axions, I will use these results to motivate a broader discussion about the role of high-power laser and XFEL facilities in fundamental particle physics. Rather than presenting a fixed roadmap, the aim is to explore how extreme-field laser experiments can complement accelerator-based searches, what kinds of weakly-coupled physics they are naturally suited to, and where their limitations lie.