Searching for Axionlike Particles under Strong Gravitational Lenses.

We establish strong gravitational lens systems as robust probes of axionlike particles (ALPs)-a candidate for dark matter. A tiny interaction of photons with ALPs induces birefringence. Multiple images of gravitationally lensed polarized objects allow measurement of differential birefringence, alleviating systematics and astrophysical dependencies. We apply this novel method to the lens system CLASS B1152+199 and constrain the ALP-photon coupling ≤9.2×10^{-11} to 7.7×10^{-8} GeV^{-1} (95% C.L.) for an ALP mass between 3.6×10^{-21} and 4.6×10^{-18} eV. A larger sample will improve the constraints.