With no conclusive evidence of dark matter yet, theorists have dreamed up a plethora of hypothetical particles. One candidate is the dark photon: Unlike regular photons, they may have a nonzero mass, but similar to photons, they’ve been proposed as a force carrier between particles.

If they exist, dark photons may couple weakly to regular photons through a process known as kinetic mixing. Detecting that process would be conclusive evidence for new physics beyond the standard model. Haipeng An (Tsinghua University), Xiaoyuan Huang (Purple Mountain Observatory), Jia Liu (Peking University), and their colleagues have now used observations from China’s Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) to derive an upper limit on the rate of kinetic mixing.

Evidence of kinetic mixing should be present in signals measured by FAST and other radio telescopes. According to dark-photon theory, the free electrons on the metal-plate dish should oscillate when interacting with dark photons, and the electromagnetic waves emitted by the oscillation would have the same frequency as the dark photons. After reflecting perpendicularly off the dish, they could be measured at the feed of the telescope.

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