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), shown in the photo, 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.
An and his colleagues examined archival FAST data, which covered the 1–1.5 GHz frequency range and are bounded by the solid red line in the figure below. The evidence is the first of its kind and didn’t show any kinetic mixing at that range, with a statistical confidence level of 95%. If dark photons do exist, their dimensionless kinetic mixing rate cannot be larger than 10−12, which at that frequency is about an order of magnitude stronger than the bound from the cosmic microwave background. (Shown in gray is the parameter space excluded by haloscopes, which are instruments that use resonant cavities to search for dark photons, axions, and other ultralight dark-matter candidates.)
The search for dark photons using radio telescopes is just beginning. In the future, FAST will have more receivers, so An and his colleagues suspect that the telescope will be able to scan a wider frequency range, marked by the red dashed line in the figure. And telescopes with dipole antennas can measure other possible mass regions. The researchers say that the Square Kilometre Array in Australia (brown shaded region) should be sensitive to dark photons up to 20 GHz, and the Low-Frequency Array in the Netherlands and other parts of Europe (blue shaded region), down to 10 MHz. (H. An et al., Phys. Rev. Lett. 130, 181001, 2023.)