Aerial view of the Five-hundred-meter Aperture Spherical Telescope (FAST), which sits in a natural basin in the Guizhou province of China. Scientists used the FAST dish to test for the probability of interactions between dark photons and standard photons. [Image: J. Dai] [Enlarge image]
For more than four decades, cosmologists have searched for dark matter in the universe—and speculated on the nature of the hypothetical material. In some models of particle physics, dark matter has its own force carrier called the dark photon, analogous to the relationship between the regular photon and electromagnetic waves.
In these models, dark photons may, on occasion, couple themselves with normal photons, which is called kinetic mixing. To seek evidence for possible interactions between dark photons and real particles, physicists from several Chinese universities have searched through observational data from one of the world’s largest radio telescopes (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.130.181001).
The research team, led by Haipeng An of Tsinghua University, found an upper limit to the kinetic mixing and suggested that interferometric arrays of radio dishes could increase the sensitivity of future investigations.
Why dark matter is “dark”
As observational astronomy grew more precise over the 20th century, scientists began to notice that distant galaxies didn’t behave the way they looked—their rotational speeds implied that they contained more matter than we could see from Earth. The discrepancy became even more noticeable when astronomy expanded beyond the visible part of the electromagnetic spectrum.
By the late 1970s and early 1980s, scientists had accumulated considerable evidence of the existence of this invisible matter, which does not appear to interact with the electromagnetic field—but its exact nature has still not been determined. The consensus among cosmologists is that dark matter consists of exotic subatomic particles.
Mixing it up in FAST
According to An and his colleagues, some scientists think that ultralight bosons—a category of hypothetical particles encompassing axions and dark photons—are a good candidate for dark matter. But how can scientists detect such particles when they cannot be seen?
Some scientists think that ultralight bosons—a category of hypothetical particles encompassing axions and dark photons—are a good candidate for dark matter.
Fortunately, the models of dark photons indicate that they may interact with free electrons in a radio telescope. In typical observations, the incoming radio waves from the universe excite the free electrons in the telescope’s metal dish, so the team wanted to see if these dark photons would also interact with free electrons via kinetic mixing. If this happened, the process would generate a monochromatic radio signal at a frequency dependent on the mass of the dark photons.
The telescope the researchers used, China’s Five-hundred-meter Aperture Spherical Telescope (FAST), currently reigns as the world’s largest filled-aperture radio telescope. In December 2020, the researchers collected 110 minutes of L-band (1 to 1.5 GHz) observational data from the telescope.
For this radio band, An’s group calculated that the value of ϵ, a dimensionless parameter quantifying the rate of kinetic mixing, can be no higher than 10−12. The researchers suggest that more powerful arrays of radio dishes, such as the Low-Frequency Array (LOFAR) in Europe and the Square Kilometer Array under construction in Australia and South Africa, may be more sensitive to dark-matter particles. Looking ahead, the team plans to analyze data from space-based radio telescopes, which are sensitive to dark-photon-induced signals at frequencies below those accessible to radio telescopes on Earth.