Astronomers have a problem. Stars and galaxies dance to an unexpected tune, their motion seemingly governed by six times the matter that can be seen. Scientists believe that the Universe is filled with a form of dark matter that far exceeds the amount of ordinary matter. There’s only one problem: There is no direct evidence for the existence of dark matter.
Over the past 50 years, physicists have tried to detect dark matter, to no avail. Many options have been considered, ranging from subatomic particles to unseen black holes. For the past few decades, the theoretical physics community has favored the idea that dark matter is made of stable particles with a mass somewhere between the mass of a proton and a few thousand times greater.
However, a group of physicists at Fermi National Accelerator Laboratory and the University of Chicago have explored a very different mass range. These scientists are looking for dark matter particles that are trillions or even quadrillion times lighter than the more traditional searches.
Ultra-light dark matter
Physicists from the BREAD collaboration (Broadband Reflector Experiment for Axion Detection) are looking for ultra-light dark matter. These researchers are looking for two classes of particles whose existence has been proposed by the theoretical community but not yet observed.
The first particle is called a “dark photon,” which could interact with dark matter particles much like regular photons interact with ordinary matter. However, if they exist, dark matter photons would not interact directly with ordinary matter, just like ordinary photons do not interact with dark matter.
However, through a quirk of quantum mechanics, it might be possible for dark photons to transform into ordinary photons, though this transformation would be rare.
In contrast, axions are thought to play a different role. In the accepted theory of the quantum world, the weak nuclear force interacts very differently with matter and antimatter. There is no a priori reason why the strong nuclear force could not also treat matter and antimatter differently. However, experimental evidence strongly suggests there is no asymmetry in how the strong nuclear force treats matter and antimatter. Axion theory was proposed as an explanation for this surprising observation. (Note: The strong nuclear force holds together the nucleus of atoms and the weak nuclear force induces some forms of radioactivity.)
The BREAD detection technique relies on dark matter or axions interacting on a metallic wall and emitting ordinary photons perpendicular to the metal. When created, those ordinary photons can be detected using conventional technology. These photons are not necessarily visible light, but could in principle be from any frequency in the electromagnetic spectrum. In the recent publication, researchers reported the outcome only for a search for dark photons by searching for a specific class of microwaves.
BREAD researchers designed a sensitive radio receiver and used it to scan the range of 10.7 to 12.5 GHz. Conceptually, this is similar to scanning the frequencies with a car radio and trying to find a broadcasting station. If dark photons were converting into ordinary photons in this frequency range, researchers would have seen a jump in signal at a particular frequency.
No signal was observed, but the researchers were able to set a limit on the existence of dark photons in the mass range of 44 to 52 microelectron volts (μeV), far lower than the range of traditional dark matter searches. The new detector was 10,000 times more sensitive than earlier measurements in this mass range.
Future experiments
Although this achievement is significant, this version of the BREAD detector is simply a device that proves the experimental approach is viable. The researchers are designing a follow-up device that is expected to considerably increase both the sensitivity and the mass range that it can explore.
While this process is underway, the researchers are using the current device to make a comparable search for axions. The detection technique is similar, but axions are expected to transform into ordinary photons when placed in a strong magnetic field. The current effort employs a 4 Tesla magnet located at Argonne National Laboratory. Again, while this effort is expected to set record-breaking performance, a larger and more powerful magnet is expected to arrive at Fermilab, which will further enhance the collaboration’s capabilities.
Dark matter, if it exists, is an elusive substance, providing very little experimental guidance as to the material’s properties. Theoretical estimates for the mass of individual dark matter particles have ranged from 1 millionth the mass of an electron to as much as 100 times the mass of the Sun. While experiments have ruled out parts of this enormous mass range, large parts remain unexplored. The BREAD collaboration hopes to stake out a leadership role in the low-mass region.
