Dark matter: new detector searches for ‘ultra-light’ particles

Astronomers have a problem. Stars and galaxies dance to an unexpected tune, their motion seemingly controlled by six times as much matter as we can see. Scientists believe that the universe is filled with a form of dark matter that far exceeds the amount of ordinary matter. There’s just one problem: there’s no direct evidence for the existence of dark matter.

For the past fifty years, physicists have tried in vain to detect dark matter. Many options have been considered, ranging from subatomic particles to invisible black holes. In recent decades, the theoretical physics community has favored the idea that dark matter consists of stable particles with a mass somewhere between the mass of a proton and a few thousand times larger.

However, a group of physicists from the 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 more traditional searches.

Ultralight 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 has not yet been observed.

The first particle is called a ‘dark photon’, which can interact with dark matter particles just as ordinary photons interact with ordinary matter. However, if they exist, dark matter photons would not interact directly with ordinary matter, just as ordinary photons do not interact with dark matter.

However, due to a quirk of quantum mechanics, it might be possible for dark photons to transform into regular photons, although 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 that there is no asymmetry in the way 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 the nucleus of atoms together and the weak nuclear force induces certain forms of radioactivity.)

The BREAD detection technique is based on dark matter or axions acting on a metal wall and emitting ordinary photons perpendicular to the metal. Once created, these ordinary photons can be detected using conventional technology. These photons are not necessarily visible light, but can in principle come from any frequency in the electromagnetic spectrum. In the recent publication, researchers only reported the outcome of a search for dark photons by looking for a specific class of microwaves.

BREAD researchers designed a sensitive radio receiver and used it to scan the range from 10.7 to 12.5 GHz. Conceptually, this is similar to scanning the frequencies with a car radio and looking for a transmitter. If dark photons in this frequency range were converted into regular photons, researchers would have seen a signal jump at a certain frequency.

No signal was observed, but the researchers were able to put a limit on the existence of dark photons in the mass range of 44 to 52 microelectron volts (μeV), much lower than the range of traditional dark matter searches. The new detector was 10,000 times more sensitive than previous measurements in this mass range.

Future experiments

While 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-on device that is expected to significantly increase both its sensitivity and the mass range it can explore.

While this process is underway, the researchers use the current device to perform a similar search for axions. The detection technique is similar, but axions are expected to turn into regular photons when placed in a strong magnetic field. The current effort uses a 4 Tesla magnet located at Argonne National Laboratory. While this effort is expected to deliver record-breaking performance, a larger and more powerful magnet is expected to arrive at Fermilab, further expanding the capabilities of the collaboration.

Dark matter, if it exists at all, is an elusive substance, offering very little experimental clues about the material’s properties. Theoretical estimates for the mass of individual dark matter particles range from 1 millionth of the mass of an electron to as much as 100 times the mass of the Sun. Although experiments have ruled out parts of this vast mass range, large portions remain unexplored. The BREAD collaboration hopes to play a leading role in the low-mass region.

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