Astronomers have a problem. Stars and galaxies dance to an unexpected tune, their motion seemingly governed by six times as much matter as 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 just one problem: There is no direct evidence for the existence of dark matter.
For the past 50 years, physicists have tried to detect dark matter, to no avail. Many options have been considered, ranging from subatomic particles to invisible black holes. For the past few 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 several thousand times larger.
However, a group of physicists at 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 (Broadband Reflector for Action Detection Experiment) collaboration are searching for ultralight 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 regular photons interact with ordinary matter. However, if they do exist, dark matter photons will not directly interact with ordinary matter, just as ordinary photons do not interact with dark matter.
However, through a quirk of quantum mechanics, it may be possible for dark photons to transform into ordinary photons, although this transformation would be rare.
In contrast, stocks 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 the reason why the strong nuclear force could not 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. Action 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 causes some forms of radioactivity.)
The BREAD detection technique relies on dark matter or stocks interacting with a metal 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 of any frequency in the electromagnetic spectrum. In the latest publication, the researchers reported only the result of a search for dark photons looking for a specific class of microwaves.
The BREAD researchers designed a sensitive radio receiver and used it to scan the 10.7 to 12.5 GHz range. Conceptually, this is similar to scanning the frequencies with a car radio and trying to find a broadcast station. If the dark photons were converted to ordinary photons in this frequency range, the researchers would have seen a jump in the signal at a certain frequency.
No signal was observed, but the researchers were able to place a limit on the existence of dark photons in the mass range of 44 to 52 microelectron volts (μeV), well below 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
Although this achievement is significant, this version of the BREAD detector is merely a device that proves that the experimental approach is viable. Researchers are designing a tracking device that is expected to significantly increase the sensitivity and range of mass it can explore.
While this process is underway, researchers are using the current device to do a comparable stock search. The detection technique is similar, but the particles are expected to convert into ordinary photons when placed in a strong magnetic field. The current effort uses a 4 Tesla magnet located at Argonne National Laboratory. Again, while this effort is expected to set record 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 on material properties. Theoretical estimates of the mass of individual dark matter particles have ranged from 1 millionth the mass of an electron to 100 times the mass of the Sun. While experiments have excluded parts of this large mass, large parts remain unexplored. The BREAD collaboration hopes to play a leading role in the low mass region.
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Image Source : bigthink.com