The Universe is driven by four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. The behavior of matter within the Universes governed by their interactions via these forces. If these four forces are truly the only forces there are, then by understanding these forces we should have a full understanding of how objects interact. So when matter behaves in a way that is odd or unexplainable, one idea that often gets considered is that there might be another force at work. Perhaps there is a fifth fundamental force we haven’t yet discovered. Take, for example, the mystery of dark matter. 

Dark matter was first proposed to account for the fact that the motion of of hydrogen in the Milky Way didn’t seem to follow the rules of gravity. Either our understanding of gravity didn’t apply on galactic scales, or our galaxy has far more mass than is visible through stars and dust. Over the last few decades other observational evidence supports the existence of dark matter, but pinning down the details has proved quite vexing. Efforts to detect dark matter directly have been unsuccessful, and creating a theoretical model that unifies dark matter with regular matter is equally elusive.

Based upon observation, we know a few facts about dark matter. It makes up about 27% of the matter in our Universe, it interacts gravitationally like regular matter, and it interacts weakly (if at all) with light. In other words, dark matter interacts via the gravitational force, but not the electromagnetic one. Perhaps the most popular model for dark matter proposes that they also interact with the weak force, making them Weakly Interacting Massive Particles (WIMPs). But if dark matter is comprised of WIMPs they should be detectible through weak interactions on Earth, and so far such interactions haven’t been observed.

So what else could dark matter be? If dark matter only interacts with other matter via the gravitational force, then one would assume it also only interacts with itself gravitationally. But there have been some interesting hints that perhaps dark matter can interact with itself in a non-gravitational way. It could be through the strong force, or it could be via some new, dark-matter force.

Every fundamental force has a corresponding force-carrying boson through which it interacts with matter. The strong force has gluons, electromagnetism and photons, etc. If there is a fifth dark-matter force, there should be some corresponding interaction boson. Several years ago Sean Carroll et al proposed an analogous force to electromagnetism known as dark electromagnetism.  Just as regular matter interacts with electromagnetism through photons, dark matter would interact through “dark photons.” Since dark photons wouldn’t interact with regular matter, the “light” from dark matter wouldn’t be seen, thus explaining its invisible nature.

According to the dark electromagnetism model, dark photons and regular photons would interact slightly through a process known as mixing, and this would have a subtle effect on particle interactions. When the model was proposed it was thought dark photons could explain a mystery in particle physics known as the g-2 anomaly, where the experimental value of a muon’s magnetic moment differs by three standard deviations from the theoretical prediction of the standard model. However subsequent experiments seem to eliminate dark photons as a viable solution.

The data seems to match the existence of a new boson. Credit: A.J. Krasznahorkay et al.

Recently studies of the decay rates of Beryllium-8 have led researchers to propose a new variation of the dark-matter force. Beryllium-8 nuclei are very unstable, so they decay into helium atoms very quickly. But if it has a lot of energy Beryllium-8 will emit a high energy photon before decaying. This photon can likewise decay into an electron-positron pair, which is much easier to detect. According to the standard model, the electron and positron are much more likely to have a similar direction, so the greater the difference in their directions, the less pairs you should see. But a team found a bump in the number when they were spaced about 140 degrees apart, which could be explained by an interaction with a new kind of boson. It’s energy is too high to be a dark photon, so the team has called it an x-boson, x being “unknown.”

It’s important to note that while this could be a big discovery, it needs to be replicated. Even if it does turn out to be legitimate there is no evidence connecting it to dark matter. The team was specifically looking for a dark-matter force boson, hence the connection to dark matter. As it stands it’s an interesting result, but there’s a lot more work to be done.

Paper: Jonathan L. Feng, et al. Protophobic Fifth-Force Interpretation of the Observed Anomaly in ⁸Be Nuclear Transitions. Phys. Rev. Lett. 117, 071803 (2016)  arXiv:1604.07411 [hep-ph]

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