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The so-called fifth force of nature is starting to look more and more real

A fifth fundamental force, perhaps discovered as part of the search for dark matter, has been reported in a new paper published in Physical Review Letters. The results still need further analysis, but they represent a step forward for an idea that has caused several months of controversy in open-source journals, and which is potentially momentous enough that it’s making physicists’ imaginations run wild. Interactions across the universe are supposed to be governed by gravitation, electromagnetism, and the strong and weak nuclear forces; so what’s this about a fifth force?

In 2015, a group of Hungarian physicists went looking for something called the “dark photon,” which is a theorized carrier of the electromagnetic force for dark matter — we know dark matter doesn’t emit regular photons, but maybe it emits its own version. The team started looking in certain radioactive decay products by firing protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that quickly decayed. These decay products should produce electrons-positron pairs, and the Standard Model says that (for some reason) we should see fewer of these pairs as the electron and positron in each pair are emitted at a wider angle.

This Hungarian team found that there was an unusually large number of pairs with angles around 140º, creating a bump in their graph of pair-frequency versus emission angle. They quickly ruled out the possibility that this was being caused by decay of any known particle, and it clearly wasn’t a dark photon. So that left two possibilities: It was a mistake, or some totally new sort of particle. The team believes the bump corresponds to a previously unknown particle that’s being emitted from the unstable beryllium atoms and quickly decaying into an electron-positron pair with the observed angle of incidence. They found that this new particle should be about 30 times heavier than an electron, or about 17 MeV (megaelectronvolts).

The original paper received little attention until a review by physicists from the University of California, Irvine. These scientists looked at the data and came to the conclusion that it didn’t contradict any known theory — meaning that while it is unknown, there’s also no reason to believe this new particle couldn’t exist. They claim that the particle is a boson, that it is not a mass-carrying particle, and that it doesn’t carry any of the four known forces. In principle, this implies that the particle is thus a force carrier for a force beyond the four currently known to exist.

If it exists, this new force is odd. It interacts only over extremely short distances, a few atomic nuclei at most, and affects only electrons and neutrons. It’s being classified as a “protophobic X boson” where “protophobic” refers to the lack of interaction with protons, and the X literally means “unknown.” Most importantly, its energy level is low enough that we won’t have to wait for the Large Hadron Collider to fit it into its busy schedule; the energy levels required to study this new force should be able to be created in a wide variety of labs around the world. According to the researchers, it was probably possible to study this as early as the 50’s or 60’s — scientists just didn’t know where to look.

The main reason they didn’t stumble upon this particle sooner (assuming it does in fact exist) is that its mass is so low, and its interactions so short-range, that they’re easy to miss. Now that the band of interest has been laid out, we can expect an oncoming glut of research into this mysterious new member of the force family — assuming, once again, that it is confirmed by later experiments.

What might this mysterious new particle mean? Beyond blowing up the Standard Model, there’s hope that the newly discovered force might act as a bridge between the light and “dark” worlds. There’s no real indication of that, and it’s mostly wishful thinking. But the protophobic nature of the particle could be a key to the different interactions it would need to have with normal matter and dark matter, respectively. Such a dark force would be useful in revealing the nature of WIMPs, the theorized mass-carrying particle that makes up dark matter. And it’s distinct from the dark photon, which would be the hypothetical electromagnetic force carrier for dark matter. Physicists may still discover the dark photon someday, too, further advancing understanding of the material that makes up the majority of the mass in the universe.

Earlier this year, scientists proved the existence of gravitational waves, which proves the possibility of direct measurement of gravity — long thought to be the only force that allows the “dark sector” to affect our world of regular matter and energy. Now, we have the potential discovery of a second force bridging the gap to the dark sector, and this one can be studied without an international funding drive. It makes your wonder just how many other fundamental qualities of the universe might be hiding in plain sight, unknown not because we haven’t built a smasher large enough, but because we simply haven’t been lucky enough to stumble upon it. By definition, we would have to find such hail Mary particles as part of unrelated studies — but, also by definition, they will offer insight into previously unknown areas of physics.

Perhaps most pressing though: what will this new particle, and force, be called? The Higgs Boson was named long before it was proven, but this particle came out of nowhere. Will it get named after its discoverers, or will it get some exotic named like “dark particle,” defined as the carrier of the “dark force”?

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