Our most advanced physics theories have been incredible in predicting and explaining the particles, interactions, and phenomena that we witness in the world, but we are quite aware that they are limited. Physicists are constantly poking at those limits, with some intriguing observations. Current physics centers on the idea of four forces of nature: gravity, electromagnetism, and the two forces of nuclear, weak and strong. Now, researchers have discovered an anomaly that hints at more evidence of the existence of a fifth force of nature.
Back in 2016, a team of Hungarian physicists published an interesting study on the radioactive decay (the process by which an unstable atomic nucleus loses energy by radiation) of beryllium, an unstable isotope. Their findings saw hints of a previously undiscovered particle they dubbed X17. The particle was not detected directly but researchers discovered an increase of electron-positron pairs, indicating that there must have been an unknown particle carrying energy away from the nucleus and decaying into the pairs. The particle is a boson (so has integer spin, like photons and the Higgs), has no electric charge, and weighs roughly 33 times that of an electron.
The same team has now seen hints of the same particle, X17, in a different decay, this time involving helium, publishing their findings in a paper on the pre-print site ArXiv. They studied the decay of an excited helium atom, and as the universe much prefers things to be in their lowest possible energy state, the helium releases that energy in the form of particles.
The team again encountered an increase of electron-positron pairs, also indicating the presence of an unknown particle. The mass of this particle is very close to the one they had observed in the case of beryllium. The team reports that they are confident in the detection, suggesting that there is only one chance in 1.6 trillion that this is a fluke.
Assuming that this is correct and there are no other unaccounted systematics, this is very exciting. This X17 particle must only interact with neutrons, otherwise it would have been detected before. And that’s only possible if it doesn’t interact using any of the four known fundamental forces: gravity, electromagnetism, strong nuclear force (which keeps atoms together), and weak nuclear force, which is responsible for some radioactive decays.
This hypothetical fifth force could be an important piece of the puzzle for physics. Dark matter and dark energy are the two most abundant substances in the universe but we have struggled to study them experimentally. If this force is real, we may be able to begin exploring a new frontier in particle physics in earnest.
The team is eagerly awaiting results from other labs to confirm or deny the existence of the particle. As reported by New Scientist, several experiments are in the works, including at CERN and the Positron Annihilation into Dark Matter Experiment in Italy, that are looking for X17 and we should know one way or the other in the next couple of years.
[H/T: New Scientist]