Have scientists discovered a fifth fundamental force of nature?

A new study has confirmed the possible discovery of a fifth fundamental force of nature, a development which could lead to a radical rethinking of our understanding of the universe.

The research conducted by theoretical physicists at the University of California, Irvine (UCI), re-analysed a 2015 Hungarian study which found evidence for a previously unknown subatomic particle, and by extension the possibility of a fifth force. The paper was published in the journal Physical Review Letters.

"If true, it's revolutionary," said Jonathan Feng, an author of the study and professor of physics & astronomy at UCI. "For decades, we've known of four fundamental forces; gravitation, electromagnetism, and the strong and weak nuclear forces. If confirmed by further experiments, this discovery of a possible fifth force would completely change our understanding of the universe, with consequences for the unification of forces and dark matter."

The 2015 study, which took place at the Hungarian Academy of Sciences, threw up an anomaly in the data which led to the discovery of a new super-light subatomic particle just 30 times heavier than an electron. The physicists concluded that this was a kind of boson – a fundamental particle - that was not predicted by the Standard Model of particle physics – a combined set of theories and equations which are our best attempt at describing the way the universe works. According to this model, each of the four known fundamental forces corresponds with a different type of boson: The strong nuclear force is 'carried' by gluons, the weak nuclear force is carried by 'w' and 'z' bosons, while the force carrier for electromagnetic forces are called photons – or particles of light.

However, the Standard Model does not explain everything, for example, no boson has yet been discovered for the gravitational force, and the model does not fully account for the mysteries of dark matter, an unidentified form of matter which, scientists say, accounts for around 85% of the universe's total mass. The Hungarian scientists initially suggested that the new particle they had chanced upon was some kind of 'dark photon', a hypothetical particle which is thought to be the force carrier for 'dark matter'.

"The experimentalists weren't able to claim that it was a new force," Feng argued. "They simply saw an excess of events that indicated a new particle, but it was not clear to them whether it was a matter particle or a force-carrying particle."

For their study, the UCI team used the data from Hungary in addition to looking back over previous experiments in the field. The evidence led them to the conclusion that the Hungarians had found neither dark photons nor standard matter particles. After synthesising all the available data, they described the mysterious particle in question as a "protophobic X boson" indicating that it could signify the presence of a fifth fundamental force of nature. Co-author of the study Timothy Tait said there was currently no other kind of boson known to science which possesses the same characteristics as the protophobic X boson. "Sometimes we also just call it the 'X boson,' where 'X' means unknown."

The theory presented by UCI is still far from being confirmed and Feng says that further experiments was needed. "The particle is not very heavy, and laboratories have had the energies required to make it since the 50s and 60s.

"But the reason it's been hard to find is that its interactions are very feeble. That said, because the new particle is so light, there are many experimental groups working in small labs around the world that can follow up the initial claims, now that they know where to look."

Feng outlined one promising direction that new research on the subject could take. He suggested that scientists could examine the possibility that the potential fifth force is linked in some way to the electromagnetic, strong and weak nuclear forces as "manifestations of one grander, more fundamental force".

He also speculates that alongside our understanding of the Standard Model of physics, there may be a 'dark sector' with its own special matter and forces. "It's possible that these two sectors talk to each other and interact with one another through somewhat veiled but fundamental interactions," he said. "This dark sector force may manifest itself as this protophobic force we're seeing as a result of the Hungarian experiment. In a broader sense, it fits in with our original research to understand the nature of dark matter."