New Study Suggests We Might Have Spotted a Fifth Force of Nature

The Universe just keeps getting stranger.

16 AUG 2016

As far as we know, there are four fundamental forces that hold our Universe together - gravity, electromagnetism, and the strong and weak nuclear forces.

But, in April last year, physicists in Hungary saw evidence of a possible fifth force of nature, one that could potentially explain some of the lingering mysteries in our Universe, such as dark matter.


Now an independent team of researchers has re-analysed the results, confirming that the anomaly seen in the data last year really could be a whole new fundamental force.

This idea is still a long way off being confirmed - as we learnt from CERN's latest announcement, sometimes promising blips in the data end up disappearing with further testing - but the study suggests that this possible new force-carrying particle is definitely worth following up on. 

"If true, it’s revolutionary," said lead researcher Jonathan Feng from the University of California, Irvine. "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."

So, a quick back story here: the strange result in question was first seen last year, when a team from the Hungarian Academy of Science fired high-energy beams of protons at lithium-7, and in the fall-out spotted the energy signature of a new super-light subatomic particle.

This new subatomic particle, they concluded, was a type of boson that was only 30 times heavier than an electron, and wasn't predicted by the Standard Model of particle physics - the best set of equations we have for understanding the Universe.

According to the Standard Model, each of the four fundamental forces has a corresponding boson - the strong force is carried by 'gluons'; the electromagnetic force is carried by particles of light, or photons; and the W and Z bosons are responsible for weak force.


We haven't yet discovered gravity's boson (that's just one of the gaps in the Standard Model, which also doesn't explain dark matter) but it's predicted to be something called the graviton.

The Hungarian team initially suggested that maybe their blip had been some kind of dark photon - a hypothetical particle responsible for carrying dark matter - but since their initial publication, international researchers have taken their data and run with it.

"The experimentalists weren’t able to claim that it was a new force," said Feng. "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."

To clear this up, Feng and his colleagues studied the original data, as well as other experiments in this area, and found theoretical evidence to strongly suggest that the blip in the data wasn't a matter-carrying particle or a dark photon.

Instead, their calculations suggested that it could be the boson for the fifth force of nature - a force that's been predicted to account for dark matter and many other mysterious things in the Universe.

What's strange about this hypothetical boson, which they call the protophobic X boson, is that it only interacts with electrons and neutrons, and at an extremely limited range, making it very hard to detect. 

"There’s no other boson that we’ve observed that has this same characteristic," said one of the researchers, Timothy Tait. "Sometimes we also just call it the 'X boson', where 'X' means unknown."

The team first floated this idea in May, uploading their analysis onto pre-print site, but it's now also been peer-reviewed and published in the journal Physical Review Letters

They've since conducted a follow-up analysis since then, uploaded to arXiv last week, which amplifies their original conclusion.

So what we have now is a strange blip that can't be explained by the Standard Model, and theoretical calculations to suggest that this blip would work as the carrier of the fifth force of nature.

But unfortunately, what we don't have are further experimental confirmations - something that researchers around the world are rushing to produce, and expect to be ready within a year.

"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," said Feng.

So what would it mean if this fifth force was verified? We're still a long way off that, but Feng suggests it could potentially be joined to the electromagnetic and strong and weak nuclear forces to manifest some kind of 'super fundamental force' - one that might interact with a dark sector with its own 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." 

(Yes, that really does sound a whole lot like the dark and light side of the Force in Star Wars.)

It's cool to contemplate, but until we have more experimental results, there's not much we can do but wait. Most of us are still smarting from the loss of the diphoton energy excess that never was, so it's too soon to commit to a hypothetical new physics-breaking discovery just yet.

But it sure is an interesting hypothesis that we'll be following closely... Watch this space.

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