Physicists discovered a fifth force of nature?

At first glance, the news is ground-breaking. A team of American particle physicists, building on some Hungarian work, announced the “possible discovery” of a fifth fundamental force of nature.

If true, this is massive. But peer a bit closer, and the claim appears to be built on some shaky foundations.

Here’s what you need to know.

What do you mean by ‘fifth force’? 

Everything we can see is governed by just four fundamental forces of nature. Three – electromagnetism, the strong nuclear force and the weak nuclear force – are explained to wonderful precision by the standard model of particle physics. The fourth force, gravity, is the realm of Einstein’s much-feted theory of general relativity.

While we haven’t added to this list in more than 60 years, there’s no theoretical reason for there not to be a fifth.

Dark matter, for instance, is an as-yet-unopened treasure trove of potential new physics. We know dark matter is out there, and that it doesn’t interact with ordinary matter. Might it not feel some kind of “dark force” instead?

So what’s all the new fuss about?

It was the search for such a dark force that led a team of physicists from the Institute for Nuclear Research in Debrecen, Hungary, to study the relaxation of excited beryllium-8 nuclei last year.

They were hoping that some dark force carriers (called “dark photons”) might interfere with the decay.

As it happened, the team measured an anomaly which could not be explained by anything in nuclear physics. The evidence, in the form of extra electrons and positrons emitted at a particular angle, seemed to indicate the fleeting appearance of a new particle (one that decayed to form each electron-positron pair).

From its quantum properties, the team surmised the particle must be a boson (a particle that carries force) – just what they were looking for.

They proposed that the particle, which they dubbed the ‘X boson’, mediates a totally new force of nature.

The Hungarian result was surprising, though, because the newfound particle was so light – 16.7 megaelectronvolts, which is just 34 times the mass of the electron. Physicists have been exploring this energy region for more than 50 years. Why hadn’t this boson turned up before?

So the matter rested, until a team of particle physicists at the University of California, Irvine, led by Jonathan Feng, performed their own analysis of the same data.

Publishing in Physical Review Letters this month, Feng’s team ruled out the dark photon possibility, as that idea conflicted with previous experiments. Instead, they proposed that the particle, which they dubbed the “X boson”, mediates a totally new force of nature. 

Their explanation of the X boson as “protophobic” (meaning it doesn’t interact with protons), very weak, and short range, explained why the particle (and the force) had never been detected until now.

Yet while the team continues to refine their theory, others are questioning the original data on which it is based.

Why the shaky foundations?

So far, the X boson has only been detected by one experiment, so it needs independent confirmation by another group.

Michigan State University particle physicist Oscar Naviliat Cuncic explained to Quanta the Hungarian group previously claimed discovery of two bosons in very similar circumstances (also involving beryllium-8 nuclei), and at different energies than the current claim.

In 2008, they claimed discovery of a new boson at 12 megaelectronvolts, then in 2012 they claimed to find another around 13.5 megaelectronvolts. These disappeared as better detectors collected better data. But what’s to say the current 16.7 megaelectronvolts boson won’t disappear too?

A recurring lesson in particle physics is not to jump to conclusions.

In December 2015, a result from the CERN’s Large Hadron Collider seemed to indicate the discovery of a new particle. Theorists published more than 500 papers explaining its origin. But by July, the blip had disappeared in the deluge of new data.

This fifth force discovery will also live or die by the hand of more data. In particular, the DarkLight experiment at the Jefferson Laboratory in Virginia will analyse the same energy range within a year.

The discovery of a fifth force of nature is an extraordinary claim, requiring extraordinary evidence. Right now, the evidence is ordinary at best.