The Sciences

Anomalous 'Fifth Force' Is Back in the News but It's Still Not a Force of Nature

Stumbling upon a particle during an experiment is one thing. Explaining what that particle is and why it behaves the way it does is quite another.

“To the best of our knowledge, the observed anomaly can not have a nuclear physics related origin.”

A group of scientists in Atomki, the nuclear physics research centre at the Hungarian Academy of Sciences, Debrecen, uploaded a pre-print paper in April 2015 with these words. They were referring to a bunch of experiments they’d run in which they found multiple non-arbitrary signs of a particle weighing about 19-times as much as a proton. They didn’t straight up claim a discovery because their equipment was not designed to detect particles. They had inferred the particle’s mass value based on the behaviour of the known particles to which the unknown particle decayed. They called the latter X17 (17 stands for the mass, 17 MeV/c2), and the incident came to be called the Atomki anomaly.

This ‘finding’ roughly coincided with a period of time in which high-energy physicists were hotly debating how to make sense of the universe. A group of rules and equations they’d pieced together over eight decades based on observations in experiments and with instruments like the Large Hadron Collider (LHC) could explain almost everything about how normal matter works. But so far it’s been pretty clueless about stuff like dark matter and questions like why the Higgs boson weighs what it does and not more. Such considerations sound esoteric, even abstruse, to the casual observer but they’re very important to physicists. However, physicists only have a few promising leads that thus far haven’t yielded any concrete results, and instruments like the LHC haven’t turned up any new particles or forces either.

Attila Krasznahorkay. Source: atomki.hu

Attila Krasznahorkay, one of the members of the Atomki group. Source: atomki.hu

Imagine you’re an adventurer in a game of Dungeons & Dragons who’s worked out that the world must contain forests, mountains, canyons, swamps, deserts and other wondrous features. But when you set off on your quest, you discover that the major road out of your little town leads directly to a dead-end inside a beautiful cave and now you don’t know how to get out on the other side.

This atmosphere of uncertainty and (a certain kind of) frustrated despair amplified news of the Atomki anomaly, leading to greater interest from physicists around the world as well as grossly exaggerated reports in the press that scientists had discovered a fifth force of nature, represented by X17. (Types of particles called bosons can be thought of as force-carriers, and if X17 is a boson, it could represent a new kind of fundamental force). The same Atomki group released a new set of results on October 23, 2019, in which they claim to have found signs of X17 for the second time. And this time, similarly exaggerated news reports repeated the ‘fifth force’ hyperbole and CNN tacked on a possible Nobel Prize for the discoverers of the particle as well. This is where it gets even more interesting.

In the results reported in April 2015, the Atomki group had bombarded beryllium atoms with protons and studied the resulting particulate debris. In the October 2019 update, the group reports bombarding helium atoms. In both cases, they claim anomalous patterns in the debris can be explained by the presence of a previously unknown particle with a mass of 17 MeV.

This is exciting because two different experiments have supposedly yielded common evidence for the existence of a new particle. If the beryllium experiment was a random fluke, it’s highly unlikely that the same fluke manifested itself in the helium experiment as well. But that doesn’t mean there wasn’t non-random interference. For example, the instruments the group used to observe the debris could be flawed in a way that produces an otherwise unaccountable spike in the data. (This is also a leading explanation for anomalous repetitive noise patterns in the IISc team’s supposed observation of room-temperature superconductivity.)

Another possibility is that the same group performed both the experiments behind the April 2015 and October 2019 reports, which means it’s possible the group could be interpreting the data to fit their theory instead of their theory to fit the data. This happens more often than you’d think, often with perfectly uncontroversial consequences and isn’t a matter of shame or disrepute as much as of normal human nature.

Although it’s notable in the Hungarians’ case that the same group reported discovering new particles in 2008 and 2012, neither of which held up to scrutiny and neither of which the group has managed to explain, according to a 2016 report in Quanta. Their new preprint paper also states, “In order to confirm the existence of the X17 particle we have conducted a search for its creation and decay” – as if the group is already sure it’s there. It could well have been a throwaway line but read against the historical context of the Atomki group’s various predictions, it seems problematic.

Maintenance work underway at the CMS detector, the largest of the five that straddle the LHC. Credit: CERN

A section of the Large Hadron Collider. Photo: CERN

But either you’re an optimistic or you’re a cynic, and one way or another, it’s exactly why reproducibility is a cornerstone of modern scientific practice. We need to wait for other scientists who aren’t associated with this group, or the Hungarian Academy of Sciences, to perform the tests the Atomki group performed in the same way, hopefully with better instruments, and check if X17 shows up again. If it exists, it should. This could take many months, if not years.

That’s why crying ‘Nobel Prize’ now is grossly premature. There’s another reason a Nobel Prize may not even be guaranteed. Finding a previously unknown particle is a momentous feat, to be sure; physicists have been looking for such particles with some of the world’s largest machines, with no luck. But simply stumbling upon a particle during an experiment is one thing – explaining what that particle is and why it behaves the way it does is quite another.

This question is more important than a Nobel Prize or any other prize. In the grand quest to discover the world (to use the D&D analogy), our adventurer has just glimpsed what seems to be a pinprick of light at the end of a long, dark corridor. She also thinks she can hear the sound of crashing waves on the other side. Since hers is a world of magic – and ours is a world of particle physics – it’d be a good idea for her to make sure it’s a legitimate way out and not some trickery before she charges at it with her greatsword. (“Any sufficiently advanced technology is indistinguishable from magic” – Arthur C. Clarke.)

In fact, yet another possible explanation is that X17 is actually a well-known particle or group of particles behaving in a way we hadn’t anticipated, and which doesn’t exactly tell us more than what we already knew about our universe. Physicists already know of such particles, collected under the much broader umbrella term of quasiparticles. For example, you probably think the proton is a particle and you’d be right – but it’s also a quasiparticle because it’s made up of a collection of smaller particles (quarks and gluons) that together behave like one larger, composite particle. In 2015, physicists reported that they’d discovered a particle called a Weyl fermion – which is actually a quasiparticle made up of a clump of electrons that behaves as if it has no mass.

But assuming X17 exists and that it isn’t a quasiparticle, what could it be? Many theoretical physicists around the world have been working on alternate theories that can explain, for example, what dark matter is and how dark matter particles could remain undetected for so long. When the Atomki group announced their first results in April 2015, they simply provided a new line of enquiry that theoreticians could pursue to hopefully uncover new behaviour, maybe even new particles, presumably hidden in the math. Some of these lines include arguments that X17 could be a pseudoscalar particle or types of particles called axions or Z bosons.

This new NASA/ESA Hubble Space Telescope image shows a beautiful spiral galaxy known as PGC 54493, located in the constellation of Serpens (The Serpent). This galaxy is part of a galaxy cluster that has been studied by astronomers exploring an intriguing phenomenon known as weak gravitational lensing. This effect, caused by the uneven distribution of matter (including dark matter) throughout the Universe, has been explored via surveys such as the Hubble Medium Deep Survey. Dark matter is one of the great mysteries in cosmology. Credit: gsfc/Flickr, CC BY 2.0

The spiral galaxy PGC 54493 is part of a galaxy cluster that astronomers have studied to explore weak gravitational lensing, a phenomenon caused by the uneven distribution of matter (including dark matter) throughout the universe. Dark matter is one of the great mysteries in cosmology. Photo: gsfc/Flickr, CC BY 2.0

Depending on which it is – and again assuming that X17 exists – the new particle could end up solving one or another longstanding open question. If X17 is an axion, for example, it could help understand dark matter or solve the strong CP problem. (It’s probably even more interesting than it might have been at this point that only on October 4 this year, other physicists reported they’d found signs of an axion in a semimetal.) There’s probably some selection bias at work here: either physicists are trying to explain a new particle as the solution for an older, longstanding (read: annoying) problem, instead of just as a new particle that won’t help them sleep better, or journalists are noticing only these attempts by physicists. Either way, it’s also a harmless exercise, so ??‍♂️.

However, the importance of the X17 results can be flipped on its head to present a problem with the finding. As the theoretical physicist Matt Strassler explained on his blog:

… X17 needs to have some unique and odd properties in order to be seen in these experiments, yet not be seen in certain other previous experiments, some of which were explicitly looking for something similar. To make equations that are consistent with these properties requires some complicated and not entirely plausible trickery. Is it impossible? No. But a number of the methods that scientists suggested were flawed, and the ones that remain are, to my eye, a bit contrived.

So even though it’s an extremely boring piece of advice and – depending on who’s talking – deflates all the excitement from a new, and rare, opportunity to speculate on what a new particle could mean, physicists agree it’s the best advice at the moment: wait. In this time, theoretical physicists will have to offer a coherent, self-consistent explanation for what the new particle could be and how it fits with existing theories of particle physics, and independent experimental physicists will have to find higher quality data that indicates the particle’s existence as well as, and with help from theoreticians, dis-indicates other explanations.

The only person harmed in this scenario is the adventurer in the cave, and (rolls die) she just walked back to her town.

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