Fifth fundamental force

10 December 2019

Interview with 

Ben Allanach, University of Cambridge

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There's some news going around recently about the discovery of a supposed “fifth fundamental force” in physics. This comes from the Atomki Institute in Hungary, where a team of scientists claim to have found evidence for a new fundamental particle that they’re calling X17. They first detected this as a blip on their instruments back in 2015, and now they claim to have repeated it. If it’s true it completely disrupts the current models of physics, which don’t have much room for a brand new particle. But lots of people are skeptical, as no independent team has yet replicated the finding. To get an independent perspective, Phil Sansom went to visit physicist Ben Allanach…

Ben - The scientists are claiming to have discovered a new particle which hasn't been found before.

Phil - That seems like quite a big deal.

Ben - That would be a huge deal if it were true.

Phil - How do they claim to have discovered this?

Ben - Okay, so they are doing nuclear collisions, you know, the hard dense centres of atoms, and they're bombarding them with protons and making heavier nuclei in excited states. These nuclear excitations can lose energy by giving out particles of light. And if the energy is high enough, the particle of light will then turn into an electron-positron pair. So by analysing the angular separation, they can tell something about what's going on.

Phil - How are they actually detecting this stuff? Because you can't put on a pair of zooming glasses and go, oh, there goes an electron.

Ben - No, that's right. They've got fancy kit to measure the electron going through it. There are basically detectors arranged around the point of collision. Positrons and electrons go through the material and leave charge deposits, and so then you read out that with electronics and you can tell where it's gone and so on. If you've got the theory right, you should be able to tell how often they come out with a particularly wide angle. And they're finding that rather large angles, 120 degrees, there are far too many of them coming out and that's consistent, they say, with the production of a new particle which is decaying into the electron and positron,

Phil - What's the name of this new particle and what does it do?

Ben - Well they've called it X17, mysteriously. "X" because they don't really know what it is, and 17 because it weighs 17 million electron volts worth of energy. What does it do? We don't know.

Phil - Why are people relating it to a, quote unquote, fifth force? What's that all about?

Ben - Actually particles and forces are kind of different sides of the same coin. When you go deep into the quantum theory, you find that forces are given by the exchange of many quantum particles. Best example probably is the electromagnetic force. You're aware of magnets repelling each other, if you put the North pole to the North pole, but really what's happening is the magnets are exchanging particles. The particle, the photon, is producing the force. So many, many exchanges, many times a second. In the same way this X17 is a particle, but exchanging it a lot would produce a force.

Phil - What would that force do?

Ben - It would give you a force between electrons and nuclei. That's all we know. It could do other things as well, but seeing as it's only been observed in interactions with nuclei and with electrons and positrons, that's all we know about its supposed effects.

Phil - What do you think? Is this the real deal or is this a little bit overhyped?

Ben - I'm sceptical for three reasons. Firstly, why haven't other experiments seen this? This isn't a particularly heavy particle. It should have been observed in other collider experiments. The other thing that makes me feel a bit nervous is that the same authors, or at least a large subset of them, have made three other discoveries of supposedly new forces in the past 20 years or so, and they're all incompatible with each other. Thirdly, there's an issue with the calibration data. So I looked at the scientific paper that they wrote today and the calibration of the angular separations is not as good quality as you'd want. So it's not a killer or anything, but it just makes you a bit suspicious that maybe there's something a bit off in the calibration that's affecting their results.

Phil - It sounds like this is almost a boy who cried wolf type situation, potentially.

Ben - Well, you never know. I mean you know, at some point they could be right. That's why good science is to check these claims with a different experimental setup, and you know different number of people with a different methodology, and then you'll know for sure, hopefully.

Phil - Well if it's not a new particle, then what is it exactly?

Ben - Well, it could be some problem with the experiment. I mean, these are difficult experiments to interpret sometimes, and you need to calibrate very precisely. Tiny uncertainties and errors in the wrong direction can lead you to the wrong conclusion sometimes. So it could be something like that if they are unlucky.

Phil - And if it is a new particle and you had first shot on naming it, what would you call it?

Ben - The heaven 17.

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