Muons behaving badly

Some new results hint at some undiscovered physics
20 April 2021

Interview with 

Ben Allanach, University of Cambridge


A view down one of the tunnels of the Large Hadron Collider.


Scientists have recently announced two independent discoveries that suggest we've got a gap in our understanding of the entities and forces that govern how the Universe operates. Both discoveries hinge on particles called muons; these are like short-lived electrons but about 200 times heavier. Scientists in the US got excited when they sent them circling around a ring guided by magnetic fields and they wobbled in a way no one had predicted. In a different series of experiments, this time at the CERN Large Hadron Collider, muons were turning up in their experiments less often than theory tells us they should. Both lines of evidence point to there being something missing in our understanding of how this branch of physics works. If it turns out to be true, it's potentially a massive step forward. To put it into context, Chris Smith spoke to Cambridge University theoretical physicist Ben Allanach, who speculates that there might be a new, undiscovered force involved…

Ben - In America, they were particularly looking at muons because you can make a very precise measurement of the wobble of their spins as they go around a storage ring. And you can also predict very precisely using the theory what that wobble should be. So by comparing the two numbers, you can see whether you've got the theory correct. And of course the American experiment is saying that perhaps we haven't got the theory correct.

Chris - So they know what they expect if our understanding of what the universe is made of, and the rules it follows, is correct, then these muons should behave in a certain way when they spin them in a ring, but they don't.

Ben - Absolutely. And they're affected by quantum fluctuations of other particles and forces through additional particles and forces that you don't know about. They will affect the spin in a way that you're not taking into account in the theory and so the two numbers won't match.

Chris - How out were the measurements in the States compared with what you would expect if the theory were correct?

Ben - Statistically they're out by what's called 4.2 Sigma. That means it's not a fluke, basically. These are very accurate measurements to parts per million. So, you know, that's something like measuring the length of your car to the width of a human hair or something - it's incredibly accurate. So in this very fine tuned world it's quite a big difference.

Chris - And contrasting that with what they've been doing at CERN - they've been making measurements also on muons but coming at this from a slightly different angle, but they've also got some interesting findings this week. How do they align with, or in the same way, not align with theories as to what we should be seeing with the behaviour of muons?

Ben - So in CERN they've been producing other kinds of particles and watching them decay into muons and electrons, and they should decay with the same rate into muons as electrons, but it seems like they're only going 85% of the time into the muons. So again, there's a problem when you have muons involved.

Chris - In other words, it looks like there is a gap in the jigsaw puzzle that is how we model what the universe is made of and the rules it follows, and it seems to hinge on the behaviour of muons. Have we got any insights from these experiments as to what is the missing element?

Ben - Currently, it's really up to people like me, the theorists, to come up with lots of ideas and then see if they make any sense. And there are lots of ideas I have to say. One of them says that the American experiment is due to a new supersymmetric particle, which is also the dark matter in the universe. So people are connecting this to other problems in physics. I, personally, I'm working on a new force and the LHC results would be due to sticking it together more when it wants to go to muons and changing the result. And this can also have appeared in the quantum fluctuation in the American experiment because this force would be coupling to muons and change the wobble of the spin that's measured there. But I have to say, you know, those are just two examples in many, and the real answer is we don't fully know. I mean, of course you need lots more experimental data to give a signpost to know really what's going on.

Chris - And if it turns out that you are right and there is an extra force - we've been comfortable with the existence of four forces so far -  electromagnetic force, which Michael Faraday was very familiar with, there's gravity, which of course Isaac Newton was very familiar with, and then more recently we've had the stronger and the weaker nuclear forces. This would add a whole new gamut to the game wouldn't it? I mean, if there is a fifth force, that rewrites physics textbooks.

Ben - You'd have to rewrite them all, yes! But what you'd want to see to be sure of this idea would be to actually produce the particle that carries this force. All forces we know of are supposed to be carried by particles. So, for example, the electric force is carried by photons, particles of light. So you'd want to actually produce and watch this force-carrying particle in order to be sure of it.

Chris - And just finally, Ben, what does this actually mean for the average person in the street though?

Ben - I think if you're interested in the universe and the way it is as we see it, I'm hoping that this is going to inspire us all to learn more about the place in which we live.


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