麻豆传媒

FOR decades, physicists have been aware of a gnawing anomaly in the behaviour of a mysterious fundamental particle, the muon. Muons are the heavier cousins of the electrons that run though power lines and bring our devices to life. But when we study muons鈥� properties in granular detail, the results differ ever so slightly from predictions. Far from being a worry, this anomaly is cause for major excitement.

The so-called standard model 鈥� a list of the fundamental particles, their properties and associated forces 鈥� works incredibly well, as far as it goes. The trouble is that most physicists believe it paints an incomplete picture. There must be other particles and forces out there, but despite our best efforts, we haven鈥檛 been able to unmask them.

The muon anomaly could be a window to this hidden world. Its prediction-defying behaviour is thought to be a sign that it is interacting with some undiscovered particle. But because the measurements of the muon are so incredibly subtle, it has long been frustratingly unclear whether the anomaly is real 鈥� it could be a statistical fluke that will fade away on closer inspection.

Soon, we should find out. Using a ring of magnets the size of a house, the Muon g-2 experiment at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, is probing their properties like never before. , based at the University of Manchester, UK. He told 麻豆传媒 about the magic of muons, the big questions they could answer and what it is like to be propelled onto international TV news.

麻豆传媒: Why are we so sure that there must be some 鈥渘ew physics鈥� out there to be discovered?

Alex Keshavarzi: In physics, there are four main unknowns. There鈥檚 the dark energy problem and the dark matter problem, these two things which we can see by their effects but can鈥檛 identify. There鈥檚 the need to amalgamate gravity with quantum mechanics. Then there is the funny something that went on in the first 3 minutes after the beginning of the universe, which somehow created an imbalance between ordinary matter and antimatter. Everything we know about physics suggests that matter and antimatter particles are always created in equal proportions, so we expect this happened at the big bang. The problem is that every constituent of matter that we see around us 鈥� ourselves, the sun 鈥� everything is made almost completely of normal matter. In the time it takes to make a cup of tea, all the antimatter in the universe disappeared and we have no idea why.

So we think there are undiscovered particles that could explain these mysteries?

It seems that at least some of the explanation must involve new forces or particles. Dark matter, for instance, must be a particle that doesn鈥檛 interact with light. There are two ways to look for this stuff. At particle accelerators such as CERN鈥檚 Large Hadron Collider near Geneva, they smash particles together and see what falls out. It鈥檚 a bit like, if you wanted to know how a clock works, you could smash it apart and look at the bits that fall out on the table. But the CERN experiments haven鈥檛 detected anything unexpected yet. An alternative way is to listen very intently to the clock so you can hear the ticks and the gears turning. That second way of doing things is called a precision measurement experiment and that鈥檚 what we鈥檙e trying at .

Why are muons so interesting?

The best way to think of a muon is that it is very much like an electron, the particle that is present in all atoms. Muons have all the same properties as electrons 鈥� except they are around 200 times heavier. In the 1900s, we revolutionised technology by understanding the electron and how to manipulate it. The muon is exciting because it鈥檚 another particle we could learn to manipulate. That could be useful in terms of technology, again, or in my field, particle physics, as a way to understand how the universe is put together.

We don鈥檛 find muons just lying around though.

A person has something like 30 muons shooting through them at any given moment. So they are common, but perhaps not by the standards we鈥檙e used to. The thing is, the electron has an infinite lifetime. But muons live for only one-500,000th of a second. The reason they are so widely studied is that, thanks to their mass, they have more energy and so they should interact more strongly with other heavy particles that might be out there. They鈥檙e a useful tool because they might help us get a glimpse of other particles that we couldn鈥檛 see via electrons.

How do we get a muon to hang around long enough to study it?

One of the tricks is to speed it up, which gives it more energy. Special relativity kicks in and you get a time dilation effect, like you see at the edge of a black hole. At Muon g-2, we speed the particles up so they have a lot more energy, taking their lifetime from 2 microseconds to 60 microseconds.

What exactly are you studying at the Muon g-2 experiment?

The muon has this quantum property called spin, which you can think of as like its own internal bar magnet. If you put that in a magnetic field, it will precess 鈥� like the way a compass needle turns if you are at the north pole. At the same time, ordinary empty space can, according to the rules of quantum mechanics, have what we call virtual particles pop up quickly out of nothing and then disappear. It鈥檚 because empty space has a tiny amount of energy and this can be briefly converted into these virtual particles. It turns out that the rate at which the muons鈥� spin precesses is determined by these virtual particles. We can calculate to incredible precision what that number should be.

When did we first twig that there might be something funny going on with muons?

We have had hints since the early 1990s. The first Muon g-2 experiment at the Brookhaven National Laboratory finished in 2003 and showed that there was a disagreement between the predicted and observed numbers. In physics, we look for a result that has what we call a five-sigma significance, or a 1 in 3.5 million chance that the result could have been obtained by chance. At this stage, we were only at four sigma.

That鈥檚 where your new experiment comes in.

Yes, we鈥檙e measuring to a much higher precision, trying to figure out whether this can be heralded as the discovery of new physics or not. I am based in Manchester now, but worked at the experiment in the US for a few years. You do have a moment of awe and wonder when you see the thing. There鈥檚 this blue ring in the middle, which is where the muons are stored. That鈥檚 20 metres across and you can go and stand in the middle. What always impressed me the most when I stood there was how you have 20 or 30 different electrical and experimental systems that all have to work together, to the nanosecond, with exactly the right power and sensitivity.

You released an important result in April 2021. Would you tell us about it?

We actually knew about it a month earlier. In particle physics experiments, it鈥檚 really important to eliminate any element of conscious or unconscious bias. So we apply a thing called blinding, which means all the numbers we鈥檙e working with are offset by some factor that is known only by a handful of people. We do the analysis, but we can鈥檛 see whether the numbers are pointing in any direction as we do it.

At the end of March 2021, we had an unblinding ceremony where the people that had hidden the offsets bring them back and we get the result out. I have to admit, I was really nervous. The new physics is the more exciting thing and I was hoping for that. And then we found that the anomaly was still there, and we had increased the probability of it being real. There is now only a 1 in 40,000 chance of this being a fluke. It鈥檚 like doing 15 coin tosses in a row and getting all heads 鈥� possible but incredibly unlikely. What really struck me was the global interest it got. Not being an experimental lead, I didn鈥檛 get on the BBC. But I did get on late-night Turkish news and on the German news.

鈥淲e have to push for precision so there is no shadow of a doubt鈥�

Did they ask any good questions that I should pinch?

A lot of people were trying to steer me towards saying something about how this is the biggest revolution in science since Albert Einstein. I had to be really careful and say, well, no, we haven鈥檛 discovered anything yet.

When might we be at that point?

The result that we released in April was based on one year鈥檚 worth of data that we took in 2018. As of now, we are on our fifth year of data-taking. So we already have three to four more years and the instrumentation has been improved along the way. If the measured value stays the same and our measurement just gets more and more precise, then we should reach that five-sigma level with our experiment. We鈥檒l release another result at roughly the end of 2022 and we might get there at that point. We should have the final results of the experiment within five years and that will be the real decider. We have to push for precision so there is no shadow of a doubt about any claims of new physics.

If the result is real, what would it mean?

It would mean that there are some virtual particles out there that we don鈥檛 know about yet. I should add that our experiment is sensitive to virtual particles, but any particles that appear in this way would also exist independently, out there in reality. Our experiment won鈥檛 tell us what those new particles are. It could be perhaps a candidate dark matter particle, maybe mediated by a new force, or some new particle that could explain the asymmetry between matter and antimatter. Other experimentalists would then need to take our data and go and make more specific searches for the particles. But it would be the first time we could say, OK, we鈥檝e definitively discovered new physics.

The hunt for new physics
Alex Keshavarzi will speak at 麻豆传媒 Live next month