Particle Physics Most Famous Anomaly (almost) Solved

Three years ago, we saw a lot of headlines  about a remarkable experiment at Fermilab in the United States. They confirmed an anomaly  of the muon, that’s a heavier version of the electron. Their measurement doesn’t fit together  with the prediction from the current theories. This discrepancy could be evidence that  the theory is wrong and that there are new particles or new forces of nature. Or  it could mean that the theory is right, but something is wrong with the calculation. So  which is it

? Theory? Or the Calculation? Well, a new paper now says that it’s  neither. Let’s have a look. The muon is one of the fundamental particles  in the standard model of partical physics. It’s unstable and decays within a few microseconds.  It has a magnetic moment that tells you how fast the particle spins in a magnetic field. Without  quantum effects, the magnetic moment of the muon, usually denoted "g" should be 2. Physicists  are interested in the quantum contribution, that’s often just called

the “g minus two”.  The g-2 of the muon can be predicted from the standard model. Just to give you an  idea, the prediction is this value whereas the experimental value is this and that’s  why people are afraid of particle physics. How does one measure this? These experiments are  basically rings of magnets in which the muons go in circles until they decay, a process that  you can also observe in Twitter discussions. But unlike Twitter threads which just  decay into obscurity, muons decay into o

ther particles of which one can measure the  momentum and from that one can infer the g-2. Already in 2001, an experiment at  Brookhaven found that the measured value of the muon g-2 deviates from  that predicted by the standard model, though it wasn’t a huge deviation. In 2021, a  follow-up experiment at Fermilab confirmed the earlier result, raising the statistical  significance to four point two sigma. Last year in August then, the Fermilab group  released more data and confirmed the anomaly

they’d previously measured. The additional data  decreased the uncertainty of the measurement, and increased the significance to 5 sigma, that’s less than a one in 3 million  chance of it being a fluctuation. That the statistical significance of the  result is now so high it means that the discrepancy between the prediction and  the observation is very unlikely to be due to a measurement error. It could mean that  something is missing in the standard model, maybe new particles or other new physi

cs. Since  the standard model contains three forces and then there’s gravity, which makes four, in  the media they often call the possibility of new physics a fifth force. I explained  in an earlier episode what that means. I’ve always suspected that the discrepancy  isn’t due to a new force, but a problem with the calculation. This is because these calculations  are really hard. It’s not just because it’s a lot of mathematics. It’s also because in parts  we don’t have the mathematics. So physic

ists just do the maths approximately on a computer  with what’s called a lattice calculation. This opens another can of worms because how do you  know that these approximations are even good. And in case you thought  that’s complicated already, there’s another complication because all these  calculations require further input that needs to come from other experiments. So first you  need to extract some numbers from other data and then that goes into your calculation  of which you then do an appr

oximation. And that last bit, the input from other  experiments seems to be the problem. The authors of the new paper have re-calculated  all these contributions to the muon g-2 and they say there is one in particular that  doesn’t fit with the data, and that’s the process in which a pair of pions are involved.  The pions are themselves pairs of quarks and the data for their properties comes from other  measurements and that seems to bring in an error. To me, this paper is like Sherlock Holmes w

ith  maths. It’s amazing detective work. But that’s not the end of the story. Now they’ll have to get  better data for this particular process and then redo the calculation to see if it fits. If it  does, there’s the question whether this wrong number was used for other calculations,  which might bring up other problems. So this correction might initiate a shift of other  results, which is going to be very interesting. Did you know there's a free and easy way to  learn more about the science beh

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