The Journal Gazette
Tuesday, May 04, 2021 1:00 am

The pace of process

Scientific riddle being slowly unraveled

Christer Watson

Sometimes I am frustrated by how slow the scientific process can be.

Example: Scientists measuring the behavior of particles have been testing various parts of the standard theory of how particles behave. It is commonly believed that this model must be wrong in some way. However, the model's predictions have done an extraordinary job in matching basically all experiments for the past several decades.

Finally, however, we may have an experiment that breaks that mold. Results of the experiment, centered near Chicago, were announced early last month. The project is called g-2 (gee minus two).

Before I explain the experiment, understand that some part of this story is wrong. That is what makes it exciting, not knowing where it is wrong. So, the story goes like this:

There is a somewhat obscure particle called the muon. This particle is similar to the electron present in atoms, except the muon is heavier and, partly as a result, decays into other particles in a few millionths of a second. Before it decays, however, it acts like a small bar magnet.

If you surround the muon with other magnets, the muon will tend to twirl around. This twirling will be a little faster or slower, depending on how strong the muon's “bar magnet” is.

The experiment involves putting a muon into this situation, then measuring how frequently the muon twirls and calculating the muon's magnetic strength. Of course, each step has to be executed really, really carefully. For example, the magnets that make the muon twirl have to create a magnetic field that is very, very smooth and regular. Any changes in the magnetic field need to be fewer than 25 parts in a million.

As the scientists built and adjusted the magnets, they added paper-thin sheets of steel to adjust the magnetic field this way and that. Think of it as carefully sanding a piece of wood, except the wood is invisible.

The simplest prediction for how strong the muon's magnet should be, using the somewhat weird units of physicists, is 2. The best prediction, however, requires accounting for a slight increase in the muon's magnet strength. This increase is caused by other particles quickly appearing and disappearing around the muon. Particles do this all the time, in generally predictable ways, so the effect on the muon can be calculated.

The better prediction is that the muon should have a magnetic strength of about 2.002.

Last month, then, scientists announced their best measurement for the muon's magnetic strength was just a little bit higher than this better prediction. The amount higher was only a sliver, 0.0000000001.

Most importantly, the uncertainty in the measurement is quite small. The scientists estimate the chance of getting this result purely from bad luck is about 0.00001%.

It is a standard practice when looking at something brand new, however, to not really fully believe it until the percentage gets a little better than this. That is partly because nature can be quite mean and trick us in subtle ways. Every scientist has seen things with a “percentage” of around 1% become true, so being skeptical and careful pays off.

One way this new experiment was careful was by performing the experiment double blind. Every scientist on the project knew the muon's predicted frequency and how much higher their measured frequency was. They wanted to avoid, subconsciously, making adjustments to things such ads the magnetic field (the invisible wood-sanding) that would keep their measurement at the higher frequency. They wanted to concentrate only on making the magnetic field smooth.

To double-blind the scientists, then, the frequency measurement was hidden by using a secret number that only two scientists, not otherwise involved in the project, would know. Every few months, the outside scientists would reveal the hidden number, the project would calculate the muon's frequency, then the whole process was reset and repeated. This hiding would ensure any adjustments were made without knowing what they were aiming for.

So the question now is: Where is the story wrong? A common hope is that the better prediction is wrong because there are some extra, previously undiscovered, particles interfering with the muon and making its magnetic strength stronger. If true, these would be first new particles discovered in decades.

That is most exciting possible answer. Eventually, we'll find out whether it is the correct answer.

Undoubtedly, though, the process will be slower than I would like.


Christer Watson, of Fort Wayne, is a visiting assistant professor of physics at Purdue University Fort Wayne. Opinions expressed are his own. He wrote this for The Journal Gazette, where his columns normally appear the first and third Tuesday of each month.

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