Science Snapshots: What the Hell is a Muon G-2 Experiment?

It looks like something important happened in the world of physics. I say "looks like" because today, I'm going to attempt to explain something that I don't understand. Like at all. We're going to be parsing out this sentence from U of Chicago: "Physicists now have a brand-new measurement of a property of the muon called the anomalous magnetic moment that improves the precision of their previous result by a factor of 2."

Scroll down for the "bottom line" if you don't want a primer in particle physics.

Let's start at the beginning. When I wanted to figure out what the hell is a muon, I needed to figure out what the hell particle physics is all about.

Alright, so in my podunk public high school I actually had a halfway decent chemistry/physics teacher. He left a very well paying job at Whirlpool just to teach high school science in bumfuck nowhere because he felt like it. I learned in school that atoms are made up of a nucleus of protons and neutrons with a cloud of electrons buzzing around them. The number of protons determined the element. If 1, hydrogen. If 6, carbon. If 85, something called Astatine. Fun fact, apparently Astatine is the rarest naturally occurring element in the Earth's crust (hard ground surrounding the lava inside).

Conceptual model of an atom with a nucleus of protons and neutrons with a cloud of electrons orbiting the nucleus. This is called the Rutherford Model. Via GeeksforGeeks.

That's all I learned. We could spend weeks talking about the forces on a ladder leaning against the wall, but gods forbid we get into anything like what's happened in particle physics since the 1910s. I guess whether or not my ass falls off a ladder is more practical, because I do fall a lot.

So those protons and neutrons are NOT the end of matter. They are also made up of things. A proton, which has a positive charge, is made up of three valence quarks. There are up quarks and down quarks, and all matter is composed of these plus electrons. A neutron, which has a neutral charge, is made up of three valence quarks also: one up and two down. Quarks also come in other flavors: strange, charm, bottom, and top (ha). Confused yet? That's fine, so am I.

Adding quarks to the Rutherford Model of an atom. Via Quora.

This is what I can figure out. Basically, up and down quarks are the lightest ones, and an up has a positive charge two thirds the charge of an electron while a down has negative one third that of an electron. So this maths out like so:

2 ups = +4/3 charge and 1 down = -1/3 charge, which equates to +1 charge for a proton

1 up = +2/3 charge and 2 downs = -2/3 charge, which equates to 0 charge for a neutron

...and of course, an electron is charge of -1.

Strange and charm quarks are in the middle of the road in terms of mass. A charm quark has a +2/3 charge and a strange quark has a -1/3 charge. And the Peckerwood's favorite quarks, bottom and top, are the heaviest (you know it!) and are also charged +2/3 and -1/3 respectively. And I'm going to stop there with quarks, because they're outside the scope of what we're talking about with muons. The bottom line is: there are a lot of elementary particles, or particles that aren't made up of anything else, like electrons. And if you're like me and only learned about protons, neutrons, and electrons, buckle up. Just look at all the elementary particles that exist!

Elementary particles in the Standard Model. Source: Wikipedia

Don't worry, I'm confused too. That was your brief introduction to the fact that protons and neutrons are made up of other things, but electrons are just themselves with nothing under the surface. Now what the hell is a muon?

A muon is like an electron, with a -1 charge, but way more massive. Like, 200 times more massive. It also is an elementary particle, meaning unlike a proton, there is nothing else below it. It's just itself, like the electron. It's a basic building block of the universe. But it doesn't seem to help build atoms, which build all the other matter we see. They are a by-product of what happens when cosmic rays collide with molecules, and for us specifically that happens in our upper atmosphere. They're unstable, usually only lasting 2.2 microseconds (that's 2 millionths of a second). But they're moving the speed of light, so they bombard us and penetrate far into the Earth when they hit the surface, and they do that a lot. You're getting penetrated by muons as you read, wondering what the hell the Captain has gotten you into today.

Now that you know all about particle physics, let's get to the point of the Fermilab experiments.

The whole point of this is that muons appear to behave in a way that is contrary to the Standard Model of Particle Physics. Specifically, they are slightly more magnetic than our current model of tiny particles predicts. For scientists, this could mean that there are possibly other particles or forces in existence that we haven't discovered yet.

So the big news is that Fermilab has repeated the experiment looking at how magnetic the muon is. The confirming experiment lowers the uncertainty of the measurements. They plan to make their third, most precise measurement in 2025, after which theoretical physicists will need to come up with some changes to the Standard Model because the Fermilab team breaks it with these findings.

Bottom line: The Fermilab team has shown the Standard Model of Particle Physics doesn't explain their observations, which means it needs to be updated. This could lead to the discovery of new elementary particles. Breaking our current models of how things work is always an exciting turning point in science, because it means we're on the brink of new discoveries!

U of Chicago article here: https://news.uchicago.edu/story/muon-g-2-experiment-reinforces-surprise-result-setting-physics-showdown

Some more info about muons here: https://www.science.org/content/article/particle-mystery-deepens-physicists-confirm-muon-more-magnetic-predicted