Muons are fundamental particles in our universe that are essentially exactly the same as electrons,
but heavier - 206.77 times heavier, to be precise.
Muons are super fascinating for a number of famous reasons, including (and this is definitely
an incomplete list) as an experimental test of time dilation, as a catalyst making fusion
possible at room temperature, and that they can be used to make muonic versions of atoms,
like muonic hydrogen (with a proton and a muon, instead of a proton and electron).
But muons decay into an electron and some neutrinos after just a few microseconds on
average, so they’re hard to store.
So to do experiments on muons, you need to have a source of muons on hand.
A small number of muons are produced naturally by cosmic rays striking our atmosphere, but
one of the best ways of consistently and reliably making a lot of muons is with a particle accelerator.
First, you take hydrogen and you strip off the electrons, leaving you with protons (protons,
as a reminder, are not fundamental particles - they consist of two up quarks and a down
quark, stuck together by mutual attraction due to the strong nuclear force).
Then you accelerate the protons really fast in an electric field (or a bunch of electric
fields) and crash them into some other atomic nuclei, like lithium or carbon or whatever.
The energy of the collision creates a whole mess of particles that goes flying out, but
most importantly for our purposes, it creates a particle called a pion - either a pion with
positive electric charge made from an up quark and an anti-down quark, or pion with negative
electric charge made of a down and anti-up quark.
If the pions keep traveling in a vacuum so that they don’t bump into anything else,
they’ll spontaneously decay after about 26 nanoseconds, most frequently turning into
a muon and a neutrino.
Which you can use for fusion, or testing special relativity, or making cool cloud chamber designs.
And that’s how you make muons - accelerate protons with enough energy that when they
crash into other nuclei, the energy of the collision creates pions, which then decay
into muons.
Which, of course, after 2.2 microseconds on average, themselves spontaneously decay.
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