25 Days of Particles: Day 12

Muon

Classification: fermion, lepton
Fundamental: yes
Family: second
Mass: 105.7 MeV
Interactions: Electromagnetic (charge -1), Strong, Weak, Gravity
Spin: 1/2
Lifetime: 2.2e-6 s

To meet and appreciate today's guest, we need to jump back in time a bit. In 1936, Carl D. Anderson and Seth Neddermeyer were studying cosmic radiation and observed negatively charged particles that had more mass than electrons but less than protons. They dubbed these particles mesotrons, using the Greek prefix 'meso' meaning 'mid.' But this was at the beginning of the discovery of the particle zoo, and physicists soon found many other mid-range mass particles that also got called mesons. The mesotron got named the mu meson to tell all the mesons apart.

But the mu mesons decayed differently than the other mesons, producing two neutrinos instead of just one, and all the other mesons turned out to be made of quarks. Particles containing two quarks were officially named mesons, and since the mu particles were not made of quarks, they were officially named muons.

Muons are the second generation's electron-type member; they have spin 1/2, charge -1, but are 200 times heavier. Since muons are electrically charged, they interact readily with other matter, but since they are fairly heavy, muons don't stop easily. In particle physics, we joke that the best identifier for muons is a meter of steel--if any particles make it through the steel, they must be muons. Muons also have a fairly long life time compared to other particles; only neutrons last longer. Particles of all types are produced when particles from space hit Earth's upper atmosphere, but most of those decay before the reach the surface. Muons, however, do make it the surface of the earth, and often through it and out the other side. This is why sensitive particle detectors are often buried underground, where all the rocks and dirt overhead can shield the experiment from muons from outer space. This also means it may be possible to use muons in accelerators one day; up until now, only the stable particles like protons and electrons and their anti-particles have been used.

The ability of cosmic ray muons to reach Earth was actually was a very interesting phenomenon when it was first discovered. The lifetime of the muons was measured, as was the average speed at which they travel. Most muons don't travel fast enough to make it to Earth before they should decay, but they show up on the surface in huge volumes anyway. This was explained by Einstein's special relativity. Muons travel fast enough that time is dilated and length is contracted for them. So to them, the upper atmosphere isn't that far from Earth surface and it doesn't take them that long to get here. The measurement of the number of cosmic ray muons was important experimental evidence in proving Einstein's theory of special relativity.

So, to sum up, muons interact easily with other matter but not as much as electrons, so they are easy to detect, and they live long enough you can detect them directly instead of needing to hunt for their decay products, and they helped demonstrate the effects of special relativity. This is why muons are awesome, and why I choose to name this blog after them. With muons I mark halfway to Christmas. Happy Holidays, everyone!