Learn About Muons

Level 1 - Muons (pronounced "myoo-ons") are elementary particles, much like electrons and quarks. Like an electron, a muon has a negative charge, but is much more massive: 207 times heavier. These particles can be kind of hard to observe, since they only exist for around 2.2 millionths of a second; that’s much faster than you can blink! Since muons are being produced all the time in the upper atmosphere, and since they travel at almost the speed of light, they still can be detected using scientific equipment, even given their brief lifespan.

Level 2 - Muons belong to a family of particles called leptons. Leptons have intrinsic properties such as charge, spin, and mass, and the muon is no exception. Out of the four fundamental forces in physics, muons are subject to three: gravity, electro-magnetism, and the lesser known weak force.

Level 3 - Muons belong to a classification of subatomic particles called Leptons. They have a charge of -1 e and a spin of ½. They have an average lifetime of roughly 2.2 μs. Most muons decay via the weak interaction into an electron and two neutrinos.

Level 1 - A muon is formed when a cosmic ray from space comes into contact with Earth’s upper atmosphere, generating a new particle: the muon. The muon then continues on almost the same path the ray was taking as it travelled to Earth, with the muon passing through all sorts of normal matter in the process, including you!

Level 2 - Cosmic protons are produced from the sun and from distant galaxies. When one is generated, it then travels through space until it collides with the Earth’s atmosphere. As the proton hits an atom in the atmosphere, another particle called a pion is generated. This pion decays into a muon, and this muon continues on its path towards the Earth’s surface, travelling in nearly the same direction as the initial cosmic proton.

Level 3 - A cosmic ray proton comes into contact with an atomic nucleus in the upper atmosphere forming a pion. This pion then decays quickly into a muon and a muon neutrino and follows the same path the proton was taking prior to the decay. This muon then has to travel to Earth to be detected. If not for the relativistic speed of this muon we would not detect it due to the half life time of approximately 2.2 μs. Due to special relativity however we detect this muon since the distance and time it travels is different in its reference frame compared to that of ours!

Level 1 - Muons and other particles are moving through you every second of the day. Though that might sound scary, it’s actually quite harmless. When you hold out your hand with your palm facing upwards, about 2 or 3 muons pass through your hand (or anything of that size) every second. That’s pretty cool to think about, right?

Level 2 - Since muons are able to penetrate hundreds of feet of solid material, they are quite useful in imaging large dense objects that are visually hidden. For instance, since a wall of rock absorbs more muons than empty air, this means that archeologists can use muon detectors to determine if there are hidden chambers in pyramids. In a similar way, volcanologists can use muon detectors to see lava pockets and obstructions in volcanos, and can use this technology to help predict eruptions.

Level 3 - The muon provides evidence supporting Einstein's Theory of Special Relativity based on the distance it travels compared to its half life. If we calculate the distance we expect the muon to travel from where it is produced in the upper atmosphere, we predict that it will decay before reaching the Earth's surface. However, we detect muons in abundance at the Earth's surface. This apparent contradiction can be explained by invoking relativity. The fast-moving muon must experience time differently than an observer on Earth. The distance it travels due to this "time dilation" is sufficient for it to reach the Earth's surface. The data from cosmic ray muons agrees with Einstein's theory.

Here are some resources that may help you learn more about cosmic ray muons:

https://home.cern/science/physics/cosmic-rays-particles-outer-space
https://radioactivity.eu.com/site/pages/muons.htm
https://www.youtube.com/watch?v=aDfB3gnxRhc
https://www.symmetrymagazine.org/article/the-stories-a-muon-could-tell

Interesting applications of muons

Muon Decay and Special Relativity https://www.youtube.com/watch?v=rVzDP8SMhPo
Muon-catalyzed Nuclear Fusion: https://www.youtube.com/watch?v=aDfB3gnxRhc
Pyramid of Khufu (PBS article, 2017): https://www.pbs.org/wgbh/nova/article/cosmic-ray-muons-reveal-hidden-void-in-the-great-pyramid/
How a Muon Accelerator Could Unravel Some of the Universe's Greatest Mysteries (The Conversation, 2020): https://theconversation.com/how-a-muon-accelerator-could-unravel-some-of-the-universes-greatest-mysteries-131415
Muon Tomography (Cern): https://cms.cern/content/muon-tomography