Science Made Simple: What Are Muons?

The Muon g-2 storage ring at Fermilab, where scientists conduct experiments to better understand the properties of the muon and probe the Standard Model of particle physics.
Credit: Reidar Hahn, Fermilab

The muon is one of the fundamental subatomic particles, the most basic building blocks of the universe as described in the Standard Model of particle physics. Muons are similar to electrons but weigh more than 207 times as much. That’s about the difference between an adult person and a small elephant. The muon is part of the lepton group. Leptons are a type of fundamental particle. This means they are not made of even smaller pieces of matter. Like other leptons, the muon is affected by only three of the four fundamental forces in the universe.

The muons that hit the Earth result from particles in the Earth’s atmosphere colliding with cosmic rays—high-energy protons and atomic nuclei that move through space at just below the speed of light. Muons exist for only 2.2 microseconds before they decay into an electron and two kinds of neutrinos. However, because they move at nearly the speed of light, muons travel far before decaying. Muons created in the atmosphere constantly hit every inch of the Earth’s surface and pass through almost any substance. They don’t stop until they penetrate far below the surface of the Earth—potentially more than a mile.

Fast Facts

  • Approximately one muon hits every square centimeter of the Earth every minute at sea level. This rate of natural background radiation increases at higher elevations.
  • Ultrasensitive detectors, including some neutrino and dark matterexperiments, are placed deep underground to minimize the effect of atmospheric muons.
  • Muons can help detect dangerous nuclear material and see into damaged nuclear power plants.
  • Scientists use muons for archeological purposes to peer inside large, dense objects such as the pyramids in Egypt.

DOE Office of Science: Contributions to the Standard Model of Particle Physics

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