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.
- 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 matter experiments, 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
The U.S. Department of Energy (DOE) has a long history of supporting research into fundamental particles. Five of the six types of quarks, one type of lepton, and all three neutrinos were discovered at what are now DOE national laboratories. Researchers supported by the DOE Office of Science, often in collaboration with scientists from around the world, have contributed to Nobel Prize-winning discoveries and measurements that refined the Standard Model.
Much of DOE’s research into muons has been through different versions of the Muon g-2 experiment, which is currently at DOE’s Fermi National Accelerator Laboratory. This research began in the 1950s at CERN in Europe, continued at DOE’s Brookhaven National Laboratory from 1997-2001, and then moved to Fermilab in 2013. This and other experiments make precision tests of the Standard Model and further improve measurements of particle properties and their interactions. Theorists work with experimental scientists to develop new avenues to explore the Standard Model. This research may also provide insight into what unknown particles and forces might explain dark matter and dark energy as well as explain what happened to antimatter after the Big Bang.