- 1. Overview
- 2. Etymology
- 3. Cultural Impact
Muon
The muon is an elementary particle similar to the electron , with an electric charge of −1 e and a spin of 1/2, but with a much greater mass . It is classified as a lepton . As is the case with other leptons, the muon is not believed to be composed of any simpler particles; that is, it is not thought to have any substructure . The muon is an unstable subatomic particle with a mean lifetime of 2.2 microseconds , much longer than many other subatomic particles. As with the decay of the neutron (outside the atomic nucleus ), the decay of the muon involves the weak interaction and is slow , because the decay force is proportional to the mass difference squared between the muon and the electron .
Muons are denoted by the symbol μ⁻ and their antiparticles are denoted as μ⁺ (see antimuon ). Muons were previously called mu mesons, but are not classified as mesons by modern particle physicists (see History of subatomic physics ), and that name is no longer used by the physics community.
Because of their relatively greater mass, muons are not as sharply accelerated when they encounter electromagnetic fields, and do not emit as much bremsstrahlung (deceleration radiation). This allows muons of a given energy to penetrate far more deeply into matter than electrons, since the deceleration of electrons and muons is primarily due to their interaction with the electric fields of atoms in matter. As muons are leptons, they do not undergo strong interaction with nuclei , and so do not participate in the nuclear reaction of ordinary matter , as protons and neutrons do.
Because muons have a very large mass and energy compared with the decay energy of radioactivity , they are never produced by radioactive decay of atoms . They are, however, produced in great quantities in high-energy interactions in normal matter, such as during the cosmic ray interactions with particles of the Earth’s atmosphere. These interactions usually first produce pions , which then most often decay to muons.
As with many other elementary particles , muons have a corresponding antiparticle of opposite charge (+1 e) but equal mass and spin: the antimuon (also called a positive muon). Muons are denoted by μ⁻ and antimuons by μ⁺ . Formerly, muons were referred to as mu-mesons, although they are not classified as mesons in modern particle physics.
Muons have a mass of 105.66 MeV/c² , which is about 207 times the mass of the electron. Since the muon’s interactions are very similar to those of the electron, a muon can be thought of as a much heavier version of the electron. Due to their greater mass, muons are not as sharply accelerated when they encounter electromagnetic fields, and do not emit as much bremsstrahlung radiation. This allows muons of a given energy to penetrate far more deeply into matter than electrons, since electrons and muons lose energy primarily by way of their interaction with the electric fields of atoms in matter.
Because muons have a very large mass and energy compared with the decay energy of radioactivity, they are never produced by radioactive decay of atoms. They are, however, produced in great quantities in high-energy interactions in normal matter, such as during the cosmic ray interactions with particles of the Earth’s atmosphere. These interactions usually first produce pions, which then most often decay to muons.
Muons are unstable elementary particles and are heavier than electrons and neutrinos but lighter than all other matter particles. They decay via the weak interaction into an electron, an electron-antineutrino, and a muon-neutrino. Due to the conservation of lepton family numbers , this decay is not allowed for the muon neutrino, which must instead decay into an electron, an electron-antineutrino, and a muon-neutrino.
Muons were discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936, while studying cosmic radiation. They noticed particles that curved differently from electrons and other known particles when passed through a magnetic field . These new particles carried a negative electric charge but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron but smaller than that of a proton. Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for “mid-”. Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. To differentiate between the different types of mesons, the mesotron was in 1947 renamed the mu meson (the Greek letter μ corresponds to m).
The existence of the muon was confirmed in 1937 by J. C. Street and E. C. Stevenson of Harvard University , using cloud chamber photographs of cosmic rays. The results showed that the particles could penetrate at least 1 cm of lead, distinguishing them from known particles.
The muon was the first elementary particle discovered that does not appear in ordinary atoms. The muon’s discovery was unexpected, and it was not clear at the time why such a particle should exist. Isidor Isaac Rabi , who was not involved in the discovery, famously quipped, “Who ordered that?” This question would not begin to be answered until the 1970s, with the development of the Standard Model of particle physics, which successfully explained the muon as a second-generation lepton.
In the Standard Model, the muon is a second-generation lepton, along with the muon neutrino. The muon is identical to the electron in all respects except for its mass, which is about 207 times greater. The muon’s greater mass is due to its interaction with the Higgs field , which gives all elementary particles their mass. The muon’s interaction with the Higgs field is stronger than the electron’s, resulting in its greater mass.
The muon’s greater mass also means that it is less stable than the electron. The muon decays via the weak interaction into an electron, an electron-antineutrino, and a muon-neutrino. The muon’s lifetime is about 2.2 microseconds, which is much longer than the lifetimes of most other subatomic particles. This is due to the fact that the muon’s decay is a weak interaction process, which is much slower than the strong or electromagnetic interactions.
The muon’s greater mass and longer lifetime make it useful in a variety of applications. Muons are used in muon tomography , a technique that uses the penetration of muons through matter to create images of the interior of objects. Muons are also used in muon spin resonance , a technique that uses the spin of muons to study the properties of materials.
The muon’s discovery was a major milestone in the history of particle physics. It was the first particle discovered that did not fit into the existing framework of the atom, and it helped to pave the way for the development of the Standard Model. The muon’s properties continue to be studied today, and it remains an important tool in the study of the fundamental forces and particles of nature.
History
The muon was discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936, while studying cosmic radiation. They noticed particles that curved differently from electrons and other known particles when passed through a magnetic field. These new particles carried a negative electric charge but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron but smaller than that of a proton. Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for “mid-”. Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. To differentiate between the different types of mesons, the mesotron was in 1947 renamed the mu meson (the Greek letter μ corresponds to m).
The existence of the muon was confirmed in 1937 by J. C. Street and E. C. Stevenson of Harvard University, using cloud chamber photographs of cosmic rays. The results showed that the particles could penetrate at least 1 cm of lead, distinguishing them from known particles.
The muon was the first elementary particle discovered that does not appear in ordinary atoms. The muon’s discovery was unexpected, and it was not clear at the time why such a particle should exist. Isidor Isaac Rabi, who was not involved in the discovery, famously quipped, “Who ordered that?” This question would not begin to be answered until the 1970s, with the development of the Standard Model of particle physics, which successfully explained the muon as a second-generation lepton.
Properties
The muon is an elementary particle with a negative electric charge and a spin of 1/2. It is classified as a lepton, and is not believed to be composed of any simpler particles. The muon has a mass of 105.66 MeV/c², which is about 207 times the mass of the electron. The muon’s interactions are very similar to those of the electron, and it can be thought of as a much heavier version of the electron.
The muon is an unstable particle with a mean lifetime of 2.2 microseconds. It decays via the weak interaction into an electron, an electron-antineutrino, and a muon-neutrino. The muon’s decay is a slow process because the decay force is proportional to the mass difference squared between the muon and the electron.
The muon has a corresponding antiparticle of opposite charge (+1 e) but equal mass and spin: the antimuon (also called a positive muon). Muons are denoted by μ⁻ and antimuons by μ⁺.
Applications
Muons are used in a variety of applications, including muon tomography and muon spin resonance. Muon tomography is a technique that uses the penetration of muons through matter to create images of the interior of objects. Muons are also used in muon spin resonance, a technique that uses the spin of muons to study the properties of materials.
Muons are also used in the study of cosmic rays. Cosmic rays are high-energy particles that originate from outside the Earth’s atmosphere. When cosmic rays interact with the Earth’s atmosphere, they produce a variety of secondary particles, including muons. The study of these muons can provide information about the properties of cosmic rays and the Earth’s atmosphere.