Muon

The muon is an elementary particle similar in many properties to the electron. Like the electron, it has a negative elementary charge and a spin of ^1⁄2. The muon and electron are subject to the electroweak interaction, but not the strong interaction. However, the muon has a mass about 200 times greater. Furthermore, unlike the electron, it decays spontaneously with an average lifetime of only about 2.2 microseconds. The formula symbol of the muon is μ . The antiparticle of the muon is the positive muon or antimuon μ . Like the positron, it is simply positively charged.

Muons were discovered in 1936 by Carl D. Anderson and Seth Neddermeyer while studying cosmic rays, and were independently detected in 1937 by J. Curry Street and E. C. Stevenson (both groups published in the same Physical Review issue in 1937). Since a center-of-mass energy of about 106 MeV is required for their production, they are not produced by radioactive decay or nuclear weapon explosions. Particle accelerators are needed for artificial production.

As leptons, the electron and muon are related particles in the Standard Model. The electron is counted to the first and the muon to the second of the three lepton families. The corresponding particle of the third family is the τ-lepton discovered in 1975.

In former times the muon was called mu-meson. At that time, "meson" (Greek, approximately the middle) - even earlier also "mesotron" - meant "medium-heavy" particle, namely with a mass between electron and proton. In the 1960s, however, the designation meson was restricted to particles with strong interactions, to which the muon as a lepton does not belong.

Cosmic rays

Muons are a major component of secondary cosmic rays. This is produced by reactions of the actual cosmic rays (mainly protons coming from space) with atomic nuclei of the upper atmosphere. Most muons are produced in the outer atmosphere: at an altitude of about 10 km, 90 percent of all muons produced in the entire atmosphere have already been produced. The reactions of the primary radiation initially produce pions and, to a lesser extent, kaons; their decay through the weak interaction produces, among other things, muons and muon neutrinos. At sea level, the particle flux density of these "cosmic" muons is around 100 per square meter per second, and the measured ratio μ is about 1.27.

At the Auger Observatory in 2016, there was growing evidence of a muon excess in cosmic rays that cannot be explained by standard models of high-energy physics, pointing either to new physics (at primary energies of cosmic rays of ¹⁰¹⁹ eV in the upper atmosphere, this corresponds to centre-of-mass energies of collision with air molecules of 110 to 170 TeV, ten times the value achievable at the LHC) or to gaps in our understanding of hadronic collision processes.

Detection of muons

With their usually high kinetic energy, muons generate long ionization tracks in matter through many successive collisions, which can be used for detection. Since they usually travel at nearly the speed of light, they generate Cherenkov radiation in water, for example.

Scintillators and semiconductor detectors are also sensitive to muons. The muons from secondary cosmic rays, for example, often account for the main part of the null effect in gamma spectrometers, because they can penetrate several meters of lead due to their high energy and can therefore hardly be shielded in the laboratory.

In particle physics experiments, muons are distinguished from other particles by various techniques:

• By measuring longer tracks, the point of origin and the direction of motion of the muons can be determined.
• By measuring tracks in magnetic fields, the ratio of charge to momentum can be determined. Together with a velocity measurement, the mass of the particle can be concluded.
• The high penetration capacity for matter can also be used for identification.