Muons

An heavy electron abundant in cosmic rays

Muons were first observed in 1936 by the Americans physicists Carl Anderson and Seth Neddermeyer. Carl Anderson discovered also three years before the positive electron

The new particle carried a negative electric charge. Its mass looked intermediate between the masses of an electron and a proton. In order to explain the forces binding protons and neutrons in the nuclei, physicists of the time were looking after the Yukawa meson, a particle having this intermediate mass predicted by the Japanese physicist Hideki Yukawa. Therefore they took the new particle as the Yukawa meson.

The Yukawa meson was not discovered until 1947. it was later on called pion or pi meson. The confusion lasted a decade until 1947. The particle discovered by Anderson and Neddermeyer appeared very different in nature, It did not interact strongly with protons and neutrons. Ii was called the muon.

The muon is considered nowadays a fundamental particle of matter, like the electron, its neutrino and the quarks constituents of the nuclear matter. Like the electron, it is not subject to strong interactions. It has also an alter ego without electric charge, the muonic neutrino or neutrino-mu.

Unlike the electron, the muon is unstable. Its average lifespan is only 2.2 millionths of a second. It has physical properties of electrons but with a mass 207 times greater : its mass energy -mc2 is 105, 66  MeV instead of 0,511 MeV. It can be viewed as a heavy unstable electron. Negative muons are noted μ-. Its antiparticle noted μ +, positively charged, appears as a heavy unstable positron.

The main source of muons is the decay of charged pions, the Yukawa mesons of the 1930’s. The pions or pi mesons themselves are created in the upper atmosphere within cosmic ray showers. Then unstable Muons then decay in turn..

The birth and death of muons are due to two successive decays. The forces behind the birth and death of a muon, which are the same, are called weak interactions or weak forces by nuclear and particle physicists. They are the forces involved also in the beta decay of nuclei.

The weak forces are explained by the role played by a very heavy unstable particle, called the W boson. The existence of the W boson, which was first introduced as an hypothesis, was confirmed by an experiment at CERN in 1983. The role of the W boson in the pi meson and muon decays are allowed by a principle of quantum mechanics at work in the world of the very small, the uncertainty principle enunciated in 1927 by the German physicist Werner Heisenberg.

Atmospheric muons

Muons are a component of cosmic rays. When a particle of high energy coming from space collides with a nucleus of the upper atmosphere, it produces a shower of particles that interact and decay in their turn. Among these secondary particles are short lived pi mesons that decay into positive or negative muons which generally live enough long to reach the surface of the earth.