Leptons

Lepton is Greek for 'small'. Leptons do NOT experience the strong nuclear force, rather any nuclear interactions they are involved in are via the weak nuclear force.

The leptons (we have to know about!) are:

    • the electron (e- or )
    • the positron (e+ or )
    • the muon (-) and
    • the antimuon (+) - basically a 'heavy electron' (about 200 times the mass)
    • the neutrino () and antineutrino () ( both with virtually no mass and zero charge)

Electron leptons have a lepton number Le of +1 and their antileptons a lepton number -1.

Muon leptons have a lepton number L of +1 and their antileptons a lepton number -1.

Leptons have zero baryon number and zero strangeness.

Electron stability

The electron is stable because there is no lighter particle into which it can decay.

Muon production

Muons are produced by the weak decay (via a w-boson) of pions into a muon and a muon antineutrino.

On earth, muons are created when a charged pion decays in the upper atmosphere. The pions are created by cosmic radiation and have a very short decay time--a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, muons in the atmosphere are moving at very high velocities, so that the time dilation effect of special relativity make them easily detectable at the earth's surface.

Muon stability

The muon differs from the electron in that it is unstable, decaying with an average lifetime of 2.2 × 10-6 seconds (2.2 microseconds) decaying into an electron, an electron-antineutrino, and a muon-neutrino.

This link takes you to an animation of this happening

Muonic atoms

Muons can be substituted for electrons in orbit around the nucleus of an atom.

The resulting atom is long-lived enough to exhibit behaviour that further supports the close resemblance between the muon and the electron.

Recent studies of muons have included the production of “muonic atoms” (ordinary atoms to which an orbiting muon is added) and muonium, which consists of an electron in orbit around a positive muon. Muonic atoms are much smaller than typical atoms because, in order to conserve angular momentum, the more massive muon must be closer to the atomic nucleus than its less massive electron counterpart.