Particle Physics Tutorial 7 - Classification of Particle (Leptons)

We can show the way the particles are classified as a tree:

From this diagram we can see that there are three classes of leptons.

Each lepton has a neutrino, an antiparticle, and an anti-neutrino.

Leptons

Leptons are fundamental particles such as the electron. They are called leptons as they are considered to be light-weights, although the muon is as massive as a meson and the tau has a greater mass than a gold atom.

Leptons do not feel the strong force.  They only feel the weak force (e.g. beta minus decay) and the electromagnetic force.

The only stable lepton is the electron.  The muon and the tau are highly unstable with very short life times.  Each has an escorting neutrino.

Neutrinos are not well understood.  While the universe is under a constant hail of neutrinos, they rarely interact with matter, so they are very difficult to measure.  During the day you get a hail from above.  At night, you are peppered with the little brutes while asleep in your bed.  But they won't interact with you (unlike the fleas on the cat...).

There are six particle-antiparticle pairs known.  Leptons (Greek – “light thing” or “small coins”) are the smallest of the fundamental particles.  They have the following properties:

• fundamental particles without structure

• they interact by the weak interaction.  If they are charged, they interact by the electromagnetic interaction, but NOT the strong interaction.

• charge and lepton number are conserved in all allowed lepton processes.

You need to know about lepton numbers:

• if it’s a lepton, the lepton number is +1;

• if it’s an anti-lepton, the lepton number is -1.

The lepton number tells us whether a particle is a lepton or not.  If L = 0, it is not a lepton.

There are three smaller categories of lepton number, Le, Lm, and Lt.  You don’t have to worry about these at this stage.  In the exam, you will often see the lepton number referred to as a quantum number.  (Quantum is Latin for "how much?".)

The names of the leptons are:

Lepton

# Mass

electron

e-

-1e

1/1800 mass of a proton

electron neutrino

ne

0

Very small

muon

m-

-1e

2/3 mass of a proton

muon neutrino

nm

0

very small

tau

t-

-1e

Mass of gold atom

tau neutrino

nt

0

Very small

Each particle has an antiparticle; for the electron, it is the positron, the muon the anti-muon, and the tau, the anti-tau

Anti Lepton

# Mass

positron

e+

+1e

1/1800 mass of a proton

Electron antineutrino

0

Very small

anti-muon

m+

+1e

2/3 mass of a proton

Muon antineutrino

0

very small

Anti-tau

t+

+1e

Mass of gold atom

Tau antineutrino

0

Very small

We show the anti-particle either by an opposite charge (e+) or by putting a bar across the symbol (, pronounced “noo-bar e”).

Consider this decay:

Notice how the lepton number and charge are conserved.  This means that the decay can proceed.  If the lepton number and charge are not conserved, the interaction will not proceed.

If leptons interact with hadrons, the hadrons are considered to have a lepton number of 0.

 Question 1 What are the symbol, the charge, and the lepton number of the particle anti-tau? Question 2 Use quantum numbers to show that  this interaction can happen: