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Fundamental particles

The fundamental particles that make up our Universe can be divided into two main categories:

(a) hadrons these particles 'feel' the strong nuclear force
(b) non-hadrons these particles do not 'feel' the strong nuclear force

These two main divisions are further divided as follows:

Mesons:   pions, kaons, eta mesons (composed of two quarks)
Baryons:   protons, neutrons, omega, sigma, lambda particles (composed of three quarks)

Leptons:   electrons, muons, tauons, neutrinos

Photons and gravitons

Note: At the present time gravitons are not known to exist.

Leptons are truly fundamental particles and cannot be broken down into smaller particles (as far as we know!). However hadrons do have an internal structure being composed of true fundamental particles called quarks.

(See: Quarks)

Some of the properties of these particles are shown in the following table.

Particle Symbol Rest energy (MeV) Charge Lifetime (s)
Electron e 0.511 -1 stable
Neutrino n very small 0 stable
Muon m 105.7 0 2.2x10-6
Pion p+ 139.6 +1 2.6x10-8
  po 135.0 0 0.8x10-16
Kaon K 495    
Eta h 549   <10-18
Proton p 938.3 1 stable
Neutron n 939.6 0 650 (free)
Lambda L 1116 0 2.6x10-10
Sigma S-1 1197 -1 1.6x10-10
  So 1192 0 6x10-20
Omega W 1672 -1 0.8x10-10

Fermions and bosons

Another way of dividing particles is to split them into two groups - one called fermions and the other bosons. Electrons, protons and neutrons are examples of fermions while photons are bosons.

In any system of identical fermions no two fermions can occupy an identical energy state. In other words they cannot have the same set of quantum numbers. They are governed by Pauli's exclusion principle.

In any system of identical bosons any number of bosons can occupy an identical energy state. They are not governed by Pauli's exclusion principle.

A hypothetical particle called the tachyon has been proposed with a velocity greater than that of light.
© Keith Gibbs 2011