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:
HADRONSMesons: pions, kaons, eta mesons (composed
of two quarks)
Baryons: protons, neutrons, omega, sigma, lambda particles
(composed of three quarks)
NON-HADRONSLeptons: 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) |
| LEPTONS |
|
|
|
|
| Electron |
e |
0.511 |
-1 |
stable |
| Neutrino |
n |
very small |
0 |
stable |
| Muon |
m |
105.7 |
0 |
2.2x10-6 |
HADRONS |
|
|
|
|
| Mesons |
|
|
|
|
| Pion |
p+ |
139.6 |
+1 |
2.6x10-8 |
| |
po |
135.0 |
0 |
0.8x10-16 |
| Kaon |
K |
495 |
|
|
| Eta |
h |
549 |
|
<10-18 |
| Baryons |
|
|
|
|
| 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.
