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particle classification, page 1 Particle Physics - Classification, 2 of 3 fundemental particle classification, page 3

The earliest classification scheme for particles was based on their masses. They can be grouped into three: leptons (from the Greek word for small), mesons (means middle in Greek) and baryons (which means heavy in Greek). Note that quarks do not fall into any of this classification. This is because they possess certain unusual properties which other particles do not have. For example, quarks can only exist in a group of three. They do not exist individually, as all particles do.

The classification based on mass is no longer valid. The overlapping of mass among the groups are quite extensive. The classification is used mainly as a convenience to describe particles with similar properties and mass clssification can only be treated as broadly correct.

There is another important quantity that is used to describe a particle's characteristic property, namely, spin. The values are usually in multiple of integrals or half-integrals. Note also that leptons are a class of fundamental particles while mesons and baryons are composite particles, or particles that made up from the fundamental particles.

Leptons     Interactions: Weak, electromagnetic     Spin: half integral

Name

Symbol

Mass (MeV/c2)

Charge

Lifetime (s)

Spin

Electron

e-

0.511

-1

stable

1/2

Muon

m-

105.7

-1

2.2 x 10-6

1/2

Tauon

t-

1784

-1

5 x 10-13

1/2

Electron neutrino

ne

< 0.000015

0

stable

1/2

Muon neutrino

nm

< 0.19

0

stable

1/2

Tauon neutrino

nt

< 18

0

stable

1/2

Leptons are fundamental particles that have no internal structure. The leptons are so small that they can be considered as points with no finite dimension. Electron neutrinos and antineutrinos are produced in the beta decay of radioactive elements. They are also produced in solar fusion process. The electron is a stable particle, but the muon and tauon decay to other leptons:

muon and tauon decays

The brackets show the mean lifetime and the 'bars' on top of the particle symbols represent antiparticles. These decays are caused by the weak interaction. The presence of neutrinos always indicate a weak interaction process (but not all weak interaction decays produce neutrinos). When similar leptons (such as electrons) approach very close to each other, the electromagnetic interactions dominate and repel each other.

particle classification, page 1 Fundamental particles classification, page 3

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