Mass is the most basic method by which to classify particles:
- the heaviest particles are called hadrons
? hadrons can be split into baryons and mesons, of which the former is heavier
- the lightest particles are called leptons
Hadrons
Hadrons are subjected to strong nuclear force. As they do not consist of quarks they are not fundamental particles.
Baryons consist of three quarks. They include:
- protons
- neutrons
The only stable baryon is the proton so all other baryons eventually decay into this state.
Mesons consist of a quark and an anti-quark. They include:
- pions
- kaons
Leptons
Leptons are subjected to weak nuclear force. They include:
- electrons
- musons
- neutrinos
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Quarks and Anti-quarks
For A level you should be aware of the following three quarks:
- up quark (u)
- down quark (d)
- strange quark (s)
| Quark | Symbol | Charge | Baryon number | Strangeness |
| up | u | +2/3e | 1/3 | |
| down | d | -1/3 e | 1/3 | |
| strange | s | -1/3 3 | 1/3 | -1 |
Baryons and mesons
Quarks can combine to form baryons and mesons.
Baryons consist always of three quarks. For example, a proton contains the quarks, up up down. An anti-proton, on the other hand, contains the quarks, anti-up anti-up anti-down:
- proton = uud
- neutron = udd
- anti-proton = ???
- anti-neutron = ???
Mesons consist always of two quarks: a quark and an anti-quark. For example, ?+ meson consist of the quarks, up and anti-down.
- pion:
? ?+ = u?
? ?– = ?d
? ? = u?
- kaon
? K+ = u?
? K– = ?s
? K = d?
Conservation laws for particle interactions
When particles interact the following are always conserved:
- charge
- baryon number
- lepton number
- strangeness