Classification of Particles & Quarks and Anti-quarks

Classification of Particles & Quarks and Anti-quarks

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 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 are subjected to weak nuclear force. They include:

  • electrons
  • musons
  • neutrinos


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 0
down d -1/3 e 1/3 0
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

? ?0 = u?

  • kaon

? K+ = u?

? K = ?s

? K0 = d?

Conservation laws for particle interactions

When particles interact the following are always conserved:

  • charge
  • baryon number
  • lepton number
  • strangeness