Classification of Particles

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Hadrons (e.g. protons and neutrons) are particles that feel the strong nuclear force. Hadrons are made of quarks and can be split into two categories: baryons or mesons.

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  • Baryons are hadrons that contain three quarks.
  • Protons and neutrons are both baryons.
    • There are other types of baryons which can be found in cosmic rays, but you don't need to know about these at A Level.
  • The proton is the only stable baryon.
    • All other baryons will eventually decay into a proton.
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  • The antiparticles of baryons are called antibaryons.
    • Examples include antiprotons and antineutrons.
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Baryon number

  • Baryon number, B, describes the number of baryons present.
    • Each baryon has a baryon number B = 1.
    • Antibaryons have a baryon number B = -1.
  • B is a quantum number.
    • This means that it can only be an integer (i.e. you can't have B = 0.5).
  • B must always be conserved.
    • Like energy and momentum, baryon number must be conserved in all interactions.
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  • Mesons are hadrons that contain a quark and an anti-quark.
  • Examples include pions and kaons.
    • There are also many other types of mesons, but you don't need to know about them at A Level.
  • Mesons can be found in cosmic rays or particle accelerators.
  • Mesons have a baryon number B = 0 because they're not baryons!
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Pions vs kaons

  • Pions are the exchange particle of the strong nuclear force between nucleons.
    • There are three versions, each with a different electric charge: π+, π0 and π-.
  • Kaons are more massive and often decay into pions. They come in many types including K+, K0 and K-.


Leptons are fundamental particles and so they are not made of any smaller particles. Leptons do NOT feel the strong nuclear force.

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Electrons and muons

  • The most common type of lepton is the electron, e-.
  • There are other types of leptons, such as the muon, μ-:
    • Muons are negatively charged and heavier than electrons.
    • Muons are unstable and always decay into electrons.
    • Muons are normally found in cosmic rays.
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  • Neutrinos are also leptons.
    • Electrons and muons each have their own neutrino: the electron neutrino, νe, and the muon neutrino, νμ.
  • Neutrinos are (almost) massless particles with zero charge, so they don't do much.
    • Just as well, because there are 100 trillion neutrinos passing through your body every second from cosmic rays!
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Lepton numbers

  • The lepton number describes the number of leptons.
  • You get a different lepton number for each type of lepton.
    • Electron lepton number, Le, is 1 for electrons and electron neutrinos.
    • Muon lepton number, Lμ, is 1 for muons and muon neutrinos.
  • Lepton numbers are quantum numbers, meaning they only come in integer numbers.
  • Each lepton number must be conserved individually.
  • Antileptons also exist. These have opposite charges and lepton numbers.


Strangeness is a fundamental property of matter, like mass, charge or baryon number.

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Quantum number

  • Strangeness is a quantum number so can only take integer values.
  • The strange quark, s, has a strangeness of -1.
  • The antistrange quark, s, has a strangeness of +1.
  • Particles made from strange quarks (such as kaons) have an associated strangeness.
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Conserved in the strong interaction

  • Unlike other quantum numbers, strangeness is only conserved in the strong interaction.
  • Strange particles, such as kaons, are produced via the strong interaction but decay via the weak interaction.
    • This is why kaons are always produced in pairs (K+ and K-).
    • The strangeness of each particle (-1 and +1) cancel out.
  • In weak interactions, such as kaon decay, strangeness can change by -1, 0 or +1.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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