Classification of particles: Hadrons are subject to the strong interaction.

The idea

Hadrons are particles made of quarks and antiquarks, and they experience the strong interaction. The strong interaction is carried by gluons and acts between quarks, which means it holds the quarks together inside protons and neutrons. Its residual effect also binds protons and neutrons together in nuclei. Hadrons are divided into baryons, such as protons and neutrons, which contain three quarks, and mesons, such as pions and kaons, which contain a quark and an antiquark. Leptons, such as electrons and neutrinos, are not hadrons and do not feel the strong interaction. The strong interaction is short range, acting over distances comparable with the size of a nucleus, and it is attractive between quarks at those separations.

What earns the marks

• Hadrons are composite particles made of quarks and/or antiquarks.
• Baryons contain three quarks, for example a proton is uud and a neutron is udd.
• Mesons contain a quark and an antiquark, for example a π⁺ is u d̄.
• The strong interaction is mediated by gluons and acts between quarks.
• The residual strong interaction binds protons and neutrons together in the nucleus.
• Leptons are not subject to the strong interaction because they do not contain quarks.
• The strong interaction is short range, of the order of nuclear dimensions.

Common mistakes and how to put them right

• Stating that leptons feel the strong interaction.

  Put it right: Leptons do not contain quarks and therefore do not experience the strong interaction.

• Confusing baryons and mesons.

  Put it right: Remember that baryons contain three quarks while mesons contain a quark–antiquark pair.

• Saying the strong interaction acts directly between protons and neutrons.

  Put it right: Gluons act between quarks, and the residual strong interaction between nucleons is a consequence of quark-level interactions.

• Describing the strong interaction as long range.

  Put it right: It is short range, acting over nuclear distances.

Exam tips

• When classifying a particle, first decide whether it is a hadron or a lepton, then whether it is a baryon or a meson.
• Use quark compositions to justify why a particle is subject to the strong interaction.
• Link the exchange particle to the interaction: gluon for strong, photon for electromagnetic, W⁺ or W⁻ for weak.
• Remember that the strong interaction is responsible for binding quarks into hadrons and for the stability of nuclei.