AQA A Level Physics

Revision Notes

2.2.1 Hadrons

Baryons & Mesons

Hadrons

  • Hadrons are the group of subatomic particles that are made up of quarks
  • Therefore, hadrons can feel the strong nuclear force
  • There are two classes of hadrons:
    • Baryons (3 quarks)
    • Mesons (quark and anti–quark pair)
  • The most common baryons are protons and neutrons
  • The most common mesons are pions and kaons

Baryons & Mesons

Hadrons may be either a baryon or a meson

Anti–hadrons

  • There are two classes of anti–hadrons:
    • Anti–baryons (3 anti-quarks)
    • Anti–mesons (quark and anti–quark pair)

Anti-hadrons

Anti-hadrons may be either an anti-baryon or an anti-meson

  • Quarks have never been discovered on their own, always in pairs or groups of three
  • Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
  • This means quarks in a baryon are either all quarks or all anti–quarks. Combination of quarks and anti–quarks don’t exist in a baryon
    • e.g.
      Wrong quark composition, downloadable AS & A Level Physics revision notes
  • The anti–particle of a meson is still a quark and anti–quark pair. The difference being the quark becomes the anti–quark and vice versa

Worked Example

The baryon Δ++ was discovered in a particle accelerator using accelerated positive pions on hydrogen targets.

Which of the following is the quark combination of this particle?

Worked example - Baryons and mesons, downloadable AS & A Level Physics revision notes

Exam Tip

  • Remembering quark combinations is useful for the exam
  • However, as long as you can remember the charges for each quark, it is easy to figure out the combination by making sure the combination of quarks adds up to the total charge of the particle (just like in the worked example!)

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