AQA A Level Physics

Revision Notes

2.2.6 Strange Quarks

Strange Particles

  • Strange particles are particles that include a strange or anti-strange quark
  • An example of these are kaons
  • Strange particles always:
    • Are produced through the strong interaction
    • Decay through the weak interaction
  • An example of a kaon production could be:

Strong Interaction Kaons, downloadable AS & A Level Physics revision notes

Kaons are produced through the strong interaction. This is shown by the gluon exchange particle.

  • An example of kaon decay could be:

Weak Interaction Kaons, downloadable AS & A Level Physics revision notes

Kaons decay via the weak interaction. This is shown by the W+ boson.


  • Strangeness, S, like baryon and lepton number, is a quantum number
  • Strangeness is conserved in every interaction except the weak interaction
  • This means that strange particles are always produced in pairs (e.g. K+ and K–)
  • S depends on whether the particle contains a strange quark, anti-strange quark, or no strange quarks
    • Particles with an anti-strange quark have S = +1
    • Particle with a strange quark have S = –1
    • Particles with no strange quark have S = 0

Strangeness, downloadable AS & A Level Physics revision notes

Only particles with a strange quark have a strangeness of +1 or –1

  • Strangeness can change by 0, +1 or –1 in weak interactions

Worked Example

The sigma baryon has a quark structure of suu. It decays to produce a proton and pion as shown in the equation below

2.2.6 Sigma Baryon Decay Equation

Prove that this decay is via the weak interaction.

Step 1: Determine the strangeness, S of each particle

      • Since sigma baryon has one s quark, it has S = –1
      • The proton and pion has no strange particles, so they have S = 0

Step 2: Determine strangeness, S on both sides of the equation

      • The sigma baryon has a S = –1 but the meson and proton have a S = 0

–1 = 0 + 0

Step 3: Comment on the conservation of strangeness

      • Since S is not conserved on both sides of the decay equation (only changed by –1), this decay is via the weak interaction
      • This is because S is conserved in all other types of interaction (strong and EM), but isn’t always conserved in weak interactions

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top