Strange Quarks (AQA AS Physics): Revision Note

Exam code: 7407

Ashika

Author

Ashika

Last updated

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

    • Are produced in quark-antiquark pairs

  • 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

  • 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.

Answer:

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

👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It's free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Did this page help you?

Ashika

Author: Ashika

Expertise: Physics Content Creator

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.

Download notes on Strange Quarks