A LevelPhysicsAQARevision NotesParticles & RadiationClassification of ParticlesStrange Quarks

Strange Quarks (AQA A Level Physics): Revision Note

Exam code: 7408

Written by: Ashika

Reviewed by: Tim

Updated on 16 September 2026

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Understand this topic with deeper explanations and understanding checks.

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:

A Feynman diagram of kaon production through the strong interaction, with a gluon as the exchange particle. A π− (d anti-u) and proton (uud) produce K⁰ (d anti-s) and Λ⁰ (uds) through gluon exchange. The text box states that gluons are exchange particles of the strong interaction (between quarks). — **Kaons are produced through the strong interaction, which is shown by the gluon exchange particle**

An example of kaon decay could be:

A Feynman diagram showing K⁰ (d and anti-strange) decaying into π⁻ (d and anti-up) and π⁺ (u and anti-d). The anti-strange emits a W⁺ boson, which is the weak-interaction exchange particle. — **Kaons decay via the weak interaction, and 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 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.

$Σ^{+} \to π^{0} + p$

Prove that this decay is via the weak interaction.

[2]

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

$- 1 = 0 + 0$ [1 mark]

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 [1 mark]
- This is because S is conserved in all other types of interaction (strong and EM), but isn't always conserved in weak interactions

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