Strange Quarks (AQA A Level Physics)

• 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:

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

• An example of kaon decay could be:

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

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

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

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