AQA A Level Physics复习笔记2.3.5 Application of Conservation Laws

• All particle interactions must obey a set of conservation laws. These are conservation of:
  • Charge, Q
  • Baryon number, B
  • Lepton Number, L
  • Strangeness, S
  • Energy (or mass-energy)
  • Momentum

   

• However, strangeness does not need to be conserved in weak interactions. It can change by either 0, +1 or –1
• Quantum numbers such as Q, B, L and S can only take discrete values (ie. 0, +1, –1, 1/2)
• To know whether a particle interaction can occur, check whether each quantum number is equal on both sides of the equation
  • If even one of them, apart from strangeness in weak interactions, is not conserved then the interaction cannot occur

   

Example of a working out what is conserved in Kaon decay. This decay must be through the weak interaction since S is not conserved

Step 1: Determine conservation of charge

0 = –1 + 1 + 0

Charge is conserved

Step 2: Determine conservation of strangeness

Λ0 has an s quark, so must have a strangeness of –1

–1 = 0 + 0 + 0

Strangeness is not conserved

Step 3: Determine conservation of baryon number

Λ0 is a baryon since it has 3 quarks, so must have a baryon number of +1

+1 = 0 + 0 + 0

Baryon number is not conserved

Step 4: Determine conservation of lepton number

Λ0 is a baryon, so must have a lepton number of 0

0 = 0 + (–1) + 1

Lepton number is conserved

Step 5: Conclusion

Since the baryon number is not conserved, this interaction is not permitted