Edexcel A Level (IAL) Physics-4.43 Conservation Laws in Particle Physics- Study Notes- New Syllabus

Using Conservation Laws to Determine Possible Particle Interactions

Whether a particle interaction, decay, or reaction can occur is determined by conservation laws. In the standard model, three key quantities must be conserved in all allowed interactions: electric charge, baryon number, and lepton number.

Conservation of Electric Charge

Law: The total electric charge before an interaction equals the total electric charge after the interaction.

Total charge (before) = Total charge (after) 

• Charge can be positive, negative, or zero.
• Charge cannot be created or destroyed.
• This law applies to all particle processes.

 Conservation of Baryon Number

Definition:

• Baryons (e.g. protons, neutrons): baryon number \( +1 \)
• Antibaryons: baryon number \( -1 \)
• All other particles: baryon number \( 0 \)

Law:

Total baryon number (before) = Total baryon number (after)

• Baryons cannot be created or destroyed on their own.
• Baryon–antibaryon pairs may be created or annihilated.

 Conservation of Lepton Number

Definition:

• Leptons (e.g. electron, muon): lepton number \( +1 \)
• Antileptons: lepton number \( -1 \)
• Non-leptons: lepton number \( 0 \)

Law:

Total lepton number (before) = Total lepton number (after)

• Leptons cannot appear or disappear without corresponding antileptons.
• Applies separately to each interaction.

How to Decide If an Interaction Is Possible

To test whether a proposed interaction is allowed:

1. Add up total charge before and after.
2. Add up total baryon number before and after.
3. Add up total lepton number before and after.
4. If all three are conserved, the interaction is possible.
5. If any one is not conserved, the interaction is forbidden.

Worked Reasoning Examples

(a) Allowed Interaction

• Charge conserved
• Baryon number conserved
• Lepton number conserved
• Interaction is possible

(b) Forbidden Interaction

• Charge conserved
• Baryon number conserved
• Lepton number not conserved
• Interaction cannot occur

Typical Situations

• Beta decay: lepton number conserved by emission of a neutrino.
• Pair production: particle and antiparticle created together.
• Annihilation: particle–antiparticle pair destroyed together.

 Key Exam Tip

• Always show conservation checks explicitly.
• State clearly which quantity is not conserved if an interaction is impossible.