Application of Conservation Laws
Physics: AQA A Level, topic 2.1.13
Changing Quark Type
Quarks can only change type via the weak nuclear force. Beta-minus and beta-plus decay are two examples of this.
Beta-minus decay
- When a neutron decays into a proton, its constituent quarks need to change from udd to uud.
- So one down quark changes into an up quark.
- Only the weak interaction can result in quarks changing type, so the decay of a neutron into a proton (beta-minus decay) must be governed by the weak nuclear force.
Beta-plus decay
- When a proton decays into a neutron, its constituent quarks need to change from uud to udd.
- So one up quark changes into a down quark.
- Only the weak interaction can result in quarks changing type. So the decay of a proton into a neutron (beta-plus decay) must be governed by the weak nuclear force.
Conservation Laws in Particle Physics
When solving problems in particle physics, you need to remember the following rules:
Charge
- Charge is always conserved.
Baryon number
- Baryon number is always conserved.
Energy
- Energy is always conserved.
Lepton numbers
- Each lepton number is conserved separately.
- Both the muon lepton number, Lμ, and the electron lepton number, Le, are always conserved.
Strangeness
- Strangeness is always conserved for strong interactions.
- Strangeness is sometimes not conserved for weak interactions because this interaction can result in a change of quark type.
- For example, in weak interactions an 's' quark can change to a 'u' quark.
- In weak interactions, only one quark can change at a time so strangeness can change by either +1, 0 or -1.
Momentum
- Momentum is always conserved.
Working in Particle Physics
Large teams of scientists are needed to confirm new knowledge.
Validation of new knowledge
- Validating new knowledge means confirming that the new knowledge is correct.
- Particle physics experiments create massive amounts of data.
- Particle physics experiments are also very expensive to run.
- These two reasons are why scientists need to work collaboratively to pool resources and expertise.
- Large teams working collaboratively are needed to validate new knowledge in particle physics.
Test yourself on Application of Conservation Laws
Explain why beta-minus decay must be governed by the weak nuclear force.
Explain why beta-plus decay must be governed by the weak nuclear force.
What change of quark occurs in beta-minus decay?
What change of quark occurs in beta-plus decay?
Which of the following quantities are ALWAYS conserved?