2.1.13

# Application of Conservation Laws

Test yourself

## 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.