Edexcel A Level (IAL) Physics-4.36 Interpreting Particle Tracks- Study Notes- New Syllabus

Conservation of Charge, Energy and Momentum in Particle Interactions

When particles interact, collide, or decay, their behaviour is governed by fundamental conservation laws. By applying these laws, particle interactions can be analysed and particle tracks interpreted.

Conservation of Charge

Principle: The total electric charge before an interaction is equal to the total electric charge after the interaction.

Total charge (before) = Total charge (after)

• Charge cannot be created or destroyed.
• Applies to all particle interactions and decays.
• Helps identify unknown particles in reactions.

Example:

• If a neutral particle decays, the charges of the decay products must sum to zero.

 Conservation of Energy

Principle: Total energy is conserved in all particle interactions.

Energy before = Energy after

• Includes kinetic energy, rest mass energy, and radiation energy.
• Energy can change form but cannot be lost.
• Mass can be converted into energy and vice versa.

Key idea:

• Some kinetic energy may appear as new particles.
• Some energy may be carried away by unseen particles.

Conservation of Momentum

Principle: Total momentum of a system is conserved provided no external forces act.

Total momentum (before) = Total momentum (after)

• Momentum is a vector quantity.
• Both magnitude and direction must be conserved.
• Very important in analysing particle tracks.

 Applying Conservation Laws to Particle Interactions

In particle collisions or decays:

• Check charge conservation to confirm possible reactions.
• Use energy conservation to determine kinetic energies or particle creation.
• Apply momentum conservation to find directions and speeds of particles.

Interpreting Particle Tracks

Particle tracks are observed in detectors such as cloud chambers and bubble chambers.

Track Features and What They Show:

• Curvature: caused by motion in a magnetic field.
• Direction of curvature: shows the sign of charge.
• Radius of curvature: related to momentum.
• Track thickness: indicates ionisation and particle type.

 Momentum and Track Curvature

For a charged particle in a magnetic field:

\( r = \dfrac{p}{BQ} \)

• Larger radius → larger momentum.
• Tighter curve → lower momentum or higher charge.

 Identifying Particle Interactions

Particle Creation:

• Single track splitting into two or more tracks.
• Total charge before equals total charge after.

Particle Annihilation:

• Two tracks meet and disappear.
• Energy carried away by radiation or unseen particles.

Particle Decay:

• A track suddenly changes into two or more tracks.
• Momentum vectors of products balance.

Using Conservation Laws Together

• Charge conservation restricts possible reactions.
• Energy conservation explains particle creation or missing energy.
• Momentum conservation explains track directions and angles.