Edexcel A Level (IAL) Physics-4.42 Antimatter- Study Notes- New Syllabus

Particles and Antiparticles: Properties and Deductions

In particle physics, every particle has a corresponding antiparticle. The antiparticle has the same mass but opposite values of certain properties. By knowing the properties of a particle, the properties of its antiparticle can be deduced, and vice versa.

What Is an Antiparticle?

Definition: An antiparticle is a particle with the same mass as its corresponding particle but with opposite values of specific quantum properties.

• Same rest mass
• Opposite electric charge
• Opposite lepton number or baryon number
• Opposite magnetic moment

Properties That Are the Same

• Rest mass
• Lifetime (if unstable)
• Speed in the same conditions

Key point: Mass does not change between a particle and its antiparticle.

Properties That Are Opposite

• Electric charge: sign is reversed
• Baryon number: sign is reversed
• Lepton number: sign is reversed
• Magnetic moment: direction is reversed

Example: Electron and Positron

• Electron charge: \( -1.6\times10^{-19}\ \mathrm{C} \)
• Positron charge: \( +1.6\times10^{-19}\ \mathrm{C} \)
• Both have the same mass.
• Electron has lepton number +1, positron has lepton number −1.

Example: Proton and Antiproton

• Proton charge: positive
• Antiproton charge: negative
• Both have the same mass.
• Proton has baryon number +1, antiproton has baryon number −1.

 Using Properties to Deduce Antiparticles

To deduce the properties of an antiparticle:

1. Keep the mass the same.
2. Reverse the sign of electric charge.
3. Reverse baryon or lepton number.
4. Reverse magnetic properties.

Annihilation and Pair Production

• When a particle meets its antiparticle, annihilation occurs.
• Their mass is converted into energy.
• High-energy photons can create particle–antiparticle pairs.

Importance in Experiments

• Opposite curvature in magnetic fields identifies antiparticles.
• Charge conservation confirms particle–antiparticle creation.
• Energy conservation explains annihilation radiation.