For every type of particle, there is a corresponding antiparticle.

The idea

Antiparticles are a fundamental feature of particle physics: each particle species has a partner with the same rest mass but opposite electric charge and opposite quantum numbers such as baryon number, lepton number and strangeness. For example, the electron e⁻ has an antiparticle called the positron e⁺; the proton p has an antiparticle called the antiproton p̄. Some neutral particles, such as the photon, are their own antiparticles. When a particle meets its antiparticle they can annihilate, converting their combined rest energy into photons, often two gamma-ray photons. Conversely, a photon with sufficient energy can produce a particle–antiparticle pair in pair production. Recognising that antiparticles exist for every particle type allows you to predict annihilation products and to interpret particle tracks in detectors.

What earns the marks

• States that each particle species has a corresponding antiparticle with the same rest mass.
• States that the antiparticle has opposite electric charge to the particle, for example electron e⁻ and positron e⁺.
• States that other quantum numbers such as baryon number, lepton number and strangeness are opposite in sign for the antiparticle.
• Recognises that some neutral particles, such as the photon, are their own antiparticles.
• Describes annihilation as a particle and its antiparticle converting their combined rest energy into photons, commonly two gamma-ray photons.
• Describes pair production as a photon converting into a particle–antiparticle pair when it has sufficient energy.
• Uses conservation laws, including conservation of charge, energy and momentum, to check whether a proposed particle reaction is possible.

Common mistakes and how to put them right

• Thinking that an antiparticle has negative mass.

  Put it right: The rest mass is identical to that of the particle; it is charge and certain quantum numbers that are opposite.

• Assuming that every antiparticle is negatively charged.

  Put it right: The positron e⁺ is positively charged, and the antiproton p̄ is negatively charged, so charge sign depends on the particle.

• Believing that neutral particles cannot have antiparticles.

  Put it right: Neutral particles can have distinct antiparticles, for example the neutron and antineutron, while some neutral particles such as the photon are their own antiparticles.

• Writing annihilation as producing a single photon without considering momentum conservation.

  Put it right: In the centre-of-momentum frame, two photons are typically produced so that total momentum can be zero.

Exam tips

• When asked to compare a particle and its antiparticle, structure your answer around rest mass, charge and at least one quantum number such as baryon number or lepton number.
• Use standard symbols correctly, including e⁻, e⁺, p, p̄, n and n̄, and state the charge with a superscript sign.
• For annihilation and pair production questions, always check conservation of charge, energy and momentum before committing to a final answer.
• If a question gives rest energies in MeV, remember that annihilation converts rest energy into photon energy, so the total photon energy equals the total rest energy of the particle–antiparticle pair.