(a) particles and antiparticles; electron–positron, proton-antiproton,…

The idea

Every particle has a corresponding antiparticle. The electron's antiparticle is the positron; the proton's is the antiproton; the neutron's is the antineutron; and each neutrino has a corresponding antineutrino. In OCR A Level Physics A, you must recognise these pairs and their symbols, for example e⁻ and e⁺, p and p̄, n and n̄, ν and ν̄. When a particle meets its antiparticle they can annihilate, producing photons. In pair production, a photon can create a particle–antiparticle pair. For the MCQ, identify the correct antiparticle partner or the particle–antiparticle pair from the options, checking charge and baryon number conservation where relevant.

What earns the marks

• Electron and positron are a particle–antiparticle pair.
• Proton and antiproton are a particle–antiparticle pair.
• Neutron and antineutron are a particle–antiparticle pair.
• Neutrino and antineutrino are a particle–antiparticle pair.
• Annihilation occurs when a particle meets its antiparticle, often producing photons.
• Pair production is the creation of a particle–antiparticle pair from a photon.

Common mistakes and how to put them right

• Thinking the positron is a proton.

  Put it right: The positron has the same mass as an electron but positive charge, while the proton is much more massive.

• Believing the antineutron has negative charge.

  Put it right: The antineutron is neutral, like the neutron, but has opposite baryon number.

• Assuming a neutrino and antineutrino are identical.

  Put it right: They are distinct antiparticles with opposite lepton number.

• Confusing annihilation with pair production.

  Put it right: Annihilation converts mass into energy (photons), while pair production converts a photon into a particle–antiparticle pair.

Exam tips

• Learn the symbols and charges for each pair, including the bar over antiparticles.
• In MCQs, check that charge and baryon number are conserved in any reaction or decay.
• Remember that neutrino–antineutrino pairs are distinct even though both have zero charge.