Students should know that the positron, antiproton, antineutron and…

The idea

Every particle has a corresponding antiparticle with the same rest mass but opposite values of charge and other quantum numbers such as baryon number and lepton number. The positron is the antiparticle of the electron, with charge +1 rather than -1; the antiproton is the antiparticle of the proton, with charge -1 rather than +1. The antineutron is the antiparticle of the neutron; although both have zero charge, they differ in baryon number sign and quark content. The antineutrino is the antiparticle of the neutrino, differing in lepton number.

For example: In beta-plus decay a proton becomes a neutron while emitting a positron and an electron neutrino, conserving charge and lepton number.

What earns the marks

• State that a particle and its antiparticle have equal rest mass but opposite charge and opposite quantum numbers such as baryon number and lepton number.
• Identify the positron as the antiparticle of the electron, with charge +1, and the antiproton as the antiparticle of the proton, with charge -1.
• Recognise that the antineutron is the antiparticle of the neutron; both have zero charge but opposite baryon number and different quark composition.
• Recognise that the antineutrino is the antiparticle of the neutrino, distinguished by opposite lepton number rather than by charge.
• Apply conservation of charge, baryon number and lepton number to a stated interaction involving a named particle and its antiparticle.

Common mistakes and how to put them right

• Treating the antineutron as a negatively charged particle because of the prefix anti-.

  Put it right by stating that the antineutron has zero charge and is distinguished from the neutron by opposite baryon number and different quark content.

• Confusing the sign of the charge for antiparticles.

  Put it right by ensuring the positron is explicitly written with a positive charge (+1 or +e) and the antiproton with a negative charge (-1 or -e).

• Assuming that an antiparticle always has opposite charge to its particle.

  Put it right by noting that neutral particles such as the neutron and neutrino have antiparticles that differ in quantum numbers rather than charge.

Exam tips

• When asked to complete a table of particles and antiparticles, check charge, baryon number and lepton number for each row rather than relying on the name alone.
• In decay equations, confirm that charge, baryon number and lepton number balance on both sides before writing the final answer.
• Use the precise terms positron, antiproton, antineutron and antineutrino rather than vague phrases such as opposite particle.