Fundamental particles: (l) decay of particles in terms of the quark model.

The idea

Particle decay can be described by showing how quark flavours change. In β⁻ decay a down quark becomes an up quark, so a neutron becomes a proton and an electron plus an electron antineutrino are emitted. In β⁺ decay an up quark becomes a down quark, so a proton becomes a neutron and a positron plus an electron neutrino are emitted. In each case the weak interaction changes quark flavour, and the emitted leptons conserve charge and lepton number. To describe a decay, write the quark content of the parent particle, show the flavour change, and list the products. For example, the neutron udd becomes the proton uud with the transformation d → u + ⁰₋₁e + ν̄. This approach explains why the nucleon's identity changes and why leptons appear even though they are not constituents of the nucleon.

What earns the marks

• Describes a neutron as udd and a proton as uud, and identifies the quark that changes in β⁻ decay.
• Describes a proton as uud and a neutron as udd, and identifies the quark that changes in β⁺ decay.
• Writes the quark-level transformation for β⁻ decay as d → u + ⁰₋₁e + ν̄.
• Writes the quark-level transformation for β⁺ decay as u → d + ⁰₊₁e + ν.
• Explains that the weak interaction changes quark flavour and that the emitted leptons conserve charge and lepton number.

Common mistakes and how to put them right

• Thinking that the whole neutron turns into a proton without any quark change.

  Put it right: The change is a single quark flavour transformation.

• Believing that electrons or positrons are quarks.

  Put it right: They are leptons produced in the decay and are not part of the nucleon.

• Mixing up the neutrino types.

  Put it right: β⁻ emits an electron antineutrino and β⁺ emits an electron neutrino.

Exam tips

• Write the quark content of the parent and daughter particles before writing the transformation.
• Check charge and lepton number on both sides of the equation.
• Use the direction of quark flavour change to decide whether the decay is β⁻ or β⁺.