The kaon as a particle that can decay into pions.

The idea

Kaons are mesons containing a strange quark or antiquark, for example K⁺ is u s̄ and K⁻ is ū s. They are produced in strong interactions, where strangeness is conserved, but they decay by the weak interaction, in which strangeness is not conserved. A common decay route is a kaon decaying into two or three pions, such as K⁺ → π⁺ + π⁰ or K⁺ → π⁺ + π⁺ + π⁻. These decays conserve charge, baryon number and lepton number, and they release energy because the total rest mass of the pions is less than the kaon rest mass. The decay products are therefore lighter mesons, and the energy difference appears as kinetic energy of the pions.

What earns the marks

• Kaons are mesons with strangeness, containing a strange quark or a strange antiquark.
• Kaons decay by the weak interaction, so strangeness is not conserved in the decay.
• A kaon can decay into two or three pions, for example K⁺ → π⁺ + π⁰.
• Charge, baryon number and lepton number are conserved in the decay.
• The total rest mass of the pions is less than the kaon rest mass, so the decay releases energy as kinetic energy.
• Kaon decay is distinct from strong production, where strangeness is conserved.

Common mistakes and how to put them right

• Writing a decay equation that does not conserve charge.

  Put it right by balancing the charges on both sides, for example K⁺ → π⁺ + π⁰.

• Assuming strangeness is conserved in kaon decay.

  Put it right by stating that the weak interaction allows strangeness to change.

• Treating pions as baryons.

  Put it right by identifying pions and kaons as mesons with baryon number zero.

Exam tips

• Check charge, baryon number and lepton number when writing or completing a kaon decay equation.
• State explicitly that the decay is mediated by the weak interaction because strangeness changes.
• Use the rest masses to argue that the decay is energetically possible rather than simply asserting it.