Strangeness can change by 0, +1 or -1 in weak interactions.

The idea

Strangeness is a quantum number assigned to hadrons to explain why certain strong interactions happen readily while others are never seen. In strong and electromagnetic interactions strangeness is conserved. The weak interaction is different: it can change the strangeness of an individual quark, altering the overall strangeness. Permitted changes are 0, +1 or −1. A change of 0 occurs when strangeness is unaltered, as in beta-minus decay. A change of +1 occurs when a strange quark is removed or an anti-strange quark is created, for example in Λ → p + π⁻. A change of −1 occurs when a strange quark is created or an anti-strange quark removed, as in K⁺ → μ⁺ + ν_μ. Changes of magnitude greater than 1 are not observed in a single weak vertex.

What earns the marks

• Strangeness is conserved in strong and electromagnetic interactions but not necessarily in weak interactions.
• A weak interaction may leave total strangeness unchanged, giving a change of 0.
• A weak interaction may increase total strangeness by 1, for example when a strange quark is removed or an anti-strange quark is produced.
• A weak interaction may decrease total strangeness by 1, for example when a strange quark is produced or an anti-strange quark is removed.
• Changes of +2, −2 or larger are not permitted in a single weak interaction.
• When checking a decay, calculate total strangeness on each side using quark content and confirm the difference is 0, +1 or −1.

Common mistakes and how to put them right

• Assuming strangeness is always conserved in every interaction.

  Put it right by stating that conservation applies to strong and electromagnetic interactions, while weak interactions may change strangeness by 0, +1 or −1.

• Treating a change of +1 and a change of −1 as interchangeable.

  Put it right by tracking the sign carefully, since creating a strange quark lowers strangeness by 1 while removing one raises it by 1.

• Believing any size of strangeness change is possible in a weak decay.

  Put it right by remembering that only 0, +1 or −1 are observed.

• Forgetting that antiparticles have opposite strangeness to their particles.

  Put it right by assigning strangeness −1 to a strange quark and +1 to an anti-strange quark before summing.

Exam tips

• Write the strangeness of each particle underneath its symbol before and after the arrow, then subtract to find the change.
• If a question asks whether a decay is possible, check charge, baryon number and strangeness in turn and state which conservation rule is violated if it is not possible.
• Learn one example each of a strangeness-conserving weak decay and a strangeness-changing weak decay so you can reason by analogy.
• Use the sign convention consistently: strange quark has strangeness −1 and anti-strange quark has strangeness +1.