Conservation of strangeness in strong interactions.

The idea

In a strong interaction the total strangeness of the reacting particles equals the total strangeness of the products. This rule follows from the fact that the strong force acts on quarks and cannot change a strange quark into a non-strange quark or create a strange quark without its antiquark. For example, in π⁻ + p → K⁰ + Λ⁰ the initial strangeness is 0 + 0 = 0, while the products give +1 + (−1) = 0, so the reaction is allowed. In contrast, a reaction such as p + p → p + Σ⁺ would have initial strangeness 0 and final strangeness −1, so it cannot proceed by the strong interaction. Strangeness is not conserved in weak interactions, which is why strange particles can decay singly, for example Λ⁰ → p + π⁻.

What earns the marks

• Total strangeness is conserved in strong interactions, so the sum of s values before equals the sum after.
• Strong interactions cannot alter quark flavour, so a strange quark remains a strange quark and a strange antiquark remains a strange antiquark.
• A reaction with unequal total strangeness on the two sides is forbidden as a strong interaction.
• Weak interactions do not conserve strangeness, so they provide the route by which strange particles decay.
• Checking strangeness conservation is a standard test for whether a proposed reaction can occur via the strong force.
• The rule applies to all strongly interacting particles in the reaction, including mesons and baryons.

Common mistakes and how to put them right

• Applying strangeness conservation to every interaction, including weak decays.

  Put it right by identifying the interaction type first and only requiring conservation for strong and electromagnetic processes.

• Adding strangeness values without regard to the sign carried by antiparticles.

  Put it right by assigning s = +1 to a strange antiquark and s = −1 to a strange quark.

• Assuming that a reaction is allowed simply because charge and baryon number balance.

  Put it right: Also checking strangeness when strange particles are present.

Exam tips

• Label each particle with its strangeness value before and after the arrow, then compare the two totals in one line.
• If a proposed reaction fails strangeness conservation, state that it cannot occur by the strong interaction rather than saying it is impossible in all circumstances.
• Link the conservation check to the interaction type named in the question, since weak processes are treated differently.