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A strange particle, $X$, decays in the following way: $X → π^– + p$. State what interaction is involved in this decay.

I know the answer to be weak interaction, but why is it weak interaction? What is the reasoning? Why could it not be a strong interaction? Is it merely because the particle is a strange particle?

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Try to think about the adjective "strange". Could this adjective be compatible with strong interactions? Doesn't the latter preserve... something? – Luboš Motl Jan 2 at 17:05
Ahh, is it because strong interactions preserve strangeness, but in this case, the RHS has 0 strangeness whilst we know that the LHS should have a strangeness? – Wk_of_Angmar Jan 2 at 17:12
@Wk_of_Angmar That's the ticket. – dmckee♦ Jan 2 at 18:09

1 Answer

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Strong interactions conserve strangeness. From the information given, X has a strangeness ≠ 0 since it is a strange particle. However, the right hand side of the equation has a strangeness = 0. Therefore, since the strangeness has not been conserved, it cannot be a strong interaction and this makes it a weak interaction.

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