So the kaon particle (K) and the sigma particle (Σ) are created very quickly through the strong interaction and decay slowly through the weak interaction.

How is this so? Is this not some kind of discrepancy? What is the explanation for this?

An internet search has lead me to determine that it could be something to do with CP violation. Could it also be to do with the fact that they tend to decay into pions and protons?

I don't want a fully complex mathematical explanation, just a short qualitative one would be great. It's from a past exam paper, and is just a short 2 mark question.


2 Answers 2


Kaons and sigmas contain strange quarks, so the "ground state" particles must decay by changing quark flavour. The strong and electromagnetic interactions cannot change flavour, but the weak interaction can, hence they can only decay weakly.

The strong interaction can easily produce $s\overline{s}$ pairs, which can subsequently pair up with lighter quarks to produce strange hadrons (including kaons and sigmas) without the need for any flavour-changing processes.


What one has to keep in mind: that the strong interaction is called strong because it is terribly binding. Quarks interact with all three, strong, weak and electromagnetic, and a strong interaction can produce quark antiquark pairs, conserving all quantum numbers, thus the strange mesons ( and baryons) will be coming in pairs, one having grabbed an s quark and another an anti-s quark. Once paired into a meson or baryon the quarks are stable, and if there were no weak interaction , they would never decay. It is the weak interaction that allows for the decay, makes accessible the lower mass energy states of the decay channels.


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