Relativistic harmonic-oscillator quark model andK→ππdecays

It is shown how the theoretical description of $K\to \pi \pi$ decays depends on the particular quark model. The relativistic MIT bag model is compared with the nonrelativistic harmonic-oscillator model and with two relativistically corrected versions of the harmonic-oscillator model. The relativistic effects in quark dynamics seem to be crucial for the understanding of $K\to \pi \pi$ transitions.