KL − KS mass difference and quark models

The matrix element $\text{〈}\text{K}\text{?}{}^0\text{|H}{}_{\mathrm{<mtext>W</mtext>}}\text{|}\text{K}{}^0\text{〉}$ is calculated in the quark MIT bag model and in several relativized versions of the quark harmonic-oscillator models for hadrons. The results of the MIT bag model depend strongly on the particular choice of parameters, while the results of the harmonic-oscillator model are relatively stable on the variation of model parameters. The harmonic-oscillator model which takes into account the c.m. motion (the RHO model) is in reasonably good agreement with the vacuum-saturation value.