# Mass difference between K0 and K0-bar and other meson-antimeson pairs

 P: 90 Mass difference between K0 and K0-bar and other meson-antimeson pairs $K^{0}$ and $\bar{K^{0}}$ (and other examples you gave) are not mass eigenstates. The mass difference which determines the rate of oscillations in these systems is the mass difference between the two mass eigenstates of the system. If you would write the mass matrix in the $K^{0}$, $\bar{K^{0}}$ basis you would get that the diagonal terms are equal ( due to CPT, as you said) but the off diagonal term (due to $K^{0}\leftrightarrow\bar{K^{0}}$ oscillations) would cause splitting in mass between the mass eigenstates. The mass eigenstates are not conjugates of eachother.
 Mentor P: 11,589 $$M=\begin{pmatrix} M_{11} & M_{12} \\ M_{21} & M_{22} \end{pmatrix}$$ +CPT => ##M_{11}=M_{22}## +CP => ##M_{12}=M_{21}## The mass matrix has two different eigenvalues, their difference depends on the relative strength of M12 to M11.