= Kaon bag parameter
{title2=$B_K$}
For relativistically normalized <kaon> states and $Q=(\bar s\gamma_\mu(1-\gamma^5)d)(\bar s\gamma^\mu(1-\gamma^5)d)$, define $\langle\bar K^0|Q|K^0\rangle=(8/3)f_K^2m_K^2B_K$. The dimensionless kaon bag parameter measures <strong interaction> corrections to the vacuum-insertion normalization. The coefficient and <matrix element> must be defined in the same <renormalization scheme> and at the same scale. Dividing by $2m_K$ gives the corresponding two-state mixing-Hamiltonian normalization.
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