= Kaon mixing square-root branch convention
{title2=$\sqrt z\sqrt{z^*}=|z|$}
In a Hermitian neutral-<kaon> <mass matrix>, independently choosing square-root signs of $z$ and $z^*$ can interchange the <eigenvalue> labels. Choose $z=re^{i\varphi}$, $a=\sqrt r e^{i\varphi/2}$ and $b=\sqrt r e^{-i\varphi/2}$, so $ab=r>0$, continuously from the desired <CP>-conserving limit. Then $(a,-b)$ is the lower-<mass> <eigenvector>. At a degenerate off-diagonal zero the <mass> eigenbasis is not fixed by diagonalization alone.
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