Mass eigenstate
= Mass eigenstate
{title2=$M|n\rangle=m_n|n\rangle$}
A <mass eigenstate> diagonalizes the Hermitian <mass> operator or <mass matrix> and has a definite <mass>. In a flavor basis with off-diagonal <mass> terms, it is a linear combination of flavor states. For unstable particles, decay propagation instead involves an effective complex matrix; its <eigenvectors> need not inherit the orthogonality of a Hermitian <mass> problem.