A fresh look at neutral meson mixing
  In this work we show that the existence of a complete biorthonormal set of eigenvectors of the effective Hamiltonian governing the time evolution of neutral meson system is a necessary condition for diagonalizability of such a Hamiltonian. We also study the possibility of probing the $CPT$ invariance by observing the time dependence of cascade decays of type $P^{\circ}(\bar{P^{\circ}})\to \{M_a,M_b\}X\to fX$ by employing such basis and exactly determine the $CPT$ violation parameter by comparing the time dependence of the cascade decays of tagged $P^{\circ}$ and tagged $\bar{P^{\circ}}$. 
M.A. Gomshi Nobary and B. Mojaveri