Field theory approach to K0-K0bar and B0-B0bar systems

Quantum field theory provides a consistent framework to deal with unstable particles. We present here an approach based on field theory to describe the production and decay of unstable $K^0-\overline{K^0}$ and $B^0-\overline{B^0}$ mixed systems. The formalism is applied to compute the time evolution amplitudes of $K^0$ and $\overline{K^0}$ studied in DAPHNE and CPLEAR experiments. We also introduce a new set of parameters that describe CP violation in $K\to \pi \pi$ decays without recourse to isospin decomposition of the decay amplitudes.