2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202677The model-independent formalism is constructed to describe decays of mixed particles without using the Weisskopf-Wigner approximation. Limitations due to various symmetries are traced for neutral $K-$mesons. As an application we show that effects of $CPT-$violation and going beyond WWA may be separated and studied independently.LaTeX, 19 pages, no figuresHigh Energy Physics - Phenomenologytext