2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/44593Quasi-degenerate neutral systems like a (K, \bar{K}) type are investigated in Quantum Field Theory (QFT). A constant mass matrix as the one used in Quantum Mechanics (QM) can only be introduced as a linear approximation to QFT. We study the phenomenological consequences of the differences between the QFT and QM treatments. The role of ``spurious'' states with zero norm at the poles is emphasized. The K_L-K_S mass splitting triggers a tiny difference between the CP violating parameters ε_L and ε_S, without any violation of TCP. Non-vanishing semi-leptonic asymmetry delta_S - δ_L does not signal TCP violation (usual claims not withstanding), while A_{TCP} keeps vanishing when TCP is good symmetry.13 pages, LaTeXHigh Energy Physics - Phenomenologytext