2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/190606It is shown that there is a double counting in the standard model of $n\bar {n}$ mixing in the medium, resulting in full cancellation of leading terms. The direct calculation of $n\bar{n}$ transition followed by annihilation is performed. The lower limit for the free-space $n\bar{n}$ oscillation time is $τ_{n\bar{n}} \sim T_{n\bar{n}}>10^{16}y$, where $T_{n\bar{n}}$ is the lifetime of neutron bound in a nucleus. This limit exceeds the previous one by 16 orders of magnitude.12 Latex pagesHigh Energy Physics - PhenomenologyNuclear Theorytext