Neutron lifetime puzzle and neutron -- mirror neutron oscillation

The discrepancy between the neutron lifetimes measured in the beam and trap experiments can be explained via the neutron $n$ conversion into mirror neutron $n^{\mathrm{\prime }}$, its dark partner from parallel mirror sector, provided that $n$ and $n^{\mathrm{\prime }}$ have a tiny mass splitting order ${10}^{-7}$ eV. In large magnetic fields used in beam experiments $n-n^{\mathrm{\prime }}$ transition is resonantly enhanced and can transform of about a per cent fraction of neutrons into mirror neutrons which decay in invisible mode. Thus less protons will be produced and the measured value ${\tau }_{\mathrm{b}\mathrm{e}\mathrm{a}\mathrm{m}}$ appears larger than $\beta$-decay time $\tau_{\beta} = \tau_{\rm trap}$. Some phenomenological and astrophysical consequences of this scenario are also briefly discussed.