  The possibility that a neutron can be transformed to a hidden sector particle
remains intriguingly open. Proposed theoretical models conjecture that the
hidden sector can be represented by a mirror sector, and the neutron n can
oscillate into its sterile mirror twin n', exactly or nearly degenerate in mass
with n. Oscillations n - n' can take place in vacuum and in the environment of
the regular matter and the magnetic field where only neutron will be subject of
interaction with the environment. We describe the propagation of the
oscillating n - n' system as a particle of the cold neutron beam passing
through the dense absorbing materials in connection with the possible
regeneration type of experiments where the effect of n -> n' -> n
transformation can be observed.
