  Models that postulate the existence of hidden sectors address contemporary
questions, such as the source of baryogenesis and the nature of dark matter.
Among the possible mixing processes, neutron-to-hidden-neutron oscillations
have been repeatedly tested with ultra-cold neutron storage and
passing-through-wall experiments in the range of small ($\delta m<2$ peV) and
large mass splitting ($\delta m>10$ neV), respectively. In this work, we
present a new constraint in the oscillation parameter space derived from
neutron disappearance in ultra-cold neutron beam experiments. The overall
limit, which covers the intermediate mass-splitting range, is given by
$\tau_{nn'}> 1$ s for $|\delta m| \in [2,69]$ peV (95\% C.L.).
