Experimental bounds on sterile neutrino mixing angles
  We derive bounds on the mixing between the left-chiral ("active") and the
right-chiral ("sterile") neutrinos, provided from the combination of neutrino
oscillation data and direct experimental searches for sterile neutrinos. We
demonstrate that the mixing of sterile neutrinos with any flavour can be
significantly suppressed, provided that the angle theta_13 is non-zero. This
means that the lower bounds on sterile neutrino lifetime, coming from the
negative results of direct experimental searches can be relaxed (by as much as
the order of magnitude at some masses). We also demonstrate that the results of
the negative searches of sterile neutrinos with PS191 and CHARM experiments are
not applicable directly to the see-saw models. The reinterpretation of these
results provides up to the order of magnitude stronger bounds on sterile
neutrino lifetime than previously discussed in the literature. We discuss the
implications of our results for the Neutrino Minimal Standard Model (the
NuMSM).
