Experimental search for neutron - mirror neutron oscillations using storage of ultracold neutrons

The idea of a hidden sector of mirror partners of elementary particles has attracted considerable interest as a possible candidate for dark matter. Recently it was pointed out by Berezhiani and Bento that the present experimental data cannot exclude the possibility of a rapid oscillation of the neutron n to a mirror neutron n' with oscillation time much smaller than the neutron lifetime. A dedicated search for vacuum transitions n->n' has to be performed at weak magnetic field, where both states are degenerate. We report the result of our experiment, which compares rates of ultracold neutrons after storage at a weak magnetic field well below 20 nT and at a magnetic field strong enough to suppress the seeked transitions. We obtain a new limit for the oscillation time of n-n' transitions, tau_osc (90% C.L.) > 414 s. The corresponding limit for the mixing energy of the normal and mirror neutron states is delta_m (90% C.L.) < 1.5x10-18 eV.