New experimental limits on neutron – mirror neutron oscillations in the presence of mirror magnetic field

Present experimental and astrophysical limits do not exclude that the neutron (n) oscillation into mirror neutron ($$n'$$), a sterile state exactly degenerate in mass with the neutron, can be a very fast process, in fact faster than the neutron decay itself, in which case it would have very interesting implications in cosmology and astrophysics. This process is sensitive to the magnetic field. Namely, if the mirror magnetic field $$\mathbf {B}'$$ exists at the Earth, $$n{-}n'$$ oscillation probability can be suppressed or resonantly amplified by the applied magnetic field $$\mathbf {B}$$, depending on its strength and on the angle $$\beta $$ between $$\mathbf {B}$$ and $$\mathbf {B}'$$. We present the results of ultra-cold neutron storage measurements aiming to check the anomalies observed in previous experiments which could be a signal for $$n{-}n'$$ oscillation in the presence of mirror magnetic field $$B'\sim 0.1$$ G. From the analysis of the experimental data new lower limits on $$n{-}n'$$ oscillation time as a function of $$B'$$ were obtained, assuming that the mirror magnetic field is constant in time: $$\tau _{nn'} > 17$$ s (95 % C.L.) for any $$B'$$ between 0.08 and 0.17 G, and $$\tau _{nn'}/\sqrt{\cos \beta } > 27$$ s (95 % C.L.) for any $$B'$$ in the interval ($$0.06\div 0.25$$) G