Direct Experimental Limit on Neutron–Mirror-Neutron Oscillations

In case a mirror world with a copy of our ordinary particle spectrum would exist, the neutron $n$ and its degenerate partner, the mirror neutron $n^{\prime }$, could potentially mix and undergo $n{n}^{\prime }$ oscillations. The interaction of an ordinary magnetic field with the ordinary neutron would lift the degeneracy between the mirror partners, diminish the $n^{\prime }$ amplitude in the $n$ wave function and, thus, suppress its observability. We report an experimental comparison of ultracold neutron storage in a trap with and without superimposed magnetic field. No influence of the magnetic field is found and, assuming negligible mirror magnetic fields, a limit on the oscillation time ${\tau }_{n{n}^{\prime }}>103\mathrm{s}$ (95% C.L.) is derived.

Figure 1

The count ratio $N_{0/\uparrow \downarrow }$ as a function of the oscillation time [see Eq. (7)]. The dashed region indicates the allowed region of Eq. (9). The error band of the curve displays the systematic uncertainties; see text. The $\mathsf{x}$ indicates the point at which the limit on ${\tau }_{n{n}^{\prime }}$ is evaluated.