We present for the first time a detailed and comprehensive analysis of the experimental results that set the current world sensitivity limit on the magnitude of the electric dipole moment (EDM) of the neutron. We have extended and enhanced our earlier analysis to include recent developments in the understanding of the effects of gravity in depolarizing ultracold neutrons; an improved calculation of the spectrum of the neutrons; and conservative estimates of other possible systematic errors, which are also shown to be consistent with more recent measurements undertaken with the apparatus. We obtain a net result of $d_{\mathrm{n}}=-0.21\pm 1.82\times 10^{-26}e\mathrm{cm}$, which may be interpreted as a slightly revised upper limit on the magnitude of the EDM of $3.0\times 10^{-26}e\mathrm{cm}$ (90% C.L.) or $3.6\times 10^{-26}e\mathrm{cm}$ (95% C.L.).

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.

A clock comparison experiment, analyzing the ratio of spin precession frequencies of stored ultracold neutrons and ${}^{199}\mathrm{Hg}$ atoms, is reported. No daily variation of this ratio could be found, from which is set an upper limit on the Lorentz invariance violating cosmic anisotropy field $b_{\perp }<2\times {10}^{-20}\mathrm{eV}$ (95% C.L.). This is the first limit for the free neutron. This result is also interpreted as a direct limit on the gravitational dipole moment of the neutron $|g_n|<0.3\mathrm{eV}/c^2\mathrm{m}$ from a spin-dependent interaction with the Sun. Analyzing the gravitational interaction with the Earth, based on previous data, yields a more stringent limit $|g_n|<3\times {10}^{-4}\mathrm{eV}/c^2\mathrm{m}$.

We performed ultracold neutron storage measurements to search for additional losses due to neutron ($n$) to mirror-neutron ($n^{\prime }$) oscillations as a function of an applied magnetic field $B$. In the presence of a mirror magnetic field $B^{\prime }$, ultracold neutron losses would be maximal for $B\approx B^{\prime }$. We did not observe any indication for $n{n}^{\prime }$ oscillations and placed a lower limit on the oscillation time of ${\tau }_{n{n}^{\prime }}>12.0\mathrm{s}$ at 95% C.L. for any $B^{\prime }$ between 0 and $12.5\mu \mathrm{T}$.

International audienceThe measurement of the neutron electric dipole moment (nEDM) constrains the contribution of CP-violating terms within both the Standard Model and its extensions. The experiment uses ultracold neutrons (UCN) stored in vacuum at room temperature. This technique provided the last (and best) limit by the RAL/Sussex/ILL collaboration in 2006: dn < 2:9 × 10-26 e cm (90% C.L.). We aim to improve the experimental sensitivity by a factor of 5 within 2-3 years, using an upgrade of the same apparatus. We will take advantage of the increased ultracold neutron density at the Paul Scherrer Institute (PSI) and of a new concept including both, external magnetometers and a cohabiting magnetometer. In parallel, a next generation apparatus with two UCN storage chambers and an elaborate magnetic field control is being designed aiming to achieve another order of magnitude increase in sensitivity, allowing us to put a limit as tight as dn < 5 × 10-28 e cm (95% C.L.), if not establishing a finite value

The Surrounding Field Compensation (SFC) system described in this work is installed around the four-layer Mu-metal magnetic shield of the neutron electric dipole moment spectrometer located at the Paul Scherrer Institute. The SFC system reduces the DC component of the external magnetic field by a factor of about 20. Within a control volume of approximately 2.5 m × 2.5 m × 3 m, disturbances of the magnetic field are attenuated by factors of 5–50 at a bandwidth from ${10}^{-3}$ Hz up to 0.5 Hz, which corresponds to integration times longer than several hundreds of seconds and represent the important timescale for the neutron electric dipole moment measurement. These shielding factors apply to random environmental noise from arbitrary sources. This is achieved via a proportional-integral feedback stabilization system that includes a regularized pseudoinverse matrix of proportionality factors which correlates magnetic field changes at all sensor positions to current changes in the SFC coils.