We present a magnetically shielded environment with a damping factor larger than one million at the mHz frequency regime and an extremely low field and gradient over an extended volume. This extraordinary shielding performance represents an improveme
nt of the state of the art in damping the difficult regime of very low-frequency distortions by more than an order of magnitude. This technology enables a new generation of high precision measurements in fundamental physics and metrology, including searches for new physics far beyond the reach of accelerator-based experiments. We discuss the technical realization of the shield with its improvements in design.
An increasing number of measurements in fundamental and applied physics rely on magnetically shielded environments with sub nano-Tesla residual magnetic fields. State of the art magnetically shielded rooms (MSRs) consist of up to seven layers of high
permeability materials in combination with highly conductive shields. Proper magnetic equilibration is crucial to obtain such low magnetic fields with small gradients in any MSR. Here we report on a scheme to magnetically equilibrate MSRs with a 10 times reduced duration of the magnetic equilibration sequence and a significantly lower magnetic field with improved homogeneity. For the search of the neutrons electric dipole moment, our finding corresponds to a linear improvement in the systematic reach and a 40 % improvement of the statistical reach of the measurement. However, this versatile procedure can improve the performance of any MSR for any application.
Physics at the Planck scale could be revealed by looking for tiny violations of fundamental symmetries in low energy experiments. In 2008, a sensitive test of the isotropy of the Universe using has been performed with stored ultracold neutrons (UCN),
this is the first clock-comparison experiment performed with free neutrons. During several days we monitored the Larmor frequency of neutron spins in a weak magnetic field using the Ramsey resonance technique. An non-zero cosmic axial field, violating rotational symmetry, would induce a daily variation of the precession frequency. Our null result constitutes one of the most stringent tests of Lorentz invariance to date.
We propose an original test of Lorentz invariance in the interaction between a particle spin and an electromagnetic field and report on a first measurement using ultracold neutrons. We used a high sensitivity neutron electric dipole moment (nEDM) spe
ctrometer and searched for a direction dependence of a nEDM signal leading to a modulation of its magnitude at periods of 12 and 24 hours. We constrain such a modulation to $d_{12} < 15 times 10^{-25} e,{rm cm}$ and $d_{24} < 10 times 10^{-25} e,{rm cm}$ at 95~% C.L. The result translates into a limit on the energy scale for this type of Lorentz violation effect at the level of ${cal E}_{LV} > 10^{10}$~GeV.
We performed ultracold neutron (UCN) storage measurements to search for additional losses due to neutron (n) to mirror-neutron (n) oscillations as a function of an applied magnetic field B. In the presence of a mirror magnetic field B, UCN losses wou
ld be maximal for B = B. We did not observe any indication for nn oscillations and placed a lower limit on the oscillation time of tau_{nn} > 12.0 s at 95% C.L. for any B between 0 and 12.5 uT.
A clock comparison experiment, analyzing the ratio of spin precession frequencies of stored ultracold neutrons and $^{199}$Hg atoms is reported. %57 No daily variation of this ratio could be found, from which is set an upper limit on the Lorentz inva
riance violating cosmic anisotropy field $b_{bot} < 2 times 10^{-20} {rm eV}$ (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 $eV/$c^2$ 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} $eV/$c^2 $m.
A new measurement of the neutron EDM, using Ramseys method of separated oscillatory fields, is in preparation at the new high intensity source of ultra-cold neutrons (UCN) at the Paul Scherrer Institute, Villigen, Switzerland (PSI). The existence of
a non-zero nEDM would violate both parity and time reversal symmetry and, given the CPT theorem, might lead to a discovery of new CP violating mechanisms. Already the current upper limit for the nEDM (|d_n|<2.9E-26 e.cm) constrains some extensions of the Standard Model. The new experiment aims at a two orders of magnitude reduction of the experimental uncertainty, to be achieved mainly by (1) the higher UCN flux provided by the new PSI source, (2) better magnetic field control with improved magnetometry and (3) a double chamber configuration with opposite electric field directions. The first stage of the experiment will use an upgrade of the RAL/Sussex/ILL groups apparatus (which has produced the current best result) moved from Institut Laue-Langevin to PSI. The final accuracy will be achieved in a further step with a new spectrometer, presently in the design phase.
