EDM (under construction)

EDM scheme EDM measurement facility.

EDM measurement facility.

An electric dipole moment (EDM) of the neutron would be a clear sign of new physics beyond known particle physics. The search for this phenomenon is considered one of the most important experiments in fundamental physics and could provide key information on the excess of matter versus antimatter in the Universe. With high measurement precision, this experiment aims to ultimately achieve a sensitivity of 10^-28 ecm for a charge distribution within the neutron. This can be interpreted as a probe of the early Universe, less than 10^-11 s after the big bang, complementary to experiments at CERN and also beyond the reach of accelerators. In the Neutron Guide Hall East, a new instrument to search for the neutron EDM is currently being commissioned.

At a later stage, ultra cold neutrons (UCN) will be provided by a source in beam tube SR-6. In the instrument’s centre, the UCN are trapped in a box, where Ramsey’s method of separated oscillatory fields is used to precisely measure Larmor precession of UCN over hundreds of seconds as an interferometer in time in a small (about 1 μT) magnetic field. A deviation of the Larmor precession in the simultaneous presence of the magnetic field and a strong electrostatic field would be an indication for an EDM. While the UCN source will provide the required neutron density of 1000 per cm³ in the instrument, it itself is optimised to control any systematic effects.

A dominating issue to achieve this precision is the quality of the magnetic environment. Currently, it provides an extended volume (~0.5 m³) of 100 pT or less absolute field and a temporal stability of < 10 fT in 1000 s without any active measures. This corresponds to about the magnetic field in between stars in the milky way.

The shielding factor against external distortions of 6 Mio at 1 MHz and > 10⁸ at 10 Hz. Inside this volume, magnetic fields of up to few μT can be applied vertically and horizontally with a homogeneity of better than up to 10^-4 relative precision. During the commissioning phase, the volume is used for optical magnetometry and precision spin-clock measurements, as well as for the comparison of precision magnetometers. To apply electrostatic fields, a bipolar 200 kV source with stabilisation on Pockels effect is implemented.