The PanEDM Experiment at ILL

Our group is part of an international collaboration called PanEDM, based at the Institute Laue-Langevin (ILL) in Grenoble, France. The goal of PanEDM is to search for an electric dipole moment of the neutron (nEDM) with a sensitivity better than 10-27 ecm, corresponding to an energy resolution of 10-42 Joule, making it one of the most precise measurements ever performed. 


A nEDM would be a manifestation of new physics beyond the standard model of particle physics, as it would be time reversal symmetry violating. With the lack of signals for new physics at the LHC, the complementary approach using precision measurements like EDM searches becomes increasingly important. Our experiment deploys Ramsey’s method of separated oscillatory fields, applied to UCN stored at room temperature in tow chambers. A change of the Larmor precession due to an electric field applied across the UCN storage chambers would be a sign of an EDM. Using one of the worldwide strongest sources of UCN, the new SuperSUN source of UCN, together with the world’s strongest magnetic shield, we hope to obtain a leading result over the next years.

Our experiment deploys Ramsey's method of separated oscillatory fields applied to UCN stored at room temperature in a double-chamber layout, combined with different means of magnetometry. Using one of the worldwide strongest sources of UCN that are currently being built, we envisage a statistical accuracy of σdstat < 5 x 10-28 ecm (3σ) with 200 days of data. Our overall systematic error budget is σdsys < 2 x 10-28 ecm, which requires a major effort in the control of spurious effects that may falsely mimic an EDM. To control systematic effects the laboratory for physics with UCN at the FRM-II provides a specialized environment for such a precision experiment. A large-scale clean room environment with a sophisticated low environmental- and magnetic distortion suppression system is currently being realized.

It houses a unique multi-coil magnetic field compensation system with 180 channels that can compensate higher order gradients at even close distances to the installation. Inside the compensated area a vibration controlled and height adjustable table is placed that supports a large scale walk-in magnetically shielded room. The room is currently being fabricated. It serves as the outermost shell of an inner magnetic shield that can be slided inside on rails. Combined the passive shields reduce external fluctuations on the level of 1 µT at quasi-DC by more than five orders of magnitude. Inside this shield, a homgeneous magnetic field is generated, together with a low-field NMR system. The innermost part is a non-magnetic vacuum system that houses the EDM measurement chambers, which are filled with ultra-cold neutrons and nuclear spin polarized vapors

In addition to the double-co-magnetometer arrangement, cells with free induction decay magnetometers are placed on top and bottom of the UCN chambers. Four out-side-accessible tubes reach through the whole experiment to allow access with magnetometers while the experiment is fully operational, e.g. to map fields online or monitor fluctuations and drifts with SQUIDs and atomic-vapor-based systems. By combining these methods, one of the most precise clock comparison experiments will be realized. A comprehensive list of systematic effects relevant for a current generation experiment is shown in C. A. Baker et al., PRL 97 (2006) 131801. Most magnetic field related effects will be significantly reduced with a high quality magnetically shielded environment. Geometric phase effects of polarized 199Hg atoms used for magnetometry are e.g. been dealt with a tunable laser based read out of magnetometers. Further, different magnetometer systems can be combined without modifying the setup. Effects that are relevant for the neutrons will be investigated using a method to adjust the energy spectrum of the polarized ultra-cold neutrons. This system is located at the beam line and can modify the spectrum without changing any parameter inside the EDM spectrometer.


  • Wurm, D. et al. The PanEDM neutron electric dipole moment experiment at the ILL. EPJ Web Conf. 219, 02006 (2019) doi: 10.1051/epjconf/201921902006.
  • Chupp, T. E., Fierlinger, P., Ramsey-Musolf, M. J. & Singh, J. T. Electric dipole moments of atoms, molecules, nuclei, and particles. Rev. Mod. Phys. 91, 015001 (2019) doi: 10.1103/RevModPhys.91.015001.
  • Altarev, I. et al. A magnetically shielded room with ultra low residual field and gradient. Review of Scientific Instruments 85, 075106 (2014) doi: 10.1063/1.4886146.
  • Altarev, I. et al. A large-scale magnetic shield with 106 damping at millihertz frequencies. Journal of Applied Physics 117, 183903 (2015) doi: 10.1063/1.4919366.


Prof. Peter Fierlinger