Realization of a high-performance laser-based mercury magnetometer for neutron EDM experiments

Abstract

At the Paul Scherrer Institute an international collaboration is searching for the permanent electric dipole moment of the neutron (nEDM). The experiment is strongly motivated by finding a new source of CP violation, which would be an important guide to an explanation of baryon asymmetry of the Universe. Ramsey’s method of separated oscillating fields is used to detect an electric field dependent shift in the Larmor frequency of stored ultra-cold neutrons. Very sensitive magnetometers are required to correct or statistical and systematic uncertainties related to magnetic field fluctuations. This work describes the realization of a laser-based mercury co-magnetometer fulfilling the performance requirements for the highest possible sensitivity of the next generation n2EDM experiment. The laser light was reliably stabilized in frequency, position and power. A magnetic field sensitivity exceeding the requirement by a factor two was demonstrated with measurements at the currently running nEDM apparatus. Detailed studies of the currently running lamp-based magnetometer system led to a significant increase of the system performance and reliability during nEDM measurements. An overall contribution to the statistical nEDM uncertainty of less than 3% induced by the co-magnetometer was achieved, which is well below the target value of 5%. The geometric phase effect is a major systematic effect in nEDM experiments introduced by the Hg co-magnetometer. The linear dependence on the electric field in linear field gradients was verified. A first direct measurement of the geometric phase effect in cubic field gradients is compared to the theoretical expectation. This measurement confirmed the importance of considering higher order field gradients and has triggered the development of new methods to compensate the resulting systematic effect in the nEDM experiment.