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Publication

A new precise measurement of the neutron electric dipole moment

Book - Dissertation

The neutron electric dipole moment (nEDM) experiment at the Paul Scherrer Institute in Switzerland is one of many experiments across the globe that are searching for the electric dipole moment of different particles, atoms and molecules. These experiments are searching for signs of time-reversal violation in fundamental systems, one of the missing ingredients that are necessary to explain why our Universe is predominantly made of matter and not antimatter. Several theories beyond the Standard Model of Particle Physics, such as supersymmetric models or multi-Higgs models, predict a neutron electric dipole moment in size very close to the sensitivity of current and next-generation experiments, making it an excellent observable to probe for physics beyond the Standard Model. The concept of the nEDM experiment is to measure the Larmor precession frequency of the neutron when exposed to parallel electric and magnetic fields. Upon reversing the direction of the electric field, the precession frequency of the neutron would be modified if its electric dipole moment is non-zero. One of the factors that have become crucial at the current level of sensitivity is the homogeneity and stability of the magnetic field while measuring the precession frequency. This work describes the addition of an array of caesium magnetometers to the nEDM experiment to measure the magnetic field and its gradients across the precession volume of the neutrons. The sensitivity and accuracy of the sensors have been characterised in detail, both as a scalar magnetometer and as a vector magnetometer, and the performance of the array in terms of monitoring the magnetic-field gradients has been determined. A procedure to optimise the homogeneity of the magnetic field was developed based on input from the caesium magnetometer array, resulting in a factor of two longer neutron spin coherence times while suppressing systematic effects, effectively increasing the nEDM sensitivity by 35%. Making use of the increased sensitivity, the nEDM experiment has taken data in 2015 and 2016. A first statistical analysis using the Cs magnetometers to correct for magnetic-field gradient drifts is presented in this dissertation, and the final value of (0.0 ± 1.1stat ± 0.2sys) × 10^{–26} e cm sets the most stringent limit up to date on the size of the neutron electric dipole moment.
Publication year:2021
Accessibility:Open