Speaker
Description
The n2EDM experiment at the Paul Scherrer Institute (PSI) aims to reach a neutron electric-dipole-moment (nEDM) sensitivity of $1\times10^{-27}~e\cdot\mathrm{cm}$. At this sensitivity, magnetic-field nonuniformities must be tightly controlled. In particular, odd-order harmonic modes contribute to a motional false EDM through the $^{199}\mathrm{Hg}$ co-magnetometer. These modes must be sufficiently reproducible to constrain the associated false-EDM contribution to below $3\times10^{-28}~e\cdot\mathrm{cm}$ [1].
We combine online and offline magnetic-field measurements to monitor the field at the required precision [2]. During neutron operation, the field is continuously monitored outside the precession chambers by the currently installed array of 56 cesium magnetometers (CsMs). During shutdown periods, a robotic mapper measures all three field components throughout the precession volume with substantially higher spatial resolution.
We present results from a magnetic-field mapping campaign conducted in 2025, comprising harmonic analyses of several field configurations of the n2EDM apparatus. To determine the reproducibility of the relevant harmonic coefficients, multiple independent maps were acquired for each configuration. Between successive measurements, the magnetic environment was reset by opening and closing the magnetic shield and subsequently degaussing it. We determined that all relevant phantom modes are reproducible up to $23~\mathrm{fT/cm}$, meeting the design requirement that their combined motional false-EDM contribution remain below $3\times10^{-28}~e\cdot\mathrm{cm}$ [1].
Besides measuring the performance of the n2EDM coil system these maps also provide an empirical basis for developing the CsM field-gradient reconstruction. The measured distributions and correlations of the harmonic coefficients can be used to select the harmonic expansion order, identify modes that are poorly constrained by the CsM array, and define prior constraints for the regression from CsM measurements to field-gradient coefficients. Validation of this mapping-informed reconstruction, including quantification of its bias and uncertainty, is ongoing. The resulting framework will support online field reconstruction and the assessment of associated systematic uncertainties during neutron operation.
References
[1]: C. Abel et al., “Generating a highly uniform magnetic field inside the magnetically shielded room of the n2EDM experiment,” Eur. Phys. J. C 85, 202 (2025). https://doi.org/10.1140/epjc/s10052-025-13902-x
[2]: N. J. Ayres et al., “The design of the n2EDM experiment,” Eur. Phys. J. C 81, 512 (2021). https://doi.org/10.1140/epjc/s10052-021-09298-z