Speaker
Description
The n2edm experiment, hosted at the PSI ultracold neutron (UCN) source, aims to measure the permanent neutron electric dipole moment, $d_n$, with a precision of at least $10^{-27}$ $e$ cm to probe charge conjugation and parity (CP) violation in beyond standard model physics [1]. It has evolved from the previous nedm experiment that took data at PSI from 2015 to 2017, resulting in the current upper limit of $|d_n| < 1.8 \times 10^{-26}$ $e$ cm to a 90 % confidence level [2]. The heart of the n2edm apparatus is two stacked storage chambers used for Ramsey spectrometry of UCNs, allowing $d_n$ to be measured in simultaneous parallel and anti-parallel electric field orientations relative to a uniform magnetic field of $B_z ≅ ±1$ μT. With the setup operating as designed, it is expected that a daily statistical sensitivity of $\sigma (d_n) = 2.6 × 10^{-26}$ e cm can be achieved. A critical condition for this level of sensitivity is the quantification of the magnetic field drift, requiring a co-magnetometer capable of measuring $B_z$ to a precision of $\sigma (B_z) = 25$ fT within a UCN precession cycle. Furthermore, systematic effects that generate false $d_n$ signatures due to harmonic modes of the magnetic field, or due to magnetic dipole contaminants must be controlled or quantified to a level of accuracy better than the final $d_n$ precision goal.
These challenges have been addressed with the gradual installation of several n2edm subsystems. For example, to correct for field drifts, we have developed a Hg-199 magnetometer that now routinely measures $\sigma(B_z) \approx 5$ fT. For in-situ determination of harmonic B-field modes, we have installed an array of 56 optically-pumped Cs magnetometers. Finally, for magnetic impurity control, we have adopted a strategy to meticulously minimise and catalogue magnetic contaminants using an offline gradiometer.
This contribution gives a broad overview of the progress and status of the n2edm experiment as a whole, while highlighting the subsystems and their relation to projected statistical and systematic uncertainty goals. It additionally discusses the key challenges that have been faced in bringing these systems from design to reality and discuss the main global limitations that we face to reach the $\sigma(d_n)= 10^{-27}$ e cm precision frontier.
Refs:
[1] Ayres, N. J., et al. "The design of the n2EDM experiment." EPJ C 81.6 (2021): 512.
[2] Abel, C, et al. "Measurement of the permanent electric dipole moment of the neutron." PRL 124.8 (2020): 081803.