Characterization of the magnetically shielded room for the neutron electric dipole moment experiment at TRIUMF

Abstract

The search for the neutron Electric Dipole Moment (nEDM) lies at the intersection of particle physics and cosmology. A discovery of a non-zero nEDM would signal new physics beyond the Standard Model due to its CP-violating nature, potentially helping explain the matter-antimatter asymmetry in the early Universe. The Standard Model predicts a neutron EDM of |dn| ∼ 10^−31 e·cm, far below current experimental sensitivity. Present limits set |dn| < 1.8 × 10^−26 e·cm (90% C.L.). The TRIUMF Ultracold Advanced Neutron (TUCAN) collaboration aims to improve this by an order of magnitude, targeting σ(dn) ≤ 10^−27 e·cm. The standard method for nEDM measurement uses polarized Ultracold Neutrons (UCN) and the Ramsey technique of separated oscillatory fields. This requires highly uniform electric and magnetic fields, achieved within a Magnetically Shielded Room (MSR), which isolates the precession field from external interference. However, magnetic field inhomogeneities are a major source of systematic error, affecting UCN spin relaxation times and mimicking false EDM signals. Accurate characterization of the magnetic field environment is essential for reducing such errors. This dissertation focuses on developing and optimizing the magnetic environment in the MSR to support the TUCAN experiment’s sensitivity goals. Initial evaluations showed that the MSR’s shielding performance was suboptimal, prompting further investigation and targeted improvements. Techniques such as magnetic shaking were used to enhance shielding, and detailed residual field mapping was conducted. A custom-built mapping system was developed to assess the magnetic environment, with simulations used to correct for systematic uncertainties, including sensor inaccuracies, misalignments, and structural imperfections. These measurements enabled a clearer understanding of field sources and helped guide optimization efforts. Overall, this work evaluates the MSR’s shielding effectiveness and readiness for nEDM measurements. The results inform ongoing improvements to the shielding and field uniformity and contribute to meeting the stringent requirements necessary for detecting or constraining the neutron EDM.