# A magic magnetic field to measure the neutron electric dipole moment

## TL;DR

This paper proposes a novel measurement method for the neutron electric dipole moment using a 'magic' magnetic field value to eliminate systematic errors, potentially enhancing sensitivity to new physics beyond the Standard Model.

## Contribution

It introduces a new experimental concept employing a specific 'magic' magnetic field to suppress false EDM signals, enabling larger chambers and improved measurement precision.

## Key findings

- Identifies a systematic effect limiting neutron EDM measurements.
- Proposes a 'magic' magnetic field value of 7.2 μT to cancel false signals.
- Designs a room-temperature experiment with a mercury co-magnetometer.

## Abstract

New sources of CP violation beyond the Standard Model of particle physics could be revealed in the laboratory by measuring a non-zero electric dipole moment (EDM) of a spin 1/2 particle such as the neutron. Despite the great sensitivity attained after 60 years of developments, the result of the experiments is still compatible with zero. Still, new experiments have a high discovery potential since they probe new physics at the multi-TeV scale, beyond the reach of direct searches at colliders. Progress in precision on the neutron EDM is limited by a systematic effect arising from the relativistic motional field $\vec{E} \times \vec{v} / c^2$ experienced by the particles moving in the measurement chamber in combination with the residual magnetic gradients. This effect would normally forbid a significant increase of the size of the chamber, sadly hindering the increase of neutron statistics. We propose a new measurement concept to evade this limitation in a room-temperature experiment employing a mercury co-magnetometer. It consists ajusting the static magnetic field $B_0$ to a `magic' value which cancels the false EDM of the mercury. The magic setting is $7.2\,\muT$ for a big cylindrical double-chamber of diameter $100$~cm.