A magic magnetic field to measure the neutron electric dipole moment

Guillaume Pignol

doi:10.1016/j.physletb.2019.05.014

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 E→×v→/c2 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 in adjusting the static magnetic field B0 to a “magic” value which cancels the false EDM of the mercury. The magic setting is 7.2μT for a big cylindrical double-chamber of diameter 100 cm.

Highlights

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
In 1950, Purcell and Ramsey [1] proposed to measure the Electric Dipole Moment (EDM) of the neutron, it is predicted to be zero if one assumes the invariance of the laws of physics under parity
Experiments to measure the neutron EDM improved in sensitivity by no less than six orders of magnitude, and yet the most recent measurement [2] is still compatible with zero: dn = (−0.21 ± 1.82) × 10−26 e cm

Summary

Introduction

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. We propose to adjust the magnetic field to suppress the subtle systematic effect that was dominant in the previous experiment [2], which is possible in a bigger apparatus with a significant increase of statistical sensitivity. In the most recent UCN experiment [2], a field of B0 = 1 μT was applied to the neutrons stored in a cylindrical chamber of diameter 47 cm and height 12 cm.

Results

Conclusion