Motional Spin Relaxation in Large Electric Fields

TL;DR

This paper investigates how motional magnetic fields caused by particle motion in electric fields lead to spin relaxation in ultra cold neutrons and helium-3 atoms, with implications for neutron electric dipole moment experiments.

Contribution

It provides a Monte Carlo simulation of spin relaxation times due to motional $v\times E$ fields for UCN and $^{3}$He at low temperatures, highlighting their relevance for nEDM searches.

Findings

Neutron relaxation times are long compared to neutron lifetime.

$^{3}$He relaxation times could impact nEDM experiments.

Monte Carlo approach effectively models spin relaxation effects.

Abstract

We discuss the precession of spin-polarized Ultra Cold Neutrons (UCN) and $^{3}$He atoms in uniform and static magnetic and electric fields and calculate the spin relaxation effects from motional $v\times E$ magnetic fields. Particle motion in an electric field creates a motional $v\times E$ magnetic field, which when combined with collisions, produces variations of the total magnetic field and results in spin relaxation of neutron and $^{3}$He samples. The spin relaxation times $T_{1}$ (longitudinal) and $T_{2}$ (transverse) of spin-polarized UCN and $^{3}$He atoms are important considerations in a new search for the neutron Electric Dipole Moment at the SNS \emph{nEDM} experiment. We use a Monte Carlo approach to simulate the relaxation of spins due to the motional $v\times E$ field for UCN and for $^{3}$He atoms at temperatures below $600,\mathrm{mK}$. We find the relaxation times for the neutron due to the $v\times E$ effect to be long compared to the neutron lifetime, while the $^{3}$He relaxation times may be important for the \emph{nEDM} experiment.