First Measurement of the Neutron $\beta$-Asymmetry with Ultracold Neutrons

TL;DR

This paper presents the first measurement of the neutron beta-asymmetry parameter using polarized ultracold neutrons, providing new experimental data on angular correlations in neutron beta decay.

Contribution

It introduces a novel experimental setup employing ultracold neutrons and measures the neutron beta-asymmetry parameter for the first time.

Findings

Measured A_0 = -0.1138 ± 0.0051 for neutron beta decay

Demonstrated polarization of ultracold neutrons using a 7 T magnetic field

Established a new method for angular correlation measurements in neutron decay

Abstract

We report the first measurement of angular correlation parameters in neutron $\beta$-decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for $\sim 30$ s in a Cu decay volume. The $\vec{\mu}_n \cdot \vec{B}$ potential of a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage (AFP) spin-flipper and enter a decay volume, situated within a 1 T, $2 \times 2\pi$ superconducting solenoidal spectrometer. We determine a value for the $\beta$-asymmetry parameter $A_0$, proportional to the angular correlation between the neutron polarization and the electron momentum, of $A_0 = -0.1138 \pm 0.0051$.