First Measurement of the NeutronβAsymmetry with Ultracold Neutrons

We report the first measurement of an angular correlation parameter in neutron $\beta$ decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for $\sim 30\mathrm{s}$ in a Cu decay volume. The interaction of the neutron magnetic dipole moment with 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 spin flipper and enter a decay volume, situated within a 1 T field in a $2\times 2\pi$ solenoidal spectrometer. We determine a value for the $\beta$-asymmetry parameter $A_0=-0.1138\pm 0.0046\pm 0.0021$.