First Measurement of the NeutronβAsymmetry with Ultracold Neutrons

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 $A_0 = -0.1138 \\pm 0.0051$.