New Result for the Neutron β -Asymmetry Parameter A 0 from UCNA

Background : The neutron  β -decay asymmetry parameter  A 0  defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for  A 0  permit an extraction of the ratio of the weak axial-vector to vector coupling constants,  λ ≡ g A / g V , which under assumption of the conserved vector current hypothesis ( g V =1 ) determines  g A . Precise values for  g A  are important as a benchmark for lattice QCD calculations and as a test of the standard model.

Purpose : The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron  β -decay angular correlation performed with UCN. This article reports the most precise result for  A 0  obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response.

Methods : UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of  A 0 .

Results : The UCNA experiment reports a new 0.67% precision result for  A 0  of  A 0 =−0.12054(44) stat (68) syst , which yields  λ = g A / g V =−1.2783(22) . Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces  A 0 =−0.12015(34) stat (63) syst  and  λ = g A / g V =−1.2772(20) .

Conclusions : This new result for  A 0  and  g A / g V  from the UCNA experiment has provided confirmation of the shift in values for  g A / g V  that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were  >10 %, whereas those in the more recent ones (published after 2002) have been of the scale of  <2 %. The impact of these older results on the global average will be minimized should future measurements of  A 0  reach the 0.1% level of precision with central values near the most recent results.