Measurement of the neutron $β$-asymmetry parameter $A_0$ with ultracold neutrons

Abstract

We present a detailed report of a measurement of the neutron β-asymmetry parameter A0, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T 2 × 2π solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report A0 = −0.11966 ± 0.00089 +0.00123 −0.00140 , from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, λ = gA/gV = −1.27590 ± 0.00239 +0.00331 −0.00377 . Complete details of the analysis are presented.

V_{ud} from neutron beta decay

“…Not yet included is a new result from PERKEO II [30], λ = −1.2755 ± 0.0013. The determination of λ = −1.27590 +0.00409 −0.00445 with UCNA [35], the only experiment on A performed with ultracold neutrons and described in impressive detail in [36], is in agreement with this result, and work to further reduce uncertainties is ongoing. An accuracy of δλ = ±0.00067 has been announced for a recent new measurement of A with the new PERKEO III spectrometer [37,38] for which a blind analysis is underway.…”

Quark tensor and axial charges within the Schwinger-Dyson formalism

“…The experimental study of these charges for the proton, however, gives smaller results compared to the naive quark model prediction, namely [3,4,5]…”