a Measurement of the Beta Decay Asymmetry of Neon -19 as a Test of the Standard Model

We have performed an accurate measurement of the parity-violating beta asymmetry of ^{19 }Ne decay. This asymmetry can be calculated in the standard electro-weak model using the measured ft value for ^{19}Ne and a value of G_{V}V_{ud }, where G_{V} is the Fermi coupling constant and V_{ud } is the u-d element of the Cabibbo-Kobayashi -Maskawa mass mixing matrix. The asymmetry is particularly sensitive to the existence of right-handed weak currents. In addition, if we assume that right-handed currents do not exist, the ^{19}Ne asymmetry and ft value provide an independent measurement of G _{V}V_{ud}.. The zero energy intercept of the asymmetry was measured to be A_0=-{0.0360} _sp{-0.0006}{+0.0008}+/-0.0003. The errors are systematic and statistical, respectively. The measured value is in good agreement with the value predicted by the standard model together with the ft values for ^{19}Ne decay and the 0^+ to 0^+ decays (A_0=-{0.0361}+/-0.0007). However, the value of V_{ud } derived from the measured asymmetry, the ^{19}Ne ft value, and mu decay violates unitarity by 1.5 sigma.. The slope of the asymmetry as a function of beta energy was measured to be {dAover dE }=(-{4.2}+/-0.7+/-0.8) times 10^{-3}/MeV. The standard model prediction for the slope is -3.5(1) times 10^{-3}/MeV. The slope is sensitive to second class currents which are not present in the standard model. The implied value of the second class form factor, d^ {II} is -60 +/- 54 +/- 60 which is consistent with the standard model value of 0. The beta asymmetry was measured from the difference in the beta emission rate parallel to and anti-parallel to the polarization of the decaying ^{19 }Ne. Polarized ^{19} Ne atoms were trapped in a thin walled cell at the center of a solenoidal magnetic field. Positrons from ^{19}Ne beta decay spiraled along the magnetic field lines and were detected in Si(Li) detectors at either end of the solenoid. The asymmetry was determined from the ratio of the rates in these two detectors. The uncertainty in this measurement is limited by systematic effects. A Monte-Carlo simulation was used to characterize and correct for systematic errors.