We measure the beta-neutrino correlation by inferring the momentum of the neutrinos from the momentum of the recoil of daughter ions from their time-of-flight. A precise measurement of the correlation coefficient can limit the existence of scalar or tensor currents from higher mass weak bosons present in some extensions to the Standard Model.
The atoms are produced here at the 88" cyclotron by bombarding a MgO pressed-powder target with protons. The resulting sodium-21 is boiled off from the target by heating the oven to roughly 1000-degreesC. The atoms are then collimated into a beam and slowed down by a Zeeman slower. Up to 8-hundred thousand atoms are confined to 0.6 mm diameter sphere using a magneto-optical trap. The atoms undergo a nuclear beta decay, and the beta-neutrino angular correlation of the nuclear decay is reconstructed from the time-of-flight of the daughter nuclei: neon atoms and ions of various charge states. Trapped atoms offer numerous advantages over the traditional sources, including:
We have recently completed a measurement of the beta-neutrino correlation coefficient in which the ion detection was triggered on positron detection. The measured coefficient is a=.5243(0.0092), a deviation from the Standard Model at the 3.6-sigma level. The Standard Model predicts a=0.558(0.003). Shown below is the observed time-of-flight spectrum. The spectrum is fitted to a monte-carlo simulation, and the beta-neutrino coefficient, a, is extracted. The details of this work can be found in Nick Scielzo's thesis.
We are currently working to understand the reason behind the deviation in the little "a" from the Standard Model. Nick Scielzo's thesis describes many of the systematics that were considered, including recoil order corrections, nuclear polarization, trap size, among others. The only systematic which was found to be significat was the dependence on trap populations. This may be due to collisions among trapped atoms: an effect we are currently working to understand. One possible solution is to trigger on the shake-off electrons instead of the positrons, eliminating the strong directional dependence.
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