Developments for a measurement of the beta-nu correlation and determination of the recoil charge state distribution in 6He beta decay

Abstract

The beta-nu of a pure Gamow-Teller beta decay such as the 6He decay is sensitive to tensor-type weak currents predicted by theories beyond the Standard Model. An experiment is developed at University of Washington aiming at measuring the coefficient a_{beta-nu} of 6He decays to the 0.1% level and looking for its deviation from the Standard-Model prediction -1/3 using laser-trapped 6He atoms. The beta particle is detected by a scintillator and a multi-wire proportional chamber, and the recoil ion is detected by a microchannel plate with delay-line anodes for position readouts. a_{beta-nu} is extracted by fitting the measured time-of-flight spectrum of the recoil ions to templates generated by Monte Carlo simulations. This dissertation describes the developments of this experiment for the intermediate goal of a 1% level a_{beta-nu} measurement, such as the detector design, Monte Carlo simulation software, and data analysis frame work. Particularly, detector calibrations are described in detail. The analysis of a 2% level proof-of-principle run in October 2015 is presented as well. Shake-off probabilities for decays of trapped 6He atoms matter for the high-precision a_{beta-nu} measurement. The charge state distribution of recoil ions is obtained by analyzing their time-of-flight distribution using the same experimental setups for the a_{beta-nu} measurement. An analysis approach that is independent of the beta-nu correlation is developed. The measured upper limit of the double shake-off probability is 2x10^{-4} at 90% confidence level. This result is ~100 times lower than the most recent calculation by Schulhoff and Drake. This disagreement is significant for the a_{beta-nu} measurement and needs to be addressed by improved atomic theory calculations.