Soo Ryong Hwang

New limit on the D coefficient in polarized neutron decay

We describe an experiment that has set new limits on the time reversal invariance violating D coefficient in neutron beta-decay. The emiT experiment measured the angular correlation J . p_e x p_p using an octagonal symmetry that optimizes electron-proton coincidence rates. The result is D=[-0.6+/-1.2(stat)+/-0.5(syst)]x10^(-3). This improves constraints on the phase of g_A/g_V and limits contributions to T violation due to leptoquarks. This paper presents details of the experiment, data analysis, and the investigation of systematic effects.

emiT: An apparatus to test time reversal invariance in polarized neutron decay

We describe an apparatus used to measure the triple-correlation term (Dσn⋅pe×pν) in the beta decay of polarized neutrons. The D coefficient is sensitive to possible violations of time reversal invariance. The detector has an octagonal symmetry that optimizes electron–proton coincidence rates and reduces systematic effects. A beam of longitudinally polarized cold neutrons passes through the detector chamber, where a small fraction undergo beta decay. The final-state protons are accelerated and focused onto arrays of cooled semiconductor diodes, while the coincident electrons are detected using panels of plastic scintillator. Details regarding the design and performance of the proton detectors, beta detectors, and the electronics used in the data collection system are presented. The neutron beam characteristics, the spin-transport magnetic fields, and polarization measurements are also described.

Searching for time reversal invariance violation in polarized neutron decay

Time reversal invariance violation is tightly constrained in the Stan- dard Model, and the existence of a T-violating effect above the predicted level would be an indication of new physics. A sensitive probe of this symmetry in the weak interaction is the measurement of the D-coefficient in neutron decay. This parameter characterizes the triple-correlation of neutron spin, electron mo- mentum, and neutrino (or proton) momentum, which changes sign under time reversal. The emiT experiment, now on line, attempts to improve the measure- ment of D, whose current average is 0.3 + 1.5 x 10 -3.