In this paper we reported the first results of a novel symmetry test performed to check the standard model's prediction of maximal parity violation in nuclear P decay. The experiment measured the ratio R of the polarization of positrons emitted in opposite directions with respect to the nuclear spin and compared the result with the standard model prediction Ro. We have since come to realize that the small transverse components of the positron polarization accepted by the polarimeter make a non-negligible contribution to Ro. Our corrected result, which is presented in detail elsewhere[1], is (b+g) =0.0003~0.0058. This correction increases our 90% confidence level lower bound on the mass of an eventual right-handed gauge boson from 225 to 250 GeV/c, leaving our main conclusions unchanged.We thank Dr. J. Govaerts for calculations which show that the rotation of the positron spin in the magnetic fields of the apparatus has a negligible eAect on the final result.
MiniBETA is a new spectrometer incorporating a low-pressure multiwire drift chamber (MWDC). It is designed for beta spectrum shape measurements and to improve knowledge on electron backscattering in the about 100 keV to a few MeV region. After the commissioning phase, which we report on here, the spectrometer will first be used for the latter purpose. This will help reducing the systematic uncertainty in beta spectrum shape measurements. The performance of the trajectory recognition algorithm was investigated using cosmic muons as the source of the primary ionization. For the initial test, the chamber was filled with a mixture of helium-isobutane (50/50) at 600 mbar. A single cell efficiency η higher than 0.98 within almost the entire cell was achieved and the single cell spatial resolution in the drift plane reached σ = 0.4 mm for most of the cells in the chamber.
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