The Nab Experiment: An ultraprecise measurement of the electron-antineutrino angular correlation coefficient in neutron beta decay

点击翻译按钮获取中文摘要

The Nab collaboration aims to make the world's most precise measurement of the electron-neutrino angular correlation parameter "a" and the Fierz interference term "b" in cold neutron beta decay. The experiment employs a novel 4 Pi field expansion spectrometer and is located at the Fundamental Neutron Physics Beam line (FNPB) of the Spallation Neutron Source (SNS) at Oak Ridge National Lab in the USA. The Nab collaboration has recently passed several significant milestones with the delivery and installation of the 4 m long superconducting spectrometer magnet on the FNPB and the delivery of several highly segmented Si detectors. The collaboration is anticipating commissioning of the full experiment at the SNS in the spring/summer of 2019, with physics data to begin shortly after for the next 2 years. The measurement of angular correlations in neutron beta-decay provides the most sensitive means to evaluate the ratio of axial to vector coupling constants (Lambda (L) = gA/gV). The precise value of "L" is very important in many applications of the theory of weak interactions, especially in astrophysics, e.g. a star's neutrino production is closely related to L2. Parameter "a'' offers an independent measure of "L'' with comparable sensitivity and radically different systematics than measurements using the correlation parameter "A", which currently provides the best precision on "L". The main components of the Nab experiment include a superconducting spectrometer, a set of highly segmented silicon (Si) detectors, and cold neutron beam monitors. The function of the spectrometer is to measure the magnitude of the proton momentum via time of flight (TOF) measurements. It is crucial that the proton silicon detector(s) be fully characterized with a known source of protons before installation into the Nab spectrometer. To that end, a test facility has been developed at the University of Manitoba to characterize the detectors for 30 keV protons. Much of the work over the past three years has focused on getting this facility ready, which includes determining the appropriate beam steering settings using an large area phosphor screen detector to image the beam as well as the building of a large area micro-channel plate (MCP) detector and its associated electronics to act as a reference detector for the silicon detector. The facility will begin characterizing the silicon detector(s) in the winter/spring of 2018-19. Since 2016, one PhD. student and three undergraduate students have received world class training in detector manufacturing/testing while working to build this facility. Given the expanded scope of the project in the coming three years, we anticipate providing additional training to one PhD and one MSc. student and anticipate an increase over previous years travel budget to assist in the commissioning, physics data taking, and analysis of the Nab experiment. Thus our projected budget request is $140k, $141k, $121k over the next three years.