The angular correlation between the beta electron and antineutrino in nuclear beta decay a is the least well-known of the group of neutron decay observables that has been studied intensely experimentally
A more precise value for it can be used to test the validity and self consistency of the Electroweak Standard Model. We are carrying out an experiment using a new method that promises reduced systematic uncertainties, and consequently a factor of five improvement in precision of a compared with previous measurements of this quantity. This new method relies on constructing an asymmetry that directly yields a without requiring precise proton spectroscopy. An important advantage of measuring a compared with other correlation coefficients is the fact that there is no need for neutron polarimetry.
Figure 1 shows a schematic of the apparatus as well as a photograph of it on our beamline. The neutron beam passes through a long vertically oriented solenoid. A proton detector atop the solenoid detects protons originating from neutron decays that occur inside the apparatus while an electron detector at the bottom detects beta electrons in coincidence. In order to conserve momentum, the unobserved electron antineutrinos must travel either upward or downward. This leads to two groups of protons for many beta energies: a fast moving group and a slow moving group. These two groups can be distinguished using the time of flight between electron and proton. The asymmetry in counts between the fast and slow groups is proportional to a. In the experiment this asymmetry is measured as a function of beta energy and a value for a is extracted. The use of two-fold coincidences leads to a significant reduction in interfering background events. The goal of the experiment is an overall relative uncertainty on a between 0.5% and 1%.
During 2009 the entire apparatus except for the beta detector was constructed, assembled and tested without neutrons at Indiana University. In parallel, the beta detector was tested at NIST using an electron beam as well as several radioactive electron sources. In late 2009 the apparatus was shipped to NIST and installed on our beamline NG-6. Since January 2010 the apparatus has been undergoing commissioning. Recently, the first time of flight versus electron energy scatter plot was obtained. The quality of the data indicates that all of the major systems (magnetic field, high voltage, grids, and detector systems) are functioning. Data collection will continue until the long shutdown which is currently scheduled for April 2011.