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.
The aCORN collaboration is 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.
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 ultimate goal of the experiment is an overall relative uncertainty on a between 0.5% and 1%.
Production data (electron energy versus proton time of flight as seen in the plot) was taken in 2012, 2013, and 2014. From these data we hope to publish a measurement of a with a relative uncertainty of about 4%. In late 2014, the experiment was relocated from our old beam line NG-6 onto our new more intense beam line NG-C. On NG-C the count rate is five times higher than on NG-6. During 2015 and 2016, enough statistics will be collected to reduce our uncertainty from 4% to 1%.