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.
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%.
In late 2009, the apparatus was shipped to NIST after being tested at Indiana University. The first “electron energy vs proton time of flight” was obtained in October 2010 (to the right an example of such a plot) and a lot of good quality data was taken until the long shutdown (April 2011 to April 2012). During this shutdown, time was spent analyzing the data and improving the apparatus, especially reducing the background on our detectors. Since April 2012, we had two successful campaigns of data collection. As of January 2013, we are now taking data that should allow us to reach a 2% measurement of a.