Is there any imbalance in the way that the electric charge of a neutron (a positive and a negative, canceling to zero) is distributed within the particle? So far, the answer has been no — and this has been tested to mind-boggling levels of precision. But pressing even further in a quest for this elusive quantity, known at the neutron Electric Dipole Moment, or nEDM, is the top priority for scientists working in a sub-field known as fundamental neutron physics, according to advisers to the Department of Energy (DOE).
On Friday, the Nuclear Science Advisory Committee endorsed an interim report that placed nEDM at the top of a list of five ranked research efforts. Krishna Kumar, of the University of Massachusetts at Amherst, leads the subcommittee that is preparing the report and on Thursday, he presented slides that show the winners and losers for this small community of physicists.
The most pressing experiment, nEDM, is one that’s starting to take shape at Oak Ridge National Laboratory (ORNL). The ORNL group is competing with a rival European collaboration in Grenoble, France to build the most sensitive detector yet. The facility (pictured) is expected to cost between $40 million and $50 million to build. The US group hopes that by 2025 they will have looked for nEDM down to levels of 4×10E-28, which would represent a factor of 100 improvement over current searches. In his presentation, Kumar recommended that physicists focus for two years on remaining R&D problems before any more money is poured into the capital costs of building it.
After nEDM, the next four most important research efforts, in order, are:
2) A continuation of UCNA, which uses ultra-cold neutrons (UCN) to measure a key coefficient (A) that helps to describe neutron decay. This experiment, at Los Alamos National Laboratory, has been scheduled for phasing out, but the report recommends pushing the experiment further to reach its planned sensitivities.
3) The completion of NPDGamma, an effort at ORNL to look for preferences in the emission of gamma rays when a neutron is captured by a proton — which if detected would be a direct measure of the strength of the weak force.
4) Investment in Nab, a new experimental effort at ORNL that would measure two other neutron decay coefficients (a and b).
5) Continuation of an effort at the National Institute of Standards and Technology (NIST) to measure the lifetime of the neutron using a beam of cold neutrons.
These five research efforts are supposed to fit within a funding envelope of about $4 million from the National Science Foundation and $9.5 million from the Department of Energy. As with many science community priority lists, it’s also interesting to see which research efforts were not ranked. On page 21 of Kumar’s presentation, you can see that about 10 projects defended their efforts to Kumar’s committee — but did not make the cut.