Is the neutron round? If not, so what?
It turns out that the "roundness" of the neutron charge distribution (measured by the electric dipole moment) is connected at a deep level to a rather fundamental question - "Why is there any matter in the Universe?" Matter and anti-matter were created in equal amounts during the Big Bang. Most (all but one part in a billion) was annihilated in subsequent matter/anti-matter collisions. Clearly (and thankfully) a small excess of matter survived to form our Universe. Theories that purport to explain this invariably predict that the neutron is not quite perfectly round.
The neutron electric dipole moment was first measured in 1950 by Smith, Purcell, and Ramsey at the Oak Ridge Reactor - the first intense neutron source. This first measurement showed that the neutron was very nearly round (to better than one part in a million). In the last sixty years the precision of the measurement has improved by more than six orders of magnitude; the neutron is known to be round to better than one part in a trillion!
The goal of the nEDM experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source is to further improve the precision of this measurement by another factor of 100.
The statistical sensitivity (sigma) of an EDM experiment depends on the electric field (E),
the number of neutrons (N) and the neutron storage time (tau):
sigma ~ 1 / (E * sqrt(N*tau))
The nEDM experiment at the Spallation Neutron Source was conceived by Golub and Lamoreaux in a seminal paper. This approach takes advantage of an amazing confluence of superfluid helium properties to substantially increase E, N, and tau:
A large density of ultracold neutrons (UCNs) can be produced through a superthermal process (8.9 angstrom neutrons are stopped by scattering off excitations in superfluid Helium).
UCNs can be stored in a material bottle for times longer than their decay lifetime.
Superfluid Helium can support strong electric fields.
Furthermore, spin-polarized Helium-3 can be used as a co-magnetometer (to minimize and control systematic errors associated with non-zero magnetic field gradients), and as a spin analyzer, producing scintillation light (signal) in response to the spin-dependent n-Helium-3 capture reaction.
The nEDM experiment is working on a focused R&D program to answer
remaining outstanding issues, as recommended by
a recent NSAC report.
results from the ongoing R&D
effort are summarized here.
Recent progress on each of the key R&D activities is discussed below.
We also hosted a
Workshop on Neutron EDM Experimental Techniques on Oct. 11 - 13, 2012;
please click here to access the Workshop presentations.
 Original nEDM Publication -- Smith, Purcell, and Ramsey (1957)
 Original Physics Reports article describing measurement technique pursued by the nEDM experiment -- Golub and Lamoreaux
 nEDM Conceptual Design Report
 NSAC Report on Fundamental Neutron Physics (2011)
 R&D Accomplishments
For More Information
The following links will let you learn more about this topic: Wikipedia Neutron Electric Dipole Moment Write-up
nEDM Public Page
The Fundamental Neutron Physics Beamline at the SNS
The Spallation Neutron Source
Report from 1950 Oak Ridge Newsletter describing initial neutron EDM measurement
A coating of deuterated tetraphenyl butadiene (dTPB) allows UCN storage, maintains 3He polarization, and converts the UV scintillation light to blue light, which can be efficiently detected by photosensors.
A large density of UCNs is produced through a superthermal process (8.9 angstrom neutrons are stopped by scattering off excitations in superfluid Helium).
Workshop on Neutron EDM Experimental Techniques -- Oct. 11 - 13, 2012
Vince Cianciolo, ORNL, Project Manager (cianciolotv at ornl.gov)
Brad Filippone, Caltech, Co-spokesperson (bradf at caltech.edu)
Martin Cooper, LANL, Co-spokesperson (firstname.lastname@example.org)