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Physicists Precisely Measure Electric Dipole Moment of Neutron

In a paper in the journal Physical Review Letters, an international team of physicists reports the most precise measurement ever made of the electric dipole moment of the neutron, a subatomic particle found in the nucleus of every atom except that of hydrogen. Their results show that the neutron has a significantly smaller electric dipole moment than predicted by various theories about why matter remains in the Universe.

Abel et al present the result of an experiment to measure the electric dipole moment of the neutron at the Paul Scherrer Institute using Ramsey’s method of separated oscillating magnetic fields with ultracold neutrons. Image credit: Gerd Altmann.

Abel et al present the result of an experiment to measure the electric dipole moment of the neutron at the Paul Scherrer Institute using Ramsey’s method of separated oscillating magnetic fields with ultracold neutrons. Image credit: Gerd Altmann.

“After more than two decades of work by researchers at the University of Sussex and elsewhere, a final result has emerged from an experiment designed to address one of the most profound problems in cosmology for the last fifty years: namely, the question of why the Universe contains so much more matter than antimatter, and, indeed, why it now contains any matter at all. Why didn’t the antimatter cancel out all the matter? Why is there any matter left?” said University of Sussex’s Professor Philip Harris.

“The answer relates to a structural asymmetry that should appear in fundamental particles like neutrons. This is what we’ve been looking for. We’ve found that the electric dipole moment is smaller than previously believed. This helps us to rule out theories about why there is matter left over — because the theories governing the two things are linked.”

“We have set a new international standard for the sensitivity of this experiment. What we’re searching for in the neutron — the asymmetry which shows that it is positive at one end and negative at the other — is incredibly tiny.”

“Our experiment was able to measure this in such detail that if the asymmetry could be scaled up to the size of a football, then a football scaled up by the same amount would fill the visible Universe.”

The experiment is an upgraded version of apparatus originally designed by researchers at the University of Sussex and the Rutherford Appleton Laboratory, and which has held the world sensitivity record continuously from 1999 until now.

“Our experiment brings together techniques from atomic and low energy nuclear physics, including laser-based optical magnetometry and quantum-spin manipulation,” said University of Sussex’s Dr. Clark Griffith.

“By using these multi-disciplinary tools to measure the properties of the neutron extremely precisely, we are able to probe questions relevant to high-energy particle physics and the fundamental nature of the symmetries underlying the Universe.”

Any electric dipole moment that a neutron may have is tiny, and so is extremely difficult to measure. Previous measurements by other researchers have borne this out.

In particular, the team had to go to great lengths to keep the local magnetic field very constant during their latest measurement.

For example, every truck that drove by on the road next to the institute disturbed the magnetic field on a scale that would have been significant for the experiment, so this effect had to be compensated for during the measurement.

Also, the number of neutrons observed needed to be large enough to provide a chance to measure the electric dipole moment. The measurements ran over a period of two years. So-called ultracold neutrons, that is, neutrons with a comparatively slow speed, were measured.

Every 300 seconds, a bunch of more than 10,000 neutrons was directed to the experiment and examined in detail.

The researchers measured a total of 50,000 such bunches.

They found the value of the neutron electric dipole moment to be dn=(0.0±1.1stat±0.2sys)*10−26e.cm.

“The results supported and enhanced those of their predecessors: a new international standard has been set,” they said.

“The size of the electric dipole moment is still too small to measure with the instruments that have been used up until now, so some theories that attempted to explain the excess of matter have become less likely. The mystery therefore remains, for the time being.”

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C. Abel et al. 2020. Measurement of the Permanent Electric Dipole Moment of the Neutron. Phys. Rev. Lett 124 (8): 081803; doi: 10.1103/PhysRevLett.124.081803

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