International research collaboration including scientists from Mainz University measures the electric dipole moment of the neutron with unprecedented accuracy

Researchers track down the mystery of the surplus of matter

28 February 2020

An international research collaboration at the Paul Scherrer Institute (PSI) involving scientists from the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) has measured a property of the neutron more precisely than ever before. In the process the scientists found out that this particle has a significantly smaller electric dipole moment than was previously known. The researchers achieved this result using the ultracold neutron source at PSI located in Villigen in Switzerland. Their results have been published in Physical Review Letters.

The Big Bang created both the matter in the universe and the antimatter – at least according to the established theory. However, since the two annihilate each other, there must have been a surplus of matter, which has remained to this day. The cause of this excess of matter is one of the great mysteries of physics and astronomy. Researchers hope to find a clue to the underlying phenomenon with the help of neutrons, the electrically uncharged building blocks of atomic nuclei. The assumption is that if the neutron has a so-called electric dipole moment (nEDM) with a measurable non-zero value, this could be due to the same physical principle that would also explain the excess of matter after the Big Bang.

50,000 measurements

The search for the nEDM can be expressed in everyday language as the question of whether or not the neutron is an electric compass. It has long been clear that the neutron is a magnetic compass and reacts to a magnetic field, or, in technical jargon, has a magnetic dipole moment. If in addition the neutron also had an electric dipole moment, its value would be very much smaller – and thus much more difficult to measure. Previous measurements by other researchers have borne this out. Therefore, the researchers at PSI had to go to great lengths to keep the local magnetic field very constant during their latest measurement, and smallest disturbances had to be accounted for and removed from the experimental data. This is the specialty of Professor Martin Fertl, physicist at the PRISMA+ Cluster of Excellence, and his research group at Mainz University: "To achieve this, we have developed and used extremely sensitive magnetometers based, among other things, on the principle of pulsed nuclear magnetic resonance."

Furthermore, the number of neutrons observed needed to be large enough to provide a chance to measure the nEDM. The measurements at PSI ran over a period of two years. So-called ultracold neutrons, i.e., neutrons with a comparatively slow speed, were measured. Every 300 seconds, an 8-second-long bundle with over 10,000 neutrons was directed to the experiment and examined. The researchers measured a total of 50,000 such bundles. To bring the neutrons onto the right way, a neutron switch had to be installed between the neutron source and the storage chamber. "This switch was constructed by nuclear chemists at Mainz University, who also closely accompanied the build-up at PSI", reported Professor Dr. Dieter Ries, who is also a member of the PRISMA+ Cluster of Excellence. He was already significantly involved in the development and characterization of the source of ultracold neutrons at PSI during his doctoral research.

The new result was determined by a group of researchers at 18 institutes and universities in Europe and the USA on the basis of data collected at this ultracold neutron source. The researchers evaluated measurement data very carefully in two separate teams, and through that obtained a more accurate result than ever before.

Search for "new physics"

"Our current result shows that the true value for nEDM is too small to be measured with the accuracy we have achieved so far – the value has therefore moved further towards zero," said Mainz physicist Professor Werner Heil, also involved at the nEDM project. "But it remains exciting to track down a finite nEDM in order to find out if it can be used to discover new physics."

Therefore, the next, more precise measurement is already being planned. The researchers expect to start the next series of measurements of the nEDM by 2021 and, in turn, to surpass the current one in terms of accuracy. "The new measurement setup is based on a lot of experience gained with the last experiment. The new setup is highly optimized in terms of the parameters of the neutron source and the minimization of systematic errors, and it will be surely groundbreaking in this sense", concluded Professor Dieter Ries.