Electric Dipole Moment of Neutron Measured More Precisely than Ever Before

At the Paul Scherrer Institute (PSI), scientists have successfully quantified a property of the neutron, an elementary particle, more accurately than ever before. During the study, they discovered that the electric dipole moment of neutron is considerably smaller than predicted earlier.

Physicists Philipp Schmidt-Wellenburg (left) and Georg Bison, scientists at the Laboratory for Particle Physics at PSI, in front of the experiment to measure a fundamental property of the neutron: its electric dipole moment. Visible are primarily the round, concentric shields made of sheet metal, which serve to shield external magnetic fields. Image Credit: Paul Scherrer Institute/Mahir Dzambegovic.

It has also turned out to be less probable that this dipole moment can be useful in explaining the origin of all matter in the universe. The team realized this outcome with the help of the ultracold neutron source at PSI. The study findings were recently reported in the Physical Review Letters journal.

Following the Big Bang, both the matter and the antimatter formed in the universe—at least based on the conventional theory. However, as both tend to mutually annihilate one another, there must have been an excess of matter, which has remained until now. The reason behind this surplus of matter is one of the major puzzles of astronomy and physics.

Scientists believe they can find a lead to the fundamental phenomenon using neutrons, atoms’ elementary building blocks that are electrically uncharged. The hypothesis: In case a so-called electric dipole moment (abbreviated nEDM) with a measurable non-zero value is exhibited by the neutron, this could be because of the same physical principle that would account for the surplus of matter following the Big Bang.

50,000 Measurements

In day-to-day language, the quest for the nEDM can be said as the question of whether or not the neutron acts as an electric compass. For a long time, it has been evident that the neutron acts as a magnetic compass and responds to a magnetic field. In technical terms, it has a magnetic dipole moment. Apart from this, if the neutron also exhibited an electric dipole moment, its value would be considerably less—and thus highly challenging to quantify.

Earlier measurements by other scientists have supported this. Thus, the PSI scientists had to put much more efforts to maintain the local magnetic field highly constant during their most recent measurement. The magnetic field was disrupted by each truck that passed by on the road adjacent to PSI, on a scale that was relevant for the experiment. Therefore, this influence had to be calculated and eliminated from the experimental data.

Moreover, the number of neutrons detected had to be sufficiently large to offer a chance to quantify the nEDM. Thus, the measurements at PSI took a period of two years. So-called ultracold neutrons (or neutrons with a relatively slow speed) were measured. For every 300 seconds, the researchers directed an 8-second-long bundle with more than 10,000 neutrons to the experiment and performed investigations. In total, they measured 50,000 such bundles.

Even for PSI with its large research facilities, this was a fairly extensive study. But that is exactly what is needed these days if we are looking for physics beyond the Standard Model.

Philipp Schmidt-Wellenburg, Researcher, nEDM Project, Laboratory for Particle Physics, Paul Scherrer Institute

Search for “New Physics”

The new outcome was ascertained by a team of researchers from 18 universities and institutes in Europe and the United States, including the University of Fribourg (Switzerland), ETH Zurich, and the University of Bern.

The researchers gathered the data from the ultracold neutron source at PSI. They had gathered measurement data there over a period of two years, and assessed it with utmost care by forming two teams. Thus, they achieved a more precise result like never before.

The nEDM project is part of the quest for “new physics” that do not fall into the so-called Standard Model. The search is also ongoing at much larger facilities like the Large Hadron Collider LHC at CERN.

The research at CERN is broad and generally searches for new particles and their properties,” explained Schmidt-Wellenburg. “We on the other hand are going deep, because we are only looking at the properties of one particle, the neutron. In exchange, however, we achieve an accuracy in this detail that the LHC might only reach in 100 years.”

Ultimately, various measurements on the cosmological scale show deviations from the Standard Model. In contrast, no one has yet been able to reproduce these results in the laboratory. This is one of the very big questions in modern physics, and that’s what makes our work so exciting.

Georg Bison, Researcher, nEDM Project, Laboratory for Particle Physics, Paul Scherrer Institute

Even More Precise Measurements are Planned

Through the latest experiment, the research team has confirmed the earlier laboratory results.

Our current result too yielded a value for nEDM that is too small to measure with the instruments that have been used up to now—the value is too close to zero. So it has become less likely that the neutron will help explain the excess of matter. But it still can’t be completely ruled out. And in any case, science is interested in the exact value of the nEDM in order to find out if it can be used to discover new physics.

Philipp Schmidt-Wellenburg, Researcher, nEDM Project, Laboratory for Particle Physics, Paul Scherrer Institute

Thus, the next, more accurate measurement has already been planned. “When we started up the current source for ultracold neutrons here at PSI in 2010, we already knew that the rest of the experiment wouldn’t quite do it justice. So we are currently building an appropriately larger experiment,” explained Bison.

The researchers at PSI predict that the next series of measurements of the nEDM will begin by 2021 and, in turn, will outclass the existing one in terms of precision.

We have gained a great deal of experience in the past ten years and have been able to use it to continuously optimise our experiment—both with regard to our neutron source and in general for the best possible evaluation of such complex data in particle physics,” added Schmidt-Wellenburg. “The current publication has set a new international standard.”

Source: https://www.psi.ch/en

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