Scientists a step closer to revealing the mysteries of matter

28 February 2020

Members of the research team in front of the neutron experiment at Paul Scherrer Institute

(Credit: Paul Scherrer Institute)

Scientists have measured a property of the fundamental particle the neutron more precisely than ever before, thanks to technology originally developed at the Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory with the University of Sussex.

The new research investigates why all the antimatter created in the Big Bang didn’t just cancel out the matter.

The team of physicists – which included the STFC’s Rutherford Appleton Laboratory in the UK, the Paul Scherrer Institute (PSI) in Switzerland, and a number of other institutions - was looking into whether or not the neutron acts like an “electric compass”. Neutrons are believed to be slightly asymmetrical in shape, being slightly positive at one end and slightly negative at the other - a bit like the electrical equivalent of a bar magnet. This is the so-called “electric dipole moment” (EDM), and is what the team was looking for.

Scientific theories about why matter remains in the Universe also predict that neutrons have the "electric compass” property, to a greater or lesser extent. Measuring it then it helps scientists to get closer to the truth about why matter remains.

The scientists found that the neutron has a significantly smaller EDM than predicted by various theories about why matter remains in the universe; this makes these theories less likely to be correct, so they have to be altered, or new theories found. In fact it’s been said in the literature that over the years, these EDM measurements, considered as a set, have probably disproved more theories than any other experiment in the history of physics.

Professor Philip Harris, Head of the School of Mathematical and Physical Sciences and leader of the EDM group at the University of Sussex, said:

“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 over 20 years ago by researchers at the University of Sussex and the Rutherford Appleton Laboratory (RAL), and which has held the world sensitivity record continuously from 1999 until now.

Dr Maurits van der Grinten, from the neutron EDM group at the Rutherford Appleton Laboratory, said:

“The experiment combines various state of the art technologies that all need to perform simultaneously. We’re pleased that the equipment, technology and expertise developed by scientists from RAL has contributed to the work to push the limit on this important parameter”

The researchers’ latest results supported and enhanced those of their predecessors: a new international standard has been set. The size of the EDM 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.

The next, more precise, measurement is already being constructed at PSI. The PSI collaboration expects to start their next series of measurements by 2021.

NOTES

Measurement of the permanent electric dipole moment of the neutron by C. Abel et al. is published in Physical Review Letters XY.

The experiment which previously held the world sensitivity record continuously between 1999 and now was originally designed by researchers at the University of Sussex and the Rutherford Appleton Laboratory (RAL), and run at the Institut Laue-Langevin (ILL) in Grenoble.

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 PSI's ultracold neutron source on apparatus adapted from the earlier experiment. The researchers collected measurement data there over a period of two years, evaluated it very carefully in two separate teams, and were then able to obtain a more accurate result than ever before.