The question that has vexed scientists and astronomers for years is why there is more matter in the Universe than antimatter. Both were formed at the time of the Big Bang, about 13.7 billion years ago. For every particle formed, an anti-particle should also have been formed. Almost immediately, however, the equal numbers of particles and anti-particles would have annihilated each other, leaving nothing but light. But a tiny asymmetry in the laws of nature resulted in a little matter being left over, spread thinly within the empty space of the Universe. This matter became the stars and planets that we see around us today.
Experiments carried out at the Institut Laue-Langevin (ILL), Grenoble, France, with its high flux reactor. Neutrons produced in the reactor can be slowed down and then stored in special traps where they bounce around like ping-pong balls. This allows long observation times and hence experiments on the fundamental properties of the neutron of outstanding accuracies.
The new result shows that the distortion in these subatomic particles is far smaller than most of the origin-of-matter theories had predicted. In relative terms, it’s less than the size of a bacterium sitting on an Earth-sized neutron would be.
ILL’s Peter Geltenbort explains: “This represents a significant breakthrough, and a real success for particle physics using neutrons…Over the years the method has been improved steadily, and pushed to its limits. It’s been said in the past that this experiment has disproved more theories than any other in the history of physics – and now it’s delivering the goods all over again.”
The only way scientists can verify their theories to explain this anomaly is to study the corresponding asymmetry in sub-atomic particles, by looking for slight “pear-shaped” distortions in their otherwise spherical forms. It has taken five decades of research to reach the stage where measurements of these particles, called neutrons, have become sensitive enough to test the very best candidate theories. Neutrons are electrically neutral, but they have positive and negative charges moving around inside them. If the centres of gravity of these charges aren’t in the same place, it would result in one end of the neutron being slightly positive, and the other slightly negative. This is called an electric-dipole moment, and it is the phenomenon that physicists have been working to find for the past 50 years. Spin-offs from the original pioneering work in this area include atomic clocks and magnetic-resonance imaging.
The apparatus employed a special type of atomic clock that used spinning neutrons instead of atoms. It applied 120,000 volts to a quartz “bottle” that was filled regularly with neutrons captured from a reactor. The clock frequency was measured through nuclear magnetic resonance.
Philip Harris and Maurits van der Grinten, of Sussex University and Rutherford Appleton Laboratory (RAL) in the UK, worked in conjunction with the ILL. The team has now expanded to include Oxford University and the University of Kure in Japan. They are busy developing a new version of the experiment that will submerge their neutron-clock in a bath of liquid helium, half a degree above absolute zero, they hope to increase their sensitivity a hundredfold.
Adapted from an ILL Press Release