The current theories and experiments maintain a total symmetry between matter and anti-matter - each particle of matter has a corresponding anti-matter particle. However, this cannot be so. Some asymmetry
should exist because otherwise when all these particles formed, they would have annihilated each other leaving behind only light.
If the matter-antimatter symmetry would have been indeed total, nothing could have taken shape in the Universe. There are various attempts to introduce such an asymmetry in the theoretical models but insofar the predictions of all these modified models failed to fit the facts. And this happened all over again. Another experiment wiped out a whole bunch of theoretical suppositions.
Physicist Dr Philip Harris, the head of the Sussex group, says: "This represents a significant breakthrough, and a real success for UK particle physics. Although there are a couple of other teams in the
world working in this same area, we're managing to stay ahead of them. 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 could test the accuracy of the matter-antimatter symmetry is to look for "distortions" in the shapes of nuclear particles. The neutron, the other particle inside the atom's nucleus besides the proton, is made out of quarks. These quarks are electrically charged -although, over-all, the neutron is neutrally charged (it's not called "neutron" for nothing). Nevertheless, the quarks move inside the neutron and the shape of the neutron isn't 100% spherical. As a result the neutron is more positively charged on one side and more negatively charged on the opposite side - this is called an electric dipole moment.
In the past 50 years measurements of the electric dipole of neutrons have found all sorts of applications ranging from atomic clocks to the nuclear magnetic resonance imaging technique. And now, after ten years of preparations, the team conducted the most sensitive measurements ever. Together with scientists from the Rutherford Appleton Laboratory and the Laue Langevin Institute in Grenoble, they built a special type of atomic clock that used spinning neutrons instead of atoms. The clock frequency was measured through nuclear magnetic resonance.
Various theories attempting to explain the creation of matter and antimatter in the aftermath of the Big Bang make various predictions about this electric dipole moment. And what the new experiment has shown is that the distortion in the subatomic particles is far smaller than most of the origin-of-matter theories had predicted. The following analogy was used: if the neutron were the size of the Earth, the distortion would still be less than the size of a bacterium. You can imagine why the existence of matter-antimatter asymmetry is difficult to pin down and prove.
"This will really help to constrain theories that attempt to go beyond our current understanding of the fundamental laws of physics", says Dr Harris. "For some of them, it's back to the drawing board; but for the better ones, it will definitely show them the way forwards."
The team is now busy developing a new version of the experiment together with Oxford University and the University of Kure in Japan. By submerging their neutron-clock in a bath of liquid helium, half a degree above absolute zero, they will increase their sensitivity a hundredfold.
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