Electrons are almost perfect spheres, and the fact that they are could help answer questions such as why antimatter is rare.

Scientists at Imperial College London made the most accurate measurement yet of the shape of the electron, in an experiment that took 10 years to complete. The data suggest the electron differs from being perfectly round by a length that is so small it is a billion trillionth of a nanometer. That is so near perfect that an electron would have to be the size of the solar system before you could see the difference.

Professor Edward Hinds, research co-author, said physicists are trying to find out if there are particles that haven't been seen yet in accelerators, which smash atoms together at high energies. The Standard Model, as it is known, has been a cornerstone of physics for decades.

But the Standard Model doesn't explain things such as why the universe is almost all matter with only a little bit of anti-matter. That's an important question, Hinds said, because most theories about the early universe - including the Standard Model - predict that there should have been an equal amount of matter and anti-matter when the universe began. But since matter and antimatter annihilate each other when they touch, that would mean that there wouldn't be anything at all in the universe except photons. Plainly that's nonsense, Hinds said.

Many theories about why matter became dominant in the universe posit that there are particles the Standard Model doesn't predict. If they exist, they would distort the shape of the electron.

To test that hypothesis, the physicists studied the electrons inside molecules of ytterbium fluoride. Using a laser, they made careful measurements of the motion of these electrons. If the electrons were not perfectly round then their motion would exhibit a distinctive wobble, distorting the shape of the molecule. The researchers saw no wobbles.

That means that the kinds of particles theorists are seeking in some models are a lot less likely, Hinds said. It doesn't mean they are wrong, but it places some lower limits on the masses of those particles.