After three months of experiments in a basement laboratory in London, scientists can confirm – with more confidence than ever – that the electron is very, very round.
In the most exquisite measurements yet, researchers declared the particle to be a perfect sphere to within one billionth of a billionth of a billionth of a centimetre. Were the electron scaled up to the size of the solar system, any deviation from its roundness would be smaller than the width of a human hair, the team said.
Abstruse as the experiment might seem, the work has profound implications for scientists wrestling with the mysteries of the cosmos. Even the slightest elongation of the electron can reveal what unknown particles might exist in nature, and even explain why matter won out over antimatter in the universe we observe.
The findings, published in the journal nature, already rule out some kinds of particles that theories suggested could pop into existence at the Large Hadron Collider at Cern, the European particle physics laboratory near Geneva.
“It’s been hard work. We’ve been working on this for a long time and we’ve had a lot of ups and downs,” said Jony Hudson, a physicist at Imperial College London. “We have measured the shape really precisely. The deviations we were looking for are much smaller than the size of the electron. It is very, very round.”
The concept of shape might seem obscure when it comes to a subatomic particle, but the rules are the same as for everyday objects. Pick up a pen, for example, and you feel its shape because electrons in the pen push back against the electrons in your hand.
And so it is with the electron itself. The particle is negatively charged, and the more evenly distributed the charge is around the centre of the particle, the more spherical it appears to be.
Scientists pursue ever more accurate measurements of the electron’s roundness because any sign of it being misshapen could herald a major discovery. One leading idea known as supersymmetry, which says that every kind of particle we know has a heavy twin, requires the electron to have a slightly distorted shape.
“What’s interesting is that the electron is so round it is becoming difficult for theories like supersymmetry to explain it,” said Hudson, whose finding already rules out the existence of some supersymmetric particles.
Evidence that the electron is misshapen on a minuscule scale might also explain why the universe we see is made of matter instead of antimatter. At the birth of the cosmos, both were made in equal measure, but some subtle difference between the two caused antimatter to disappear. If the electron is elongated, it will behave differently to its antimatter counterpart, the positron. For example, each would wobble differently in an electric field.
“There must be a difference in the behaviour of matter and antimatter that we’ve not observed, and amazingly, the shape of the electron might just be enough to explain how the matter-antimatter imbalance built up over billions of years,” Hudson said.
His team studied the roundness of electrons by measuring how much, or how little, the particles wobbled in an electric field. The rounder the electron, the less wobble it will display. In the experiment, electrons were anchored to a molecule called ytterbium fluoride and examined with a laser beam. Each measurement took only one thousandth of a second.
Running their experiment non-stop for more than three months, Hudson’s team took 25 million measurements of electrons and averaged them out. They found no sign of the electron wobbling in the field, meaning it is more spherical than any previous experiment had shown. “To the best of our knowledge, with the experimental precision we have, the electron appears to be round,” Hudson said.
In an accompanying article, Aaron Leanhardt at the University of Michigan, Ann Arbor, said the work provided a window “on the high-energy soul of the cosmos”.
“This work has important ramifications for the types of particles that can be discovered at high-energy accelerators, and may eventually help to explain the composition of the observable universe,” Leanhardt wrote.
With improvements to their equipment, Hudson’s team hopes to make even more precise measurements of the electron in the coming years. “If we could make it 10 times better we could pretty conclusively rule in or out supersymmetry, and that would be a huge result for us,” he said.