Clara Moskowitz in Nature:
Scientists are unanimous that their current theory of physics is incomplete. Yet every effort to expose a deeper theory has so far disappointed. Now the most sensitive test yet of the shape of an electron — a property that could expose underlying 'new physics' — has failed to find hints of anything novel. The finding rules out a number of favoured ideas for extending physics, including some versions of a popular idea called supersymmetry. The result came from a search for the so-called electric dipole moment in the electron. A familiar example of a dipole is a bar magnet, which is shaped like a dumbbell with a north and a south pole. Electrons are traditionally thought of as spherical, but if they had dipole moments, they would be slightly squashed. “It’s a question of: Does the electron look the same no matter which way you look at it?” explains physicist Jony Hudson of Imperial College London. “The dipole moment is physicists’ technical way to describe if it’s symmetric or not.”
The Standard Model of particle physics, which describes all the known particles in the Universe, predicts a practically zero electric dipole moment for the electron. Yet theories that include additional, yet-to-be-detected particles predict a much larger dipole moment. Physicists have been searching for this dipole moment for 50 years. Now a group called the ACME collaboration, led by David DeMille of Yale University and John Doyle and Gerald Gabrielse of Harvard University, has performed a test 10 times more sensitive than previous experiments, and still found no signs of an electric dipole moment in the electron. The electron appears to be spherical to within 0.00000000000000000000000000001 centimeter, according to ACME’s results, which were posted on the preprint site arXiv. “It’s a surprise,” says Ed Hinds, also of Imperial College London, who worked with Hudson on the previous best limit, set in 2011. “Why on Earth is it still zero?”
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