Physicists are measuring whether the electron is perfectly round or slightly squashed, using molecules cooled to a few millionths of a degree above absolute zero. This matters because the electron’s shape—technically its electric dipole moment—is tied to the fundamental symmetries of nature. The Standard Model of particle physics predicts the electron is nearly perfectly round, but many theories that try to explain dark matter or why the Universe contains more matter than antimatter predict a measurable squashing. No experiment has yet found one, so a non-zero result would be a direct sign of new physics. The team has built an instrument that traps heavy molecules, polarises their spins, and watches how those spins precess in an electric field. Cooling the molecules to microkelvin temperatures lets them observe the spin for longer, dramatically improving measurement precision. The project aims to measure the electron’s roundness at least ten times better than any previous attempt, with upgrades planned for a further tenfold improvement. This is fundamental science with no immediate practical application. But a discovery here would reveal new particles or forces beyond the reach of the largest particle colliders, reshaping our understanding of the Universe’s origins. Past fundamental measurements of this kind have led to technologies like atomic clocks and MRI.
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Our understanding of the fundamental particles, forces and symmetries of nature is far from complete. Our best theory of particle physics - the Standard Model - explains the particles and forces observed in experiments, yet fails to explain many cosmological observations about the Universe. We have little understanding of dark matter or dark energy, and no explanation for the observed asymmetry between matter and antimatter. Many theories have been constructed to address these shortcomings, but none are yet supported by experimental evidence. We aim to shed light on these important problems in particle physics and cosmology by measuring the roundness of electrons with exquisite precision. The roundness of a particle is quantified by its electric dipole moment (EDM). A non-zero EDM violates time-reversal symmetry, implying that nature has an arrow of time at a fundamental level. In almost all theories, this symmetry needs to be violated to explain the preponderance of matter over anti-matter in the Universe. In the Standard Model, the predicted value of the electron EDM is extremely tiny, whereas most new theories, including most forms of supersymmetry, predict values many orders of magnitude larger. Furthermore, some theories of dark matter predict EDMs that oscillate with a magnitude and timescale that can be measured. Thus, EDM measurements offer extraordinary potential for new discoveries - they tell us about the fundamental symmetries essential to understanding the origins and nature of our Universe, they distinguish decisively between the Standard Model and its extensions, and they offer clues about the nature of dark matter. We have built an instrument that measures the electron EDM using electrons in heavy molecules. The molecules are spin-polarized and the spin precession rate in an electric field is measured to determine the EDM. We cool the molecules to a few microkelvin so that we can watch the spin precess for a long time. This makes the measurement far more precise and is the key advance over previous measurements. The goals of the project are to bring the instrument to full sensitivity, study and control systematic effects that could give false results, then determine the electron roundness at least ten times better than ever before. We will also build the upgrades needed for a further factor of ten improvement. Our result will be sensitive to new particles with masses beyond the reach of the greatest colliders, and to interactions that orchestrated the matter-antimatter asymmetry in the Universe.
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