The standard model of particle physics accurately describes all particle physics measurements made in the laboratory. However, it is unable to answer many central questions that arise from cosmological observations, such as the nature of dark matter and why matter dominates over antimatter throughout the Universe. Theories containing particles and interactions beyond the standard model may explain these phenomena. Such particles can come into virtual existence in the vacuum and then interact with known particles to modify their properties. For example, the existence of very massive particles whose interactions violate time-reversal symmetry, as needed to explain the cosmological matter–antimatter asymmetry, gives rise to an electric dipole moment along the spin axis of the electron. To date, no electric dipole moments, of the electron or other fundamental particles, have been observed.
An upgraded version of the ACME experiment, ACME II, has recently set a new experimental limit on the electron’s electric dipole moment. We obtained this result by measuring the electron spin precession in a superposition of quantum states of electrons subjected to a huge intramolecular electric field. The sensitivity of our measurement is one order of magnitude better than any previous work and probes for the existence of new particles with mass far beyond the direct reach of the Large Hadron Collider.