An Improved Measurement of the Electron Magnetic Moment

Abstract

A single isolated electron in a Penning trap yields a new measurement of the electron magnetic moment g/2 = 1.001 159 652 180 59 (13). Combined with the Standard Model calculation, this yields an independent determination of the fine structure constant α−1 = 137.035 999 166 (16). Comparison of the measured g-factor and the predicted g-factor using an independent measurement of the fine structure constant yields the most stringent test of the Standard Model. A new dilution refrigerator–superconducting solenoid system with significantly improved stability has been constructed. The new system has a more robust mechanical joint, which improves the long-term stability of the magnetic field. A Helium-3-based cryogenic NMR probe has been invented and used to optimize the homogeneity and the drift rate of the cryogenic bore magnet. The achieved low drift rate and robustness enables measurement of the g-factor at many widely different fields for the first time. The statistical uncertainty has been improved by a factor of 4 because of the newly developed system. The large systematic shift—microwave cavity correction—is studied in an eight times wider parameter range. A new correction model and characterization methods have been proposed. The g-factor is measured at 11 fields to confirm the new systematic correction method. Three developments to improve the systematic error and precision for future measurements have been proposed and demonstrated. A new trap with an order of magnitude better anharmonicity is proposed and designed. The new trap is smaller than the current trap to suppress the microwave cavity correction. A new scheme to measure g-factor with direct measurement of the spin frequency is proposed and demonstrated. A superconducting quantum limited detector has been also developed to achieve 20 times narrower linewidth. All developments promise an improved measurement of the electron’s g-factor. The constructed system has been also used to search dark photon dark matter. The single trapped electron is used as a background-free detector at 0.6 meV. A new limit on dark photon is set with a week of data. The search demonstrates the sensitivity of the single electron and guarantees a future search in the 0.1–1 meV range. The newly constructed system and a better understanding of the systematic error allow tests of the Standard Model and theories beyond it in many aspects. The same technique can be applied for the positron’s g-factor measurement, which will be the most precise test of CPT in the lepton sector.