Electron Magnetic Moment, Fine Structure Constant, Mass Ratios, Laser Spectroscopy and QED

Harvard University, Cambridge MA

Investigators

Abstract

This project represents a continuation of a long series of experiments that pose ever-increasingly more stringent tests of fundamental quantities and symmetries, including: 1. the electron magnetic moment (often called its g value) 2. the fine structure constant alpha 3. most stringent test of CPT (charge-parity-time) invariance with leptons 4. the ratio of the masses of the proton and electron 5. the proton and antiproton magnetic moments Technological spinoffs of this research have been incorporated into ion cyclotron resonance spectroscopy and magnetic resonance imaging, and the improvement of promising new detectors continues, demonstrating broader applicability. A better value of alpha is crucial to determining a number of the set of fundamental constants that are needed by a technological society. Quantum electrodynamics (QED) incorporates alpha into its description of the interaction of matter and light. It is the prototype quantum field theory, serving as the model which theoretical efforts to describe strong interactions and gravity seek to emulate. An outstanding achievement of QED is the predicted relationship between the electron magnetic moment g and alpha, which allows alpha to be determined by measuring g, Given the success of calculating the parameters describing this relationship, QED theory now contributes 43 times less uncertainty to g than do previous measurements. A sufficiently improved measurement of g will thus determine alpha 43 time more accurately. An independent measurement of alpha, along with the measurement of g, will test QED to an unprecedented level of accuracy the most precise comparison of any theory and experiment. The experimental precision, with the theoretical accuracy expected in calculations underway, should allow distinguishing between inconsistent values of alpha measured from the quantum Hall effect, the ac Josephson effect, and from a neutron measurement. An iodine clock and optical comb developed for this work, technologies of much broader applicability for communications, will make possible absolute frequency measurements and more accurate measurements of frequency intervals.