Lepton Magnetic Moments and Fine Structure Constant

Harvard University, Cambridge MA

Investigators

Abstract

The most precise prediction made to date by a fundamental physical theory is that of a "magnetic moment," the strength of the magnet within the fundamental particle of electricity (the electron) and its antimatter counterpart (the positron). So far, measurements of the magnetism of these particles agree with prediction to a very high precision--much more precisely than those who formulated the theory ever expected. This is despite the fact that the same theory has serious problems: it predicts that no universe would survive after a big bang, and it has not been able to explain why the universe is made of matter rather than antimatter. What is wrong in our mathematical description, and the source of the fundamental imbalance between the properties of matter and antimatter, have yet to be discovered. This project will investigate such problems by measuring an electron's or positron's magnetism even more precisely than before. To do so, a single elementary particle will be suspended for months at a time. Batteries and magnets will keep the charged particle from colliding with any apparatus. Cooling the apparatus to nearly absolute zero will make a nearly perfect vacuum. To measure the magnetism, the separations of the lowest energy levels of the system will be probed by stimulating transitions between these levels using radio waves, and measuring the frequency of the waves that make these transitions occur most rapidly. This project promises to improve the measurement precision by an order of magnitude or more by stimulating two transitions simultaneously. Methods developed as part this project so far are being used to stabilize the magnets in magnetic resonance imaging (MRI) and to analyze the constituents of modern pharmaceuticals via ion cyclotron resonance (ICR) analysis. In more technical detail, a single electron or positron will be suspended in the electric and magnetic fields of a cylindrical Penning trap. Refrigeration below 0.1 kelvin will allow cryopumping to produce a nearly perfect vacuum and eliminate blackbody photons from the cylindrical cavity formed by the metal trap electrodes so the electron can radiate down to a cyclotron ground state. Electromagnetic driving forces will stimulate further cooling of the particle, and others will stimulate changes in its cyclotron and spin state. These one-quantum changes will be detected using quantum non-demolition methods that keep repeated detections from changing the quantum states of interest. Spontaneous emission of the particle's cyclotron motion will be inhibited, using a combination of the choice of the magnetic field strength and the cavity size, to give averaging times long enough for detecting a single quantum state of a single particle. The magnetic moment in natural units is essentially the measured ratio of the particle's spin and cyclotron frequencies, both of which will be measured simultaneously to greatly reduce the effect of tiny but unavoidable drifts of the magnetic field. The measured magnetic moments, the most precisely measured properties any elementary particle, will test of the most precise prediction of the standard model of particle physics at an unprecedented precision.