MRI: Development of an Instrument for Ultra-High Resolution 1S-2S Spectrosopy of Exotic Hydrogenic Atoms

University Of California-Riverside, Riverside CA

Investigators

Abstract

Our most precise information about the structure and interactions of atoms is ultimately based on precise measurements of the light that is emitted and absorbed by various atoms and molecules. The spectrum of colors (or "resonant frequencies") of this light provides the basic input to calculations and measurements that are used for chemical identification, for reaction rate estimates, and for predictions of stability of new structures in a broad range of applications from medicine to defense and quality control in manufacturing. Recently, measurements on different types of hydrogen atoms (consisting of a central heavy proton and a planet-like electron or muon) are in disagreement about the radius of the proton. The discrepancy suggests one of 3 possibilities: (1) the theory of how charged particles interact, known as quantum electrodynamics or QED, is incorrect; (2) our knowledge of the structure of the proton is incorrect; or (3) there is some new kind of interaction or force of nature yet to be discovered. To decide whether the mystery is due to a problem with QED or not, two scientists from the University of California Riverside (UCR) propose to develop a novel instrument that will enable a measurement of the resonant frequency of positronium at ultimately 1000X more accuracy than the current state of the art. Because positronium is the simplest possible atom, consisting of an electron bound to an anti-electron, it should be described perfectly by QED theory. New measurements with the proposed instrument will decide whether QED theory is the problem and set significantly higher limits on the discrepancy in our understanding of the structure of the proton. The new instrument will lead to an improvement in the spectroscopy of unstable atoms at the frontier of the field of precision spectroscopy. The purely leptonic atom positronium (Ps) is uniquely well-suited for testing bound-state quantum electrodynamics (QED) and provides the understanding and background by which we may extract non-QED physics out of precision atomic measurements on heavier leptons and hadrons. Few have dared to try measurements on positronium at the few parts per trillion level that would allow insight into physics such as the proton charge radius and higher level recoil effect corrections in muonium, and that might show differences between light and heavy leptons. The ideal level spacing for a precision measurement on positronium is the 1S-2S interval at approximately 1 233 607 216 MHz. Knowledge of the first 10 digits of this interval has stood for 20 years with an uncertainty of ±3 MHz. The proposed instrument will implement several new techniques that would dramatically improve the accuracy of Ps atom spectroscopy and potentially other high resolution spectroscopy experiments by high precision individual atom trajectory analysis allowing line-centers to be measured to kHz precision. The laser frequency metrology during the transit of the atoms through the laser field will be accomplished with sub-kHz relative and absolute accuracies by recording the instantaneous deviations from the line-center of a thermally-stabilized ultra-low expansion glass reference cavity that is calibrated using a GPS-disciplined RF-referenced frequency comb. By providing ultrastable environmental, mechanical, and vibrational isolation, the overall specifications of the instrument will be sufficient to produce a narrow linewidth (~2 MHz), a reduction in systematic errors to enable up to 1000X increase in accuracy, and a 10X higher count rate to enable precision measurements in manageable measurement durations (months vs years).