Precision Measurement of 1S-2S Interval in Positronium

University Of California-Riverside, Riverside CA

Investigators

Abstract

This project will improve the accuracy of positronium (Ps) atom spectroscopy. Spectroscopy is important because the colors of light (or "spectrum") absorbed by atoms is a unique fingerprint that is useful for chemical identification in a broad range of applications found in medicine, defense, and industry. Spectroscopy can also reveal details about sub-atomic particles and their fundamental interactions. Ps is a unique atom for this purpose. Ps is formed by an electron bound with an anti-electron (positron). Because Ps does not contain any protons or neutrons, spectroscopy of Ps atoms is an excellent test of bound-state quantum electrodynamics, a cornerstone theory for modern atomic physics. This project will thus promote the progress of science. This project will also help expand the nation's high technology work force by training students from diverse backgrounds to develop and implement high precision spectroscopy techniques. The purely leptonic positronium (Ps) atom is uniquely well-suited for testing bound-state quantum electrodynamics (QED). High accuracy Ps spectroscopy at the few parts per trillion level will provide background information that can be used to extract non-QED physics out of precision atomic measurements with heavier leptons and hadrons. In this way, Ps spectroscopy can shed light on the proton charge radius puzzle. It can also help constrain higher level recoil effect corrections in muonium. The previous precision measurement of the Ps 1S-2S interval, 1,233,607,216.4 +/- 3.2 MHz, performed by the coPI and S. Chu has stood for 25 years. The uncertainty in this measurement came from positronium atoms spending too little time in the laser beam (transit time broadening), atoms moving too fast (second-order Doppler shift), laser intensity too high (AC Stark shift), and metrology limited at the ~1 MHz level. This project will implement several new techniques to improve accuracy and precision. A position-sensitive time-of-flight detector will record the trajectory and speed of every detected atom as it passes through a larger laser beam profile, and thereby reduce the uncertainty due to second-order Doppler shifts, AC Stark Shifts, and transit-time broadening. A frequency-comb and a frequency-beat technique will be used to monitor the instantaneous laser frequency as positronium atoms transit the excitation beam. These innovations should reduce the uncertainty in the measurement by a factor of 300, with the goal of reaching a 10 kHz uncertainty. This work will train graduate students in a combination of atomic physics, positron science, laser spectroscopy, and metrology. This training will benefit the students as well as the Nation by providing highly skilled Ph.D.'s for the scientific workforce. UCR is a Department of Education Designated Hispanic Serving Institution (HSI) with 30% of Physics majors and 15% of graduate students from under-represented ethnic minority groups. Several undergraduate researchers and a high school teacher will also be involved in these experiments, and this will help to attract more participants to STEM disciplines. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.