Precision Measurement of the 1S-2S Interval in Positronium

General audience abstract:

The positronium atom (Ps) is an exotic short-lived atom formed by an electron and its anti-particle, the positron. This project will improve the accuracy of measurements of the structure of Ps, revealing detail about the fundamental interactions between the electron and positron. Unlike more conventional atoms Ps does not contain any protons or neutrons; thus spectroscopy of Ps atoms is an excellent test of “bound-state quantum electrodynamics,” the cornerstone theory for modern atomic physics.

This project pushes the state-of-the-art in the ultra-high precision measurement of short lived atoms and molecules and opens the door to better understanding of transient chemical species that are produced as a reaction is taking place. Graduate students will build and operate the apparatus designed especially for this experiment. Undergraduate students will assist in the experiments, mostly during the summers.

Local high school teachers will attend a week-long University of California Riverside Summer Academy for High School teachers at which teachers are exposed to research, visit the labs, and are inspired by lectures and demonstrations that explain how to incorporate research within the high school physics curriculum. The PI will visit high school Physics teachers and classrooms and work with local county Board of Education science projects to advance junior and senior high school curriculum development under the new Next Generation Science Standards and state science standards.

This project will thus help expand the nation's high technology work force by training students in cutting edge techniques and increase the diversity of participants in STEM fields, starting with the pipeline from junior high school physical science classrooms. Technical audience abstract:

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 as well as the recent shifting of the value of the Rydberg constant (by about 7 times the previously accepted experimental uncertainty).

Whenever a measured constant is shifted by such a large amount, then there is an opportunity for closer examination to yield new experiments and new physical understanding in how to measure the constants. Higher accuracy 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 Co-PI 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.

The University of California Riverside is a Department of Education Designated Hispanic Serving Institution (HSI) with 50% of Physics majors and 10% of domestic 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.