1S-2S Spectroscopy of Positronium

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The interactions of the fundamental particles inside atoms underpins atomic physics, chemistry, and the structure of all matter. The understanding of these interactions is based on precise measurements of the light that is emitted and absorbed by various atoms and molecules. The precise set of colors (or "resonant frequencies") of this light is a unique fingerprint that is used for chemical identification in a broad range of applications from medicine to defense and quality control in manufacturing. The precise measurement of the fingerprint can also tell us details about the fundamental particles that make up atoms. Recently, a measurement of a particular resonant frequency in hydrogen (an electron bound to a proton) and muonic hydrogen (a heavy version of the electron, the muon, bound to a proton) has revealed a mystery: the diameter of the proton appears to be different depending on which atom it is in. This mystery arises if one assumes that the electron and muon interact with the proton by the well-accepted theory called bound-state quantum electrodynamics (QED) and that any differences must be attributed to the nuclear structure. Assuming that there have been no mistakes, there are 3 possibilities: QED is incorrect, the nuclear structure is incorrect, or there is some new kind of interaction which has not been taken to account. 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 measure the resonant frequencies of positronium, the simplest atom (consisting of an electron bound to an anti-electron) that should be described perfectly by bound-state QED theory. The new techniques developed for these measurements will lead to an improvement in the spectroscopy of unstable atoms which are at the frontier of the field of precision spectroscopy. The scientists involved with the project have a strong record in increasing diversity in physics. The PI plays a special role in the local community K-12 system, advising on increasing the number of students taking high school physics and increasing the pool of physical science/engineering majors in higher education and in teacher training. The co-PI has established a pipeline for recruiting local area junior college transfers to UCR, engaging them in research, and mentoring their eventual applications to graduate schools.The purely leptonic atom positronium is uniquely well-suited for testing bound-state quantum electrodynamics (QED) and provides the understanding and background by which one 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 in 10^12 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 1.234 PHz. Last measured in collaboration with S. Chu, knowledge of the first 10 digits of this interval has stood for 20 years with an uncertainty of ±3.2 MHz. The proposed experiment implements a new technique that would dramatically improve the accuracy of Ps atom spectroscopy and potentially other high resolution spectroscopy experiments, for example in muonium, by individual atom trajectory analysis. A position sensitive time-of-flight detector will record the trajectory and speed of every detected atom and thereby remove second-order Doppler shifts, AC Stark Shifts, and better account for transit-time broadening. The new detection method will allow for reducing the laser intensity and the speed of the detected atoms that heretofore have limited the measurement precision. The basic features of the analysis and development of the methods to analyze and reduce uncertainties will be accomplished using a wavemeter with ±2 MHz metrology relative to an ultrastable reference cavity. This method will be sufficient to produce narrow line widths (~1 MHz) and a vastly improved signal-to-noise ratio in the 3 year period of the proposal. A 100 fold better measurement and exploration of the systematic effects at 1000 fold better accuracy will be achieved with the acquisition of a laser frequency comb reference.