Pinpointing with a Broad Beam: h/m and the Fine Structure Constant

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The fine structure constant characterizes the strength of the electromagnetic interaction and is ubiquitous in physics. It has been measured using methods from across physics, whose agreement is a powerful confirmation of the consistency of theory and experiment. Using well-measured fundamental constants (the Rydberg constant and atomic mass ratios), the fine structure constant can be related to the recoil velocity of an atom that has scattered a photon. This project will use this relationship in order to measure the fine structure constant with the highest precision ever. This will allow the research team to test several extensions of the standard model, such as dark matter candidates, and probe for a substructure of the electron, to test if the electron is indeed an elementary particle. The measurement of the recoil velocity (which is inversely proportional to the atomic mass) can also be used as an ultra-precise realization of the unit of mass and thus a cornerstone of the revised international system of units. This experiment is based on an atom interferometer. It uses a broad laser beam which comes extremely close to the ideal case of an infinite electromagnetic plane wave. A powerful pulsed laser will be available for this project, as well as an atomic fountain accommodating the beam. A new geometry for the atom interferometer will be used, vertically offset simultaneous conjugate interferometers (OSCIs). This will cancel the effects of vibrations, gravity, and even the gravity gradient. The project will measure h/m using a combination of Bragg diffraction and Bloch oscillations with cesium atoms at an accuracy of 0.02 ppb. From h/m, the fine structure constant will be determined with a target accuracy of 0.039 ppb, limited by the accuracy of mAt/me. It is hoped that that improvements in the cesium or electron mass measurement will reduce the error in the fine structure constant to 0.02 ppb. Already now, such measurements of the fine structure constant set stringent limits on several proposed particles. For example, scalar and pseudoscalar bosons, vector bosons, dark photons, hidden-sector photons, and axial-vector bosons would all be discoverable. This tests the standard model of particle physics broadly and deeply.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.