Prof. B. Lee Roberts, Boston University

"Exploring Terra Incognita with the World's Largest Penning Trap"

Abstract: The Standard Model provides a very precise prediction of the muon’s magnetic anomaly aμ = (gμ - 2)/2, the deviation from 2 of the gyromagnetic ratio gμ. In his seminal 1926 paper, P.A.M. Dirac predicted that for electrons ge = 2, but experiments then revealed that ge was slightly larger than 2. The reason was to be found in Quantum Mechanics, and the first radiative correction to ge , calculated by Julian Schwinger, explained a deviation of order 0.1 %. Today, the Standard Model predicts the value of aμ to a precision of ± 0.36 parts per million (ppm). Dedicated experiments have measured aμ to ± 0.35 ppm precision. Therefore, precision measurements of the anomaly provide a stringent test of the Standard Model’s completeness, since Nature knows about all forces that could contribute to the muon’s magnetism, including those from New Physics that has not yet been discovered.

I will briefly review the intellectual history that began with the discovery of spin and the g-factor of the electron and its role on the development of Modern Physics. I will then focus on the new measurement of the muon magnetic anomaly that was recently reported by Fermilab experiment E989. The result determined from the first data set collected in 2017 has a precision of 0.46 ppm, and agrees well with the previous result obtained at Brookhaven National Laboratory at the beginning of this century. The combined experimental value exhibits tension with the Standard Model value.