Description:

In his seminal 1926 paper, P.A.M. Dirac predicted that the g-factor for electrons is ge = 2. Subsequent more precise experiments revealed that ge was slightly larger than 2. The reason was to be found in quantum mechanics, and the first radiative correction to ge, ae = a/2pi (ae =(ge -2)/2 is called the electron’s magnetic anomaly) calculated by Julian Schwinger, explained a deviation of order 0.1 %. Today the Standard Model predicts the value of the muon’s magnetic anomaly aμ to a precision of ± 0.36 parts per million (ppm). This calculation requires the calculation of the quantum contributions to aμ from all the known forces: the strong, electromagnetic and the weak forces. The Fermilab experiment E989 has measured aμ to ± 0.2 ppm precision, a factor of four more precise than what was obtained at Brookhaven National Laboratory. Additional data collected will reduce this uncertainty by another factor of two. Measurements of the muon magnetic anomaly provide a stringent test of the Standard Model’s completeness, since nature knows about all forces/virtual particles that could contribute to the muon’s magnetism, including those from New Physics that have not yet been discovered.

I will first review the intellectual history that began with the discovery of spin and the g-factor of the electron and its role in the development of modern physics. I will then focus on our new measurement of the muon magnetic anomaly, which was recently reported by Fermilab experiment E989. The result determined from the data sets collected in 2019 and 2020 has a precision of ±0.2 ppm and agrees well with our first Fermilab Run 1 data set. I will also discuss the comparison of our experimental result with the Standard Model value.

Lee Roberts, Professor, Boston University

Host: Gerald Gabrielse