Sean Foster, BU: Measurement of the muon anomalous precession frequency in Runs 2 & 3 of the muon g-2 experiment at Fermilab

Abstract: In this defense, I present a measurement of the muon anomalous precession frequency for the Runs 2 & 3 data of the E989 Muon g-2 Experiment at Fermilab. The muon anomalous precession frequency is one of two key inputs, the other being the magnetic field, used to precisely determine the muon magnetic anomaly, aμ. In April 2021, the E989 collaboration reported its first measurement of aμ to an unprecedented precision of 460 parts-per-billion (ppb). This result is in agreement with the previous measurement performed by the E821 collaboration at Brookhaven National Laboratory, and the combined experimental value is in tension with the Standard Model prediction at 4.2 sigma, a possible hint of new physics. The first result from E989 was based on the Run-1 data, which was collected in 2018 and comprises 6% of the experiment’s target statistics; the Run-1 result was statistics-limited. The Runs 2 & 3 data were collected in 2019-2020 and constitute a four-fold increase in statistics compared to Run-1 and consequently, a factor of two reduction in the measurement’s statistical uncertainty. This reduced statistical uncertainty, as well as continued understanding of systematic effects, will result in an even more precise measurement of aμ and help clarify the observed tension between theory and experiment. The precession frequency analysis presented in this defense includes a number of improvements compared to Boston University's Run-1 analysis: the implementation of a more robust pileup-subtraction procedure, the implementation of a kernel ratio method, the adoption of the statistically optimal asymmetry-weighted method, and investigations that led to an improved understanding of an outstanding residual slow effect. In this defense, I will motivate a measurement of the muon magnetic anomaly, describe the experimental principle, give an overview of the E989 experiment, and present a precession frequency analysis with full systematic uncertainty evaluation for the Runs 2 & 3 data. The combined precession frequency measurement has a statistical uncertainty of 201 ppb and a systematic uncertainty <60 ppb, constituting the most precise determination of the muon anomalous precision frequency to date.