The Precision Magnetic Field Analysis for the Fermilab Muon g-2 Experiment

Abstract

There is presently a disagreement between the theoretical prediction and experimental measurement of the muon’s anomalous magnetic moment, aµ = (gµ − 2)/2, aµ(expt.) − aµ(theor.) = (2,706 ± 726) × 10^−12. Such a discrepancy could be a signal for new physics. The goal of Fermilab E989 is to make a more precise measurement of the muon’s magnetic moment to 140 ppb to shed light on the current discrepancy. Part of the new measurement requires a precision measurement of the magnetic field averaged by the muon motion as they circulate in a 14 m diameter storage ring.

This work describes one of two independent analyses of measurements from the magnetometer systems to calculate the precision field map in the Run 1 data sets. It also lays out the framework for averaging the magnetic field in both time and space, weighted by the muon distribution. The field is precisely mapped using a trolley that carries 17 NMR probes around the muon storage region. The field’s behavior in the times between the trolley scans is interpolated using an array of 378 NMR probes positioned around the outside of the muon storage region. The results are maps of the magnetic field as functions of time and space in the storage ring. Conservative estimates of the systematic uncertainties are also made for each data set in Run 1, with suggested improvements in both the data taking methods and future analyses.