Magnetometry for the muon g-2 experiment

The Muon g-2 Experiment at Fermilab aims to measure the muon anomalous magnetic moment with a precision of 140 parts per billion (ppb). The collaboration has published the latest measurement based on the first three Runs (collected from 2018 to 2020) in August 2023 with a precision of 200 ppb. The experiment accumulated three more years of data, from 2020 to 2023, which are currently being analyzed. This additional statistic is sufficient to achieve and possibly exceed the goal of 100 ppb of final statistical uncertainty. As the statistical error reduces, increasing attention is dedicated to studying systematic uncertainties. Among them, one source is a magnetic transient generated by the fast kickers. To center the muon orbit into its final position in the storage ring, three kickers emit a 120 ns magnetic pulse of 240 G, right after injection. This, however, induces eddy currents in the kicker aluminum structure that last for several microseconds. To measure the 10 mG magnetic perturbations generated by the eddy currents, the INFN team developed a laser magnetometer based on the Faraday effect. This article describes the technical principles, operations, and data analysis of this sensitive device.