The calibration system of the Muon g–2 experiment

The Muon g–2 experiment at Fermilab (E989) plans to measure the muon anomalous magnetic moment to a precision of 140 parts per billion (ppb), which corresponds to a total uncertainty of 1.6×10−10. To achieve this level of precision the experiment must detect more than 1.8×1011 decay positrons by using the 24 calorimeters distributed around the muon storage ring. Each calorimeter consists of 54PbF2 crystals read out by SiPMs. The response of each of the 1296 channels must be calibrated and monitored to keep uncertainties due to gain fluctuations at the sub-per mil level in the time interval corresponding to one beam fill (700μs) and at the sub-percent level on longer time scales. These requirements are much more demanding than those needed by most high energy physics experiments. This paper presents a novel laser-based calibration system that distributes light to all calorimeter cells, while allowing one to correct for laser intensity fluctuations and to monitor the distribution chain stability at unprecedented levels of accuracy. Results on the system performance during the first few months of stored muon operation in 2018 are also presented.