Measurement of the positive muon anomalous magnetic moment to 0.7 ppm
The 2000 run halved the world’s uncertainty on the muon’s magnetism, and the gap with theory started to show.
G. W. Bennett et al., Muon g−2 Collaboration, including S. Giron. Physical Review Letters 89, 101804 (2002).
In 2000 the experiment ran with four times more data than the year before. About four billion high-energy positrons were recorded by the ring’s 24 calorimeters.
Four independent analyses agreed to within 0.4 ppm. The result, precise to 0.7 parts per million, cut the combined world uncertainty roughly in half and sat about 2.6 standard deviations above the Standard Model prediction of the time.
My part Co-author. The positrons were counted by the calorimeters I helped build.
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What changed in 2000
Twelve beam bunches per cycle, a new inflector magnet, a sweeper magnet to cut background, and extra detectors to monitor lost muons.
The magnetic field
A trolley carrying 17 NMR probes mapped the field inside the vacuum chamber; about 150 fixed probes tracked it between maps. The field was known to 0.2 ppm.
Counting positrons
Twenty-four lead/scintillating-fiber calorimeters, read out by 400 MHz waveform digitizers, recorded each positron’s energy and time.
Four analyses, one answer
Different teams fit the data in different ways, each handling the beam’s coherent oscillations and pulse pileup on its own. They agreed within 0.4 ppm.
Comparison with theory
The Standard Model value then was 11 659 177(7) × 10−10, a difference of about 2.6 times the combined uncertainty.
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