In the latest twist to the US Brookhaven laboratory’s high-precision muon magnetism experiment, researchers announced in July a result slightly at odds with the Standard Model of particle physics.
The Brookhaven experiment measures the quantity g-2 for the muon, where g is the particle’s so-called gyromagnetic ratio – the ratio of its magnetic moment to its spin angular momentum. According to classical Dirac quantum theory, this should be exactly two. However, additional small effects can change this value, leading to a non-zero g-2.
The quantity g-2 is a very sensitive probe for testing the Standard Model, so when in 2001 the Brookhaven team announced a result at odds with Standard Model calculations, the particle physics world took notice. However, when the theoretical calculations were redone, a small error was found and agreement between theory and experiment was restored.
The original Brookhaven measurement was based on an analysis of 109 muon decays accumulated in 1999. The result announced in July includes a second data sample, accumulated in 2000, which contains four times as much data as the first. The new measured value of g-2 reinforces the earlier result with a total error of 0.7 ppm compared with the 1.3 ppm for the 1999 data measurement. Along with further refinement of the theoretical calculations, it shows a slight discrepancy with the current Standard Model value, differing by between 1.6 and 2.6 times the estimated error of the measurement. This is too small a discrepancy to claim new physics. Given the precision achieved by the Brookhaven experiment, however, it is bound to attract speculation.