I discuss the history of the muon (g-2)(g−2)
measurements, beginning with the Columbia-Nevis measurement that
observed parity violation in muon decay, and also measured the muon
gg-factor
for the first time, finding g_\mu=2gμ=2.
The theoretical (Standard Model) value contains contributions from
quantum electrodynamics, the strong interaction through hadronic vacuum
polarization and hadronic light-by-light loops, as well as the
electroweak contributions from the WW,
ZZ
and Higgs bosons. The subsequent experiments, first at Nevis and then
with increasing precision at CERN, measured the muon anomaly
a_\mu = (g_\mu-2)/2aμ=(gμ−2)/2
down to a precision of 7.3 parts per million (ppm). The Brookhaven
National Laboratory experiment E821 increased the precision to 0.54 ppm,
and observed for the first time the electroweak contributions.
Interestingly, the value of a_\muaμ
measured at Brookhaven appears to be larger than the Standard Model
value by greater than three standard deviations. A new experiment,
Fermilab E989, aims to improve on the precision by a factor of four, to
clarify whether this result is a harbinger of new physics entering
through loops, or from some experimental, statistical or systematic
issue.
K-meson neutrinoless double muon decay as a probe of neutrino masses and mixings
