Speaker
Description
The Muon g−2 Experiment at Fermi National Accelerator Laboratory has concluded its six-year data-taking campaign with a measurement of the muon's anomalous magnetic moment, a_μ, at a precision of 0.127 parts per million. Because a_μ arises entirely from quantum fluctuations, it is sensitive to all the particles and interactions of the Standard Model, as well as to possible new degrees of freedom that are too heavy or too weakly coupled to be produced directly. To fully exploit this sensitivity, however, a theoretical prediction of comparable accuracy is required.
In this course, I will first explain why lepton anomalous magnetic moments, particularly the muon's, are such effective probes of fundamental physics, and describe how a_μ is measured, from producing polarized muons to extracting their anomalous spin-precession frequency in a storage ring. I will then review the Standard Model prediction: the QED contributions, now known to five loops; the electroweak corrections; and the hadronic contributions, which dominate the theoretical uncertainty. After discussing the dispersive, data-driven determination of the leading-order hadronic vacuum polarization contribution and the tensions that have emerged among the underlying e⁺e⁻ → hadrons measurements, I will give a brief introduction to lattice QCD. I will then present a hybrid calculation of this contribution, in which about 96% is computed from first principles on the lattice and the small remainder is obtained from e⁺e⁻ → hadrons data. Combined with all other Standard Model contributions, this result yields a prediction that agrees with experiment within one standard deviation, thereby testing the Standard Model at the level of 0.4 parts per billion on the muon magnetic moment, g_μ=2(1+ a_μ). I will conclude with the open questions that remain and the prospects for further progress on both the experimental and theoretical fronts.