Abstract
The anomalous magnetic moment of the muon (g − 2)μ is one of the most precisely measured quantities in particle physics (0.54 ppm). There is a long-standing discrepancy of 3-4 standard deviations between the direct measurement of (g − 2)μ and its theoretical evaluation. This theoretical prediction is subdivided into three contributions: QED, weak and hadronic. The QED and weak parts can be determined in perturbative approaches with very high precision. Thus, the hadronic uncertainty dominates the total theoretical uncer- tainty. Within the hadronic uncertainty, the largest contribution stems from the vacuum polarization term, which can be evaluated with the measurement of the inclusive hadronic cross section in e + e − annihilation. The second largest contribution to the hadronic uncer- tainty stems from the so-called Light-by-Light amplitudes. They have to be evaluated via theoretical models. These models require transition form factor measurements as input. Existing and future measurements of the relevant hadronic cross sections and transition form factors are presented.
Highlights
There are various ways to test the Standard Model (SM) of particle physics
One particular test at this high precision frontier is the measurement of the anomalous magnetic moment of the muon aμ = 0.5 · (g − 2)μ
There is a discrepancy of 3-4 standard deviations between the direct measurement of (g − 2)μ and its theoretical evaluation [1, 2]
Summary
There are various ways to test the Standard Model (SM) of particle physics. Many approaches include searches for new particles or phenomena at the high energy frontier. As an alternative to the energy scan experiments, the study of the Initial State Radiation (ISR) events at flavor-factories allows independent measurements of exclusive hadronic cross sections. This method allows high statistics e+e− experiments running at a fixed center-of-mass (c.m.) energy to access processes at lower effective c.m. energies by studying events with a high energy photon emitted from the initial state. The second largest contribution to the hadronic uncertainty stems from the so-called hadronic Light-by-Light (LbL) amplitudes They have to be evaluated via theoretical models. Existing and future measurements of (g − 2)μ, the relevant hadronic cross sections, and transition form factors are presented
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