Abstract
After a brief introduction on ongoing experimental and theoretical activities on (g - 2)μ, we report on recent progress in approaching the calculation of the hadronic light-by-light contribution with dispersive methods. General properties of the four-point function of the electromagnetic current in QCD, its Lorentz decomposition and dispersive representation are discussed. On this basis a numerical estimate for the pion box contribution and its rescattering corrections is obtained. We conclude with an outlook for this approach to the calculation of hadronic light-by-light.
Highlights
The measured value of the anomalous magnetic moment of the muon aμ, obtained by the BNL E821 experiment [1], represents a puzzle for the standard model (SM): it differs by about three standard deviations from the calculated value
Taken at face value this is a serious discrepancy, but before claiming a real crisis for the SM or plain discovery of new physics, it is important to make sure that systematic effects, either on the theory or on the experimental side, have not been underestimated. This requires redoing the measurement, ideally in a completely new setting. This is the aim of the Muon g − 2 experiment [4] which has started to run at Fermilab and aims to reduce the final uncertainty reached by the BNL E821 experiment by about a factor four
After a quick overlook at ongoing lattice calculations of the hadronic light-by-light (HLbL) contribution to the (g − 2)μ we have concentrated on the dispersive approach and briefly summarized its basic steps
Summary
The measured value of the anomalous magnetic moment of the muon aμ, obtained by the BNL E821 experiment [1], represents a puzzle for the standard model (SM): it differs by about three standard deviations from the calculated value (see e.g. [2, 3]). Taken at face value this is a serious discrepancy, but before claiming a real crisis for the SM or plain discovery of new physics, it is important to make sure that systematic effects, either on the theory or on the experimental side, have not been underestimated This requires redoing the measurement, ideally in a completely new setting. This is the aim of the Muon g − 2 experiment [4] which has started to run at Fermilab and aims to reduce the final uncertainty reached by the BNL E821 experiment by about a factor four. The reason is simple: the calculation of the HVP is based on an exact relation, derived from analyticity and unitarity, which allows one to express this contribution as an integral over the measurable cross section σ(e+e− → hadrons). The status of the calculations of the HVP contribution is covered by the talk given by Christoph Lehner [24]
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