Organizers: Aida El-Khadra, David Hertzog, Martin Hoferichter

The deviation between measurement and Standard-Model (SM) prediction of the anomalous magnetic moment of the muon is one of the few current persistent hints for physics beyond the SM. Because the muon g-2 arises from quantum-mechanical loop contributions in the SM, it is sensitive to virtual effects of new particles, and places important constraints on SM extensions. To leverage the anticipated reduction in the experimental errors of a factor of 4 at the Fermilab E989 experiment, and determine unambiguously whether or not New-Physics effects contribute to this quantity, the theoretical errors must be made more reliable and reduced to a commensurate level. The dominant sources of uncertainty in the SM prediction of the muon g-2 are from the hadronic corrections, in particular, from the hadronic vacuum polarization (HVP) and hadronic light-by-light scattering (HLbL) contributions. This workshop served as the third plenary meeting of the Muon g-2 Theory Initiative, formed to map out strategies for obtaining the best theoretical predictions for these hadronic corrections in advance of the experimental results.

With the release of the first results from E989 expected soon, a major thrust of the meeting was devoted to finalizing a white paper that will summarize the status of the theory prediction before the public release of the E989 measurement. In detailed discussion sessions, strategies for converging on the conclusions were developed, including a procedure for summarizing the available information into the SM prediction for the muon g-2. In particular, for HVP from data-driven dispersive analyses, the strategy accounts for the tension between the KLOE and BaBar 2pi data. For HLbL from dispersive analyses, the leading contributions are now under good control, with the error budget dominated by estimates of the subleading corrections. Even with a conservative procedure to estimate the uncertainties, the precision of the SM prediction will be comparable to experiment after the first E989 release. For the dispersive determinations of HVP and HLbL, new data are required in moving forward, priorities of which were discussed at the workshop. In addition, for HLbL strategies were discussed to address the subleading contributions.

Calculations of HVP and HLbL in lattice QCD are in agreement with the phenomenological evaluations, but not yet competitive in precision. Still, many examples were discussed for which already now lattice calculations provide valuable cross checks. Moreover, there is a clear path for improvements that will allow lattice QCD to challenge phenomenology in the coming years, and the required strategies were discussed in detail at the workshop.

In addition to the purely hadronic aspects, there were several talks on the anomalous magnetic moment of the electron, promising developments that in the next couple of years will allow for a gain in sensitivity by an order of magnitude.