Status of hadronic light-by-light scattering in the muon g-2

Understand

We give an update on the status of the hadronic light-by-light scattering contribution to the muon g-2.

• We review recent work by various groups, list some of the open problems and give an outlook on how to better control the uncertainty of this contribution.
• This is necessary in order to fully profit from planned future muon g-2 experiments to test the Standard Model.
• Despite some recent developments, we think that the estimate a_{mu}^{HLbL} = (116 \pm 40) x 10^{-11} still gives a fair description of the current situation.