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We summarize our recent new evaluation of the pion-exchange contribution tohadronic light-by-light scattering in the muon g-2. We first derive a newshort-distance constraint on the off-shell pion-photon-photon form factor atthe external vertex in a_\mu which relates the form factor to the quarkcondensate magnetic susceptibility in QCD. We then evaluate the pion-exchangecontribution in the framework of large-N_C QCD using an off-shell form factorwhich fulfills all short-distance constraints and obtain the new estimatea_{\mu}(LbyL;\pi^0) = (72 \pm 12) x 10^{-11}. Updating our earlier results forthe contributions from the exchanges of the eta and eta-prime using simplevector-meson dominance form factors, we get a_{\mu}(LbyL; PS) = (99 \pm 16) x10^{-11} for the sum of all light pseudoscalars. Combined with availableevaluations for the other contributions to hadronic light-by-light scatteringthis leads to the estimate a_{\mu}(LbyL; had) = (116 \pm 40) x 10^{-11}. Thecorresponding contributions to the anomalous magnetic moment of the electronare also given.

Hadronic light-by-light scattering in the muon g-2: a new short-distance constraint on pion exchange