We compute the isospin-violating part \( {a}_{\mu}^{\textrm{HVP},38} \) of the hadronic-vacuum-polarization (HVP) contribution to the muon (g − 2) in lattice QCD at the SU(3)f-symmetric point where Mπ = MK ≃ 416 MeV. All diagrams involving internal photons are evaluated in coordinate space, employing a Pauli-Villars-regulated photon propagator with a cutoff scale Λ well below the lattice cutoff. The counterterm (mu – md), whose Λ dependence is consistent with the expected logarithmic behaviour, is calibrated using the experimental kaon mass splitting as input. The bare electromagnetic contribution at fixed Λ is compared to a phenomenological estimate based on the kaon-loop and pseudoscalar-pole contributions to the forward light-by-light amplitude. An extension of these calculations to physical pion and kaon masses appears promising.
