Large two-loop contributions to g - 2 from a generic pseudoscalar boson

We calculate the dominant contributions to the muon g - 2 at the two-loop level due to a generic pseudoscalar boson that may exist in any exotic Higgs sector in most extensions of the standard model. The leading effect comes from diagrams of the Barr-Zee type. A sufficiently light pseudoscalar Higgs boson can give rise to contributions as large as the electroweak contribution which is measurable in the next round of g - 2 experiments. Through the contribution we calculate here, the anticipated improved data in the recent future on the muon g - 2 can put the best limit on the possible existence of a light pseudoscalar boson in physics beyond the standard model.