Large two-loop contributions to g-2 from a generic pseudoscalar boson - art. no. 091301

We calculate the dominant contributions to the muon g-2 at the two-loop lev el due to a generic pseudoscalar boson that may exist in any exotic Higgs s ector in most extensions of the standard model. The leading effect comes fr om diagrams of the Barr-Zee type. A sufficiently light pseudoscalar Higgs b oson can give rise to contributions as large as the electroweak contributio n which is measurable in the next round of g-2 experiments. Through the con tribution we calculate here, the anticipated improved data in the recent fu ture 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.