Hadronic light-by-light contribution to $a_\mu$: extended Nambu-Jona-Lasinio, chiral quark models and chiral Lagrangians

Abstract

This talk discusses our old work on the hadronic light-by-light contribution to the muon anomalous magnetic moment and some more recent contributions. I discuss the various contributions starting with pseudo-scalar meson exchange, the quark- and pion-loop, as well as scalar and $a_1$-exchange. For the $\pi^0$-exchange I point out a possible large enhancement when only connected contributions are included. For the quark-loop I include some comments about the more recent estimates of this contribution. The pion-loop is discussed in more detail, in particular I discuss our unpublished work on including effects from $a_1$ and the polarizability.