  The CP violating Dashen phase in QCD is predicted by chiral perturbation
theory to occur when the up-down quark mass difference becomes sufficiently
large at fixed down-quark mass. Before reaching this phase, all physical
hadronic masses and scattering amplitudes are expected to behave smoothly with
the up-quark mass, even as this mass passes through zero. In Euclidean space,
the topological susceptibility of the gauge fields is positive at positive
quark masses but diverges to negative infinity as the Dashen phase is
approached. A zero in this susceptibility provides a tentative signal for the
point where the mass of the up quark vanishes. I discuss potential ambiguities
with this determination.
