ANOMALOUS THRESHOLDS AND THE ISGUR-WISE FUNCTION

The original de Rafael-Taron bound on the slope of the Isgur-Wise func tion at zero recoil is known to be violated in QCD by singularities ap pearing in an unphysical region. To be consistent, quark models must h ave corresponding singularity structures. In an existing relativistic quark-loop model, the meson-quark-antiquark vertex is such that the re quired singularity is an anomalous threshold. We also discuss the impl ications of another anomalous threshold, whose location is determined by quark masses alone.