Z -disks in {C} P²

Abstract We study locally flat disks in (CP²) ^: = ({C} P²) B⁴ with boundary a fixed knot K and whose complement has fundamental group Z. We show that, up to topological isotopy relative to the (rel. ) boundary, such disks necessarily arise by performing a positive crossing change on K to an Alexander polynomial one knot and capping off with a Z -disk in D⁴. Such a crossing change determines a loop in S³ K and we prove that the homology class of its lift to the infinite cyclic cover leads to a complete invariant of the disk. We prove that this determines a bijection between the set of rel. boundary topological isotopy classes of Z -disks with boundary K and a quotient of the set of unitary units of the ring Zt^ 1/ (K). Number-theoretic considerations allow us to deduce that a knot K S³ with quadratic Alexander polynomial bounds 0, 1, 2, 4, or infinitely many Z -disks in (CP²) ^. This leads to the first examples of knots bounding infinitely many topologically distinct disks whose exteriors have the same fundamental group and equivariant intersection form. Finally, we give several examples where these disks are realized smoothly.