Colorful boojums at the interface of a color superconductor

Abstract

We study junctions of vortices, or boojums, at the interface between color and hadronic superconducting/superfluid phases. This type of interface could be present in the interior of neutron stars, where an inner core made of quark matter in the color-flavor-locked phase is surrounded by an outer shell of superconducting protons and superfluid neutrons. We study the fate of magnetic (proton) and superfluid (neutron) vortices as they enter the color-flavor-locked phase. We find that proton vortices terminate on Dirac monopoles of the massless magnetic field, and magnetic fluxes of massive gauge field spread along the surface and are screened by surface superconducting currents. On the other hand, three neutron vortices join at a boojum and are split into three color-magnetic vortices with different color-magnetic fluxes canceled out in total. The color-magnetic vortices host confined color-magnetic monopoles when strange quark mass is taken into account. We also present a simple numerical model of the shape of the neutron boojum.