Coupling a Cosmic String to a TQFT
A common framework of particle physics consists of two sectors of particles, such as the Standard Model and a dark sector, with some interaction between them. In this work, we initiate the study of a qualitatively different setup in which one of the sectors is a topological quantum field theory (TQFT). Instead of particles, the physics of a TQFT only manifests itself in non-trivial spacetime topologies. Topological defects provide a natural place to investigate such effects. In particular, we consider two possible ways in which axionic cosmic strings can interact with a Zn TQFT. One of them, by extending the structure of the axion coupling, leads to specific predictions for the localized degrees of freedom on the cosmic string, which can in turn effect their evolution and leave observable signals. The second approach, by gauging a discrete subgroup of the axionic shift symmetry, leads to dramatic changes in the string spectrum. We stress that the scenario considered here should be regarded as a plausible way for new physics to arise since it can be the low energy effective field theory for quite generic scenarios at high energies. To demonstrate this point and further illustrate the physical implications, we constructed such UV completions for both of the cases of couplings to TQFTs. The detailed prediction for observable signals of such scenarios needs further investigation. At the same time, our results demonstrate that there are rich new phenomena in this scenario.