SymTFT approach to 2D orbifold groupoids: ’t Hooft anomalies, gauging, and partition functions

A bstract We use the 3D SymTFT approach to study the generalized symmetries and partition functions of 2D CFTs in various orbifolded and fermionic phases. These phases can be realized by the sandwich construction in the associated 3D SymTFTs with different gapped boundaries that encode the data of symmetries in the 2D CFTs. We demonstrate that the gapped boundaries can all be identified with the (fermionic) Lagrangian algebra in the 3D SymTFT, and thus use them to establish webs of dualities of the boundary CFTs in different phases on the level of partition functions. In addition, we introduce the concept of “para-fermionic Lagrangian algebra” which enables us to construct the partition functions of para-fermionized CFTs on the 2D boundary. Finally, we provide many important examples, including a 3D SymTFT viewpoint on gauging non-invertible symmetries in 2D CFTs.