Modularity of logarithmic parafermion vertex algebras

The parafermionic cosets $C_k=\mathrm{C}\mathrm{o}\mathrm{m}(H,L_k({\mathfrak{s}\mathfrak{l}}_2))$ are studied for negative admissible levels $k$, as are certain infinite-order simple current extensions $B_k$ of $C_k$. Under the assumption that the tensor theory considerations of Huang, Lepowsky and Zhang apply to $C_k$, all irreducible $C_k$- and $B_k$-modules are obtained from those of $L_k({\mathfrak{s}\mathfrak{l}}_2)$, as are the Grothendieck fusion rules of these irreducible modules. Notably, there are only finitely many irreducible $B_k$-modules. The irreducible $C_k$- and $B_k$-characters are computed and the latter are shown, when supplemented by pseudotraces, to carry a finite-dimensional representation of the modular group. The natural conjecture then is that the $B_k$ are $C_2$-cofinite vertex operator algebras.