# Congruence Subgroups and Super-Modular Categories

## TL;DR

This paper investigates the properties of super-modular categories, especially their representations of the theta subgroup of SL(2,Z), and proves a conjecture relating their kernels to congruence subgroups in certain cases.

## Contribution

It introduces a super-modular analogue of the Ng-Schauenburg Congruence Subgroup Theorem and proves it for categories with minimal modular extensions.

## Key findings

- Proves the conjecture for super-modular categories with minimal modular extensions.
- Establishes a connection between super-modular categories and congruence subgroups.
- Suggests all super-modular categories may satisfy the conjecture through their extensions.

## Abstract

A super-modular category is a unitary pre-modular category with M\"uger center equivalent to the symmetric unitary category of super-vector spaces. Super-modular categories are important alternatives to modular categories as any unitary pre-modular category is the equivariantization of a either a modular or super-modular category. Physically, super-modular categories describe universal properties of quasiparticles in fermionic topological phases of matter. In general one does not have a representation of the modular group $\mathrm{SL}(2,\mathbb{Z})$ associated to a super-modular category, but it is possible to obtain a representation of the (index 3) $\theta$-subgroup: $\Gamma_\theta<\mathrm{SL}(2,\mathbb{Z})$. We study the image of this representation and conjecture a super-modular analogue of the Ng-Schauenburg Congruence Subgroup Theorem for modular categories, namely that the kernel of the $\Gamma_\theta$ representation is a congruence subgroup. We prove this conjecture for any super-modular category that is a subcategory of modular category of twice its dimension, i.e. admitting a minimal modular extension. Conjecturally, every super-modular category admits (precisely 16) minimal modular extensions and, therefore, our conjecture would be a consequence.