A non-commuting twist in the partition function

TL;DR

This paper introduces a novel non-commuting twist in the computation of the partition function for orbifold theories, revealing connections to dihedral symmetries and the Mathieu group M_{24} in the context of BPS state counting.

Contribution

It presents the first analysis of twisted indices with non-commuting twists in orbifold models, especially for CHL models with dihedral symmetries, and links the results to Mathieu group M_{24}.

Findings

Twisted partition functions are dominated by the untwisted sector.

The generating functions for twisted BPS states relate to Mathieu group M_{24}.

Non-commuting twists affect the structure of the BPS spectrum.

Abstract

We compute a twisted index for an orbifold theory when the twist generating group does not commute with the orbifold group. The twisted index requires the theory to be defined on moduli spaces that are compatible with the twist. This is carried out for CHL models at special points in the moduli space where they admit dihedral symmetries. The commutator subgroup of the dihedral groups are cyclic groups that are used to construct the CHL orbifolds. The residual reflection symmetry is chosen to act as a `twist' on the partition function. The reflection symmetries do not commute with the orbifolding group and hence we refer to this as a non-commuting twist. We count the degeneracy of half-BPS states using the twisted partition function and find that the contribution comes mainly from the untwisted sector. We show that the generating function for these twisted BPS states are related to the Mathieu group M_{24}.