# Dyon degeneracies from Mathieu moonshine

## TL;DR

This paper constructs Siegel modular forms from twisted elliptic genera of K3 orbifolds related to Mathieu moonshine, providing explicit formulas and confirming their role as generating functions for 1/4 BPS dyons in string theory.

## Contribution

It completes the construction of Siegel modular forms and twisted elliptic genera for all CHL compactifications associated with Mathieu moonshine, including explicit expressions for all classes.

## Key findings

- Siegel modular forms satisfy properties for BPS dyon counting
- Inverse forms have integer Fourier coefficients with correct signs
- Construction covers all 7 CHL compactifications

## Abstract

We construct the Siegel modular forms associated with the theta lift of twisted elliptic genera of $K3$ orbifolded with $g'$ corresponding to the conjugacy classes of the Mathieu group $M_{24}$. We complete the construction for all the classes which belong to $M_{23} \subset M_{24}$ and two other classes outside the subgroup $M_{23}$. For this purpose we provide the explicit expressions for all the twisted elliptic genera in all the sectors of these classes. We show that the Siegel modular forms satisfy the required properties for them to be generating functions of $1/4$ BPS dyons of type II string theories compactified on $K3\times T^2$ and orbifolded by $g'$ which acts as a $\mathbb{Z}_N$ automorphism on $K3$ together with a $1/N$ shift on a circle of $T^2$. In particular the inverse of these Siegel modular forms admit a Fourier expansion with integer coefficients together with the right sign as predicted from black hole physics. Our analysis completes the construction of the partition function for dyons as well as the twisted elliptic genera for all the $7$ CHL compactifications.