Mathieu Moonshine and symmetries of K3 sigma models
  A recent observation by Eguchi, Ooguri and Tachikawa (EOT) suggests a relationship between the largest Mathieu group M24 and the elliptic genus of K3. This correspondence would be naturally explained by the existence of a non-linear sigma-model on K3 with the Mathieu group as its group of symmetries. However, all possible symmetry groups of K3 models have been recently classified and none of them contains M24. We review the evidence in favour of the EOT conjecture and discuss the open problems in its physical interpretation. 
Roberto Volpato
hep-th