Large loop-coupling enhancement of a heavy pseudoscalar from a light   dark sector
  The small background and the sensitivity to charged particles via a leading order loop coupling make the diphoton channel a privileged experimental test for new physics models. We propose a simple archetypal scenario to generate a sharp di-photon resonance as a result of threshold enhancements in the effective coupling between a heavy pseudoscalar particle and new vector-like leptons. We therefore study three different scenarios consistent with the current experimental limits and deviating from the Standard Model at the 2~$\sigma$ level. The model also introduces a natural dark matter candidate able to match the observed dark matter abundance and comfortably respect the current direct detection constraints. 
Stefano Di Chiara, Andi Hektor, Kristjan Kannike, Luca Marzola, and   Martti Raidal
hep-ph