Abstract
We show how to define Chern-Simons matter theories with boundary. Rather than imposing boundary conditions, we introduce new boundary degrees of freedom from the beginning and show how they can be used to cancel the gauge non-invariance of the Chern-Simons action. We apply this method to the ABJM theory with boundary. By imposing also boundary conformal invariance, we determine the required boundary action. This result allows us to derive the action for the multiple self-dual strings living on M5-branes. © SISSA 2010.