Boundary Dynamics of Asymptotically Flat 3d Gravity Coupled to Higher Spin Fields

We construct a two-dimensional action principle invariant under a spin-three extension of BMS3 group. Such a theory is obtained through a reduction of Chern-Simons action with a boundary. This procedure is carried out by imposing a set of boundary conditions obtained from asymptotically flat spacetimes in three dimensions. When implementing part of this set, we obtain an analog of chiral WZW model based on a contraction of sl(3,?) × sl(3,?). The remaining part of the boundary conditions imposes constraints on the conserved currents of the model, which allows to further reduce the action principle. It is shown that a sector of this latter theory is related to a flat limit of Toda theory. © The Authors.