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String theories as the adiabatic limit of Yang-Mills theory

We consider Yang-Mills theory with a matrix gauge group $G$ on a direct product manifold $M=Σ_2\times H^2$, where $Σ_2$ is a two-dimensional Lorentzian manifold and $H^2$ is a two-dimensional open disc with the boundary $S^1=\partial H^2$. The Euler-Lagrange equations for the metric on $Σ_2$ yield constraint equations for the Yang-Mills energy-momentum tensor. We show that in the adiabatic limit, when the metric on $H^2$ is scaled down, the Yang-Mills equations plus constraints on the energy-momentum tensor become the equations describing strings with a worldsheet $Σ_2$ moving in the based loop group $ΩG=C^\infty (S^1, G)/G$, where $S^1$ is the boundary of $H^2$. By choosing $G=R^{d-1, 1}$ and putting to zero all parameters in $ΩR^{d-1, 1}$ besides $R^{d-1, 1}$, we get a string moving in $R^{d-1, 1}$. In arXiv:1506.02175 it was described how one can obtain the Green-Schwarz superstring action from Yang-Mills theory on $Σ_2\times H^2$ while $H^2$ shrinks to a point. Here we also consider Yang-Mills theory on a three-dimensional manifold $Σ_2\times S^1$ and show that in the limit when the radius of $S^1$ tends to zero, the Yang-Mills action functional supplemented by a Wess-Zumino-type term becomes the Green-Schwarz superstring action.