RST model

The Russo-Susskind-Thorlacius model or RST model in short is a modification of the CGHS model to take care of conformal anomalies. In the CGHS model, if we include Faddeev-Popov ghosts to gauge-fix diffeomorphisms in the conformal gauge, they contribute an anomaly of -24. Each matter field contributes an anomaly of 1. So, unless N=24, we will have gravitational anomalies. To the CGHS action, the following term

$S_{\text{RST}}=-{\frac {\kappa }{8\pi }}\int du\,dv\,{\sqrt {-g}}\left[R{\frac {1}{\nabla ^{2}}}R-2\phi R\right]$

is added, where κ is either $(N-24)/12$ or $N/12$ depending upon whether or not ghosts are considered. The nonlocal term leads to nonlocality.