I would like to understand the following statement found on page 6 second paragraph of 0305098.
The authors discuss the quantization of the level $k$ of the WZW model for compact (and non-compact) manifold. They notice that, on the compact case, $k$ is quantized and the (classical) tensionless limit is not valid anymore. That makes them quiet happy because:
This should not come as a surprise since massless particles on a compact manifold are problematic
Could someone give me a reference to understand this statement?