The Lorentz Anomaly via Operator Product Expansion
  The emergence of a critical dimension is one of the most striking features of string theory. One way to obtain it is by demanding closure of the Lorentz algebra in the light-cone gauge quantisation, as discovered for bosonic strings more than fourty years ago. We give a detailed derivation of this classical result based on the operator product expansion on the Lorentzian world-sheet. 
Stefan Fredenhagen, Jens Hoppe, Mariusz Hynek
hep-th