The Hagedorn spectrum and large $N_c$ QCD in 2+1 and 3+1 dimensions
  We show that a Hagedorn spectrum (i.e., spectrum where the number of hadrons grows exponentially with the mass) emerges automatically in large $N_c$ QCD in 2+1 and 3+1 dimensions. The approach is based on the study of Euclidean space correlation functions for composite operators constructed from quark and gluon fields and exploits the fact that the short time behavior of the correlators is known in QCD. The demonstration relies on one critical assumption: that perturbation theory accurately describes the trace of the logarithm of a matrix of point-to-point correlation functions in the regime where the perturbative corrections to the asymptotically free value are small. 
Thomas D. Cohen and Vojt\v{e}ch Krej\v{c}i\v{r}\'ik
hep-th hep-ph nucl-th