  Hagedorn states are the key to understand how all hadrons observed in high
energy heavy ion collisions seem to reach thermal equilibrium so quickly. An
assembly of Hagedorn states is formed in elementary hadronic or heavy ion
collisions at hadronization. Microscopic simulations within the transport model
UrQMD allow to study the time evolution of such a pure non-equilibrated
Hagedorn state gas towards a thermally equilibrated Hadron Resonance Gas by
using dynamics, which unlike strings, fully respect detailed balance.
Propagation, repopulation, rescatterings and decays of Hagedorn states provide
the yields of all hadrons up to a mass of m=2.5 GeV. Ratios of feed down
corrected hadron multiplicities are compared to corresponding experimental data
from the ALICE collaboration at LHC. The quick thermalization within t=1-2 fm\c
of the emerging Hadron Resonance Gas exposes Hagedorn states as a tool to
understand hadronization.
