Chapter 8 Hadronization We now turn to a notoriously difficult problem of strongly coupled QCD: hadronization. Hadronization is the process of the formation of hadrons from gluons and quarks. It is widely believed that this is how hadrons were formed in the very early stages of the universe, as the quark-gluon plasma cooled sufficiently. Nowadays, those kind of energies and densities are only reached in extreme natural conditions, such as in the cores of neutron stars. They can also be reproduced in the laboratory, typically at high energy particle colliders of the type found at CERN, FermiLab or DESY, and most recently with the Relativistic Heavy Ion Collider (RHIC) in Brookhaven, USA. All these colliders are based on the simple premise of colliding two par- ticles together at very high energies, and seeing what the result is. Richard Feynmann likened the process to smashing two Swiss watches together to figure out how the watches work. It is a crude method, but has been very successful. Of primary interest are the hard processes which occur immedi- ately after the collision: these processes can be calculated using Feynman diagrams and have been used to verify the predictions of QCD to within an error of a few percent. However, the vast majority of the collisions after 106