Effective field theory approach to parton-hadron conversion in high energy QCD processes

A QCD-based effective action is constructed to describe the dynamics of confinement and symmetry breaking in the process of parton-hadron conversion. The deconfined quark and gluon degrees of freedom of the perturbative QCD vacuum are coupled to color singlet collective fields representing the nonperturbative vacuum with broken scale and chiral symmetry. The effective action recovers QCD with its scale and chiral symmetry properties at short space-time distances, but yields at large distances (r≳1 fm) to the formation of symmetry-breaking gluon and quark condensates. The approach is applied to the evolution of a fragmenting qq¯ pair with its generated gluon distribution, starting from a large hard scale ${\mathit{Q}}^2$. The modification of the gluon distribution arising from the coupling to the nonperturbative collective field results eventually in a complete condensation of gluons. Color flux tube configurations of the gluons in between the qq¯ pair are obtained as solutions of the equations of motion. With a reasonable parameter choice, the associated energy per unit length (string tension) comes out ≃1 GeV/fm, consistent with common estimates.