Universal features of QCD dynamics in hadrons and nuclei at high energies

QCD has been called the perfect theory [1]; as a renormalizable field theory whose validity could extend up to the grand unification scale, it provides the mechanism for generating nearly all the mass of the visible universe. The current quark masses are the only external parameters in the theory. Quenched QCD, without dynamical quarks, explains the hadron spectrum to an accuracy a of 10%. These lattice results suggest that gluons play a central role in the structure of matter. The role of glue in QCD is best understood in the asymptotic weak coupling regimes of the theory where analytical computations are feasible. Much of the discussion in perturbative QCD (pQCD) has been in the Bjorken-Feynman asymptotics where Q −→ ∞, s −→ ∞ and xBj ≡ Q/s = fixed. The machinery of precision physics in QCD such as the operator product expansion and factorization theorems are derived in this limit of the theory. The progress in this direction has been truly remarkable [3]. In DIS for instance, both coefficient functions and splitting functions have been derived to next-to-next-to leading order (NNLO). What does the hadron look like in the Bjorken-Feynman asymptotics? The DGLAP evolution equations tell us that the gluon distribution grows rapidly with increasing Q at small xBj. However, the phase space density (in a particular gauge and frame), decreases rapidly with increasing Q. The proton become more “dilute” even though the number of partons increases; the typical size of resolved partons decreases as 1/Q, faster than the increase in the number through QCD evolution. The more dilute the hadron, the cleaner will be the QCD background for new physics beyond the standard model. Much of the current focus in QCD is in quantifying this background. It would be unfortunate however if this were the only focus in QCD studies because the theory, even in the weak coupling domain, contains rich and non-trivial dynamics. We speak here of the Regge-Gribov asymptotics where xBj −→ 0, s −→ ∞ and Q = fixed. This regime of strong color fields is responsible for the bulk of multiparticle production in QCD. What does the hadron look like in the Regge-Gribov asymptotics ? The BFKL equation, which resums the leading logarithms in x, indicates that the gluon distributions grow even more rapidly in this asymptotics. Unlike the Bjorken-Feynman case, the phase space density in the hadron grows rapidly as well. The stability of the theory requires that the phase space densities