PROCEEDINGS OF RIKEN BNL RESEARCH CENTER WORKSHOP (VOL. 71)

KHARZEEV, D. ; STASTO, A. ; TUCHIN, K. ; VOGELSANG, W.

Brookhaven National Laboratory

关键词: Gluons;    Modifications;    Partons;    Quantum Chromodynamics;    Renormalization;   

DOI  :  10.2172/15015156
RP-ID  :  BNL--73793-2005
RP-ID  :  AC02-98CH10886
RP-ID  :  15015156

美国|英语

来源: UNT Digital Library

【 摘 要 】

The high energy limit of Quantum Chromodynamics is one of the most fascinating areas in the theory of strong interactions. Over a decade ago the HERA experiment at DESY in Hamburg provided strong evidence for the rise of the proton structure function at small values of the Bjorken variable x. This behavior can be explained as an increase of the gluon density of the proton with energy or correspondingly with smaller values of x. This increase can be attributed on the other hand to the large probability of gluon splitting in QCD. The natural framework for describing the gluon dynamics at small x is the Balitskii-Fadin-Kuraev-Lipatov formalism developed some 30 years ago. It predicts that the gluon density grows very fast with increasing energy, as a power with a large intercept. This increase has to be tamed in order to satisfy the unitarily bound. Over two decades ago, Gribov, Levin and Ryskin proposed the mechanism called the parton saturation, which slows down the fast rise of the gluon density. This formalism accounts for an additional gluon recombination apart from the pure gluon splitting. It leads to the very interesting non-linear modification of the evolution equations for the gluon distributions. Since then, much progress has been made in the theoretical formulation of the parton saturation. Currently the most complete theory for parton saturation is the Color Glass Condensate (CGC) with the corresponding renormalization group functional evolution equation, the JIMWLK equation, which describes the nonlinear evolution of the gluon density at small values of x and in the regime where gluon fields are strong. The simpler form of the JIMWLK equation, the Balitskii-Kovchegov (BK) equation has been successfully used to explain the experimental data on proton structure function. The models, which include the parton saturation, have been applied to explain the experimental data at Tevatron and RHIC. In the latter case the Color Glass Condensate can be thought of as an initial stage for the subsequent formation of the Quark Gluon Plasma. Despite its success in describing various observables, the parton saturation phenomenon still needs deeper understanding and improvements; in particular, the existence or limitations on geometrical scaling, the edge effects in the high energy collisions, or impact parameter dependence. In particular it has been recently realized that the current evolution equations of CGC, the JWIWLK equations miss some of the important contributions coming from the resummation of the so-called Pomeron loops. These terms are known to provide sizeable corrections to the asymptotic high energy behavior. Also, the CGC formalism was constructed within the leading logarithmic approximation, and it is known that the corrections which go beyond this order are large.