MAKING GLUE IN HIGH ENERGY NUCLEAR COLLISIONS Page: 2 of 9
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However, for x << 1 (corresponding to the transverse momentum range
AQCD << Pt << 's-) the factorization formula for energy and multiplicity dis-
tributions breaks down. Simply put, this is because partons in one nucleus can
resolve more than one parton in the other . The parton densities in the nu-
clei become very large and may even saturate at sufficiently small x. The regime
of high parton densities is a novel regime in QCD where, although the coupling
constant may be small, the fields strengths are large enough for the physics to be
The precise x value at which the above mentioned leading twist factorization
breaks down is not clear. There are hints from from HERA that parton saturation
is already seen in the data for x ~ 10-4 and Q2. ~ 4 GeV2 . If this result is
robust, similar effects may be seen in nuclei at larger values of x, even x ~ 10-2.
Their relevance for RHIC (and especially LHC) cannot then be ignored.
The effects of high parton densities in the central rapidity region of nuclear
collisions can be studied in a model which is based on an effective field theory (EFT)
approach to QCD at small x . The model describes the time evolution of gauge
fields in a nuclear collision. It takes into account, self-consistently, interference
effects (which are also responsible for shadowing in deeply inelastic scattering) that
become important at small x. Another nice feature is that it provides a space-time
picture of the nuclear collision. This feature would be extremely useful if the gauge
fields at late times were to provide the initial conditions for a parton cascade  or
for hydrodynamic evolution if it can be determined that the matter produced has
The model is formulated in the infinite momentum frame P+ - oo and light
cone gauge A+ = 0. It contains a dimensionful parameter 2, defined to be the
color charge squared per unit area,
A1/3f 1 1 N
p r2 = J2XO dx q(x Q2) + 2 g(x, Q2) (1)
fro x0 2N, NC2 1
Here q, g stand for the nucleon quark and gluon structure functions at the resolution
scale Q of the physical process of interest. Also, above xo = Q/'s-. Using the
HERA structure function data, Gyulassy and McLerran  estimated that < 1
GeV for LHC energies and < 0.5 GeV at RHIC. Thus a window of applicability
for weak coupling techniques does exist, and higher order calculations will tell us
if it is smaller or larger than the simple classical estimate.
An interesting property of the light cone gauge is that final state interactions are
absent! Kovchegov and Mueller  showed that the effects of final state interactions,
as seen in a covariant gauge computation, are already contained in the nuclear
wavefunction in light cone gauge. This non-trivial observation is at the heart of
the approach described in this talk. Finally, we should alert the reader to alternative
approaches to the one described here pursued in Refs. - .
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KRASNITZ,A. & VENUGOPALAN,R. MAKING GLUE IN HIGH ENERGY NUCLEAR COLLISIONS, article, November 20, 1998; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc708940/m1/2/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.