EXPLORING THE POLARIZATION OF GLUONS IN THE NUCLEON. Page: 4 of 12
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Figure 1. LO toy calculation of the Q2-
_1.0 . 1ooo evolution of the contributions to the proton
Q2 [GeV'] spin.
experiments are dedicated to a direct determination of the spin-dependent gluon distribution
Ag(x, Q2) - g+ (x, Q2) - g-(x, Q2) , (2)
where g+ (g-) denotes the number density of gluons in a longitudinally polarized proton with
same (opposite) sign of helicity as the proton's, and where x is the gluon's light-cone momentum
fraction. The field-theoretic definition of Ag is
Ag(x, Q2) =47rxP+ dA eAxp+ (P, SIG+u(0) G+(An)IP, S) ,Q2 (3)
written in A+ = 0 gauge. G " is the QCD field strength tensor, and G" its dual. The integral
of Ag(x, Q2) over all momentum fractions x becomes a local operator only in A+ = 0 gauge and
then coincides with AG(Q2) [2, 7]. The COMPASS experiment at CERN and the HERMES
experiment at DESY attempt to access Ag(x, Q2) in charm- or high-pT hadron final states in
photon-gluon fusion y*g -> qq. A new milestone has been reached with the advent of the first
polarized proton-proton collider, RHIC at BNL [8, 9]. RHIC will provide precise and detailed
information on Ag, over a wide range of x and Q2, and from a variety of probes.
2. Model estimates of Ag
Before we discuss in some detail the phenomenology of Ag in polarized high-energy scattering, let
us briefly address some of the available theoretical expectations for Ag and its integral. As was
first pointed out in , it is possible to estimate the operator matrix element corresponding
to AG in non-relativistic quark and bag models. In such models, for example, baryon mass
splittings result from lowest-order exchange of transverse gluons, and the associated forces
are spin-dependent. One obtains estimates [11, 12] for AG(Q2 ~ 1 GeV2) of about 0,2 to
0.3. In a sense, these are "natural" values since they are of the order of the proton spin
itself. Very recently, for the first time model calculations of the x-dependence of Ag have
been presented . The resulting distribution is positive everywhere and of moderate size.
The more and more precise experimental constraints on Ag will likely motivate further model
investigations, which ultimately might lead to new insights into QCD. Likewise, it is to be hoped
that lattice calculations, which are becoming ever more powerful, will be able to address gluonic
observables in nucleon structure in the future .
Other considerations, based in part on perturbation theory, led to the prediction of a very
large gluon polarization in the nucleon. The peculiar evolution pattern of AG(Q2) c 1/as(Q2)
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STRATMANN,M. & VOGELSANG,W. EXPLORING THE POLARIZATION OF GLUONS IN THE NUCLEON., article, October 22, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc888123/m1/4/: accessed December 11, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.