BNL-NT-07/12

Exploring the polarization of gluons in the nucleon
Marco Stratmann
Radiation Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
E-mail: marco@ribf.riken.jp
Werner Vogelsang)
BNL Nuclear Theory, Brookhaven National Laboratory, Upton, NY 11973, USA
E-mail: wvogelsang@bnl.gov
Abstract. We give an overview of the current status of investigations of the polarization of
gluons in the nucleon. We describe some of the physics of the spin-dependent gluon parton dis-
tiibution and its phenomenology in high-energy polarized hadronic scattering. We also review
the recent experimental results.
1. Introduction
For many years now, spin has played a very prominent role in QCD. The field of QCD spin
physics has been driven by the hugely successful experimental program of polarized deeply-
inelastic lepton-nucleon scattering (DIS) [1]. One of the most important results of this program
has been the finding that the quark and anti-quark spins (summed over all flavors) provide only
about a quarter of the nucleon's spin, AE ~~ 0.25 in the proton helicity sum rule [2, 3, 4]
1 1
= 2AE(Q2) + AG(Q2) + Lq(Q2) + Lg(Q2) ,                           (1)
implying that sizable contributions to the nucleon spin should come from the gluon spin
contribution AG(Q2), or from orbital angular momenta Lq,g(Q2) of partons. Here, Q is the
resolution scale at which one probes the nucleon. The Q2-dependence of the various contributions
to the proton spin is predicted in perturbative QCD through its evolution equations [4, 5, 6].
To lowest order (LO), the quark and anti-quark spin contribution AE(Q2)/2 does not depend
on Q2. Figure 1 shows a LO toy calculation of the Q2-evolution of the contributions to the
proton spin in Eq. (1), assuming that at an initial scale Qo = 1 GeV we have AE = 0.25,
AG = Lq = 0.2, Ls = -0.025. The rise of AG oc log(Q2) or 1/a,(Q2) (compensated by an
opposite evolution of L9) is an important prediction of QCD and awaits experimental testing.
For the initial conditions chosen here, the evolution leads to large positive values of AG. We
note that at asymptotic Q2 the total quark and gluon angular momenta, ZAE+Lq and AG+L9,
respectively, become roughly equal [4].
To determine the gluon spin contribution on the right-hand-side of Eq. (1) has become a major
focus of the field. Like AE, it can be probed in polarized high-energy scattering. Several current
1 Invited plenary talk presented at the "Second Meeting of the APS Topical Group on Hadronic Physics",
Nashville, Tennessee, October 22-24, 2006