Gluon Polarization in the Nucleon

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Gluon Polarization in the Nucleon

• Werner Vogelsang
• RBRC BNL Nuclear Theory
• PANIC 05, 10/24/2005

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Outline
? The parton distribution ?g(x,Q2)
? ?g and the proton spin
? ?g and the axial anomaly
? Expectations for ?g
? Phenomenology of ?g in pol. high-energy scatt.
? Conclusions
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The parton distribution ?g(x,Q2)
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Main goal of QCD spin physics
To understand the spin structure of hadrons in
terms of quarks and gluons
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Gluon distributions
This is the quantity that appears in pol.
high-energy scattering !
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Properties of parton distributions
all these operators need renormalization ?
scale dependence / evolution of parton
distributions
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Glück, Reya, Stratmann, WV 96 / 00
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Evolution of first moment
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?g(x,Q2) and the proton spin
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Jaffe, Manohar Ratcliffe Ji Teryaev Jaffe,
Bashinskii Ji, Hoodbhoy, Lu
where
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LO evolution Ratcliffe Ji, Tang, Hoodbhoy
Hägler, Schäfer
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?g and the axial anomaly
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How does the QCD axial anomaly enter nucleon
spin structure ?
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singlet
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Proposal by Altarelli, Ross Altarelli,
Stirling Efremov, Teryaev
idea is that axial charge can be small if ?G
is large
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How large ?
early estimates (late 80s)
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perturbative-anomaly scenario controversial
in the literature
Jaffe, Manohar
is huge gluon spin contribution better than
small quark one ?
in any case, has given boost to attempts to
measure ??G
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Expectations for ?g
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Ideally
Lattice low-energy models confinement,
Nucleon Structure seen by hard probes
asymptotic freedom
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constituent quark model estimates
Jaffe Barone, Calarco, Drago
? ambient gluon fields
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?G on the lattice ?
Non-local operator, except for A 0 gauge
Attempt by Mandula 90
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Phenomenology of ?g in polarized
high-energy scattering
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?g can be probed in other reactions in
polarized high-energy scattering
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important to have adequate (and proven)
theoretical framework
? higher-order (NLO) corrections
? often sizable, reduce scale dependence
? comparison to data for unpolarized cross
sections
need variety of probes
eventually, will need a CTEQ-style global
analysis
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RHIC offers best possibilities to probe ?g
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Calculations are already successful in
unpolarized pp at RHIC
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Ellis, Karliner
Theory good enough ?
its a stretch to conclude anything about the
integral ?G
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Conclusions Outlook
gluon polarization promises to be an important
piece for our understanding
carried by tremendous efforts in experiments
RHIC, HERMES, COMPASS -- eRHIC ?
complementarity of lepton-nucleon scattering
and pp !
improvements in models, lattice ?
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N. Saito
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AB scheme