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Gluon Polarization and the Axial Anomaly Puzzle

Explore the physics of gluon polarization and the theoretical puzzles surrounding it, including the axial anomaly contribution and the calculation of ΔG. Discover the connection between gauge invariance, photon helicity, and the IMF limit in QCD.

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Gluon Polarization and the Axial Anomaly Puzzle

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  1. XiangdongJi University of Maryland/SJTU Physics of gluon polarization Jlab, May 9, 2013

  2. Gluon polarization • Ever since the EMC “spin crisis”, the gluon polarization has been one of the most important pursuits of hadron physics community • HERMES • COMPASS • RHIC SPIN • “HERA-N” • “EIC…”

  3. Physics arguments • ΔG is an obvious contribution to the spin of the proton. • Can contribute to the quark helicity through axial anomaly (Altarelli & Ross, Carlitz, Collins, & Mueller,…) • Its contribution to the spin grows like 1/αS • However, there are a number of theoretical puzzles about it!

  4. Axial Anomaly • It was argued that Δq probed by DIS is not entirely due to the quark contribution. There is a gluon anomaly contribution. This contribution is proportional to (αS /2π)ΔG • For the anomaly to have a large contribution ΔG must be on the order few unit of hbar. • Thus, ΔG could be large even at non-perturbative scale.

  5. Physics: Feynman parton picture • A fast moving proton is a beam of free quarks and gluons. • The gluon partons have well-defined helicity ± 1 and densities g±(x) in wavelength • Gluon helicity distribution is g(x) = g+(x) – g-(x) and • G = ʃdx g(x) is the fraction of the proton helicity carried in the gluon. 1/2 +1 or -1

  6. QCD expression • The total gluon helicity ΔG is gauge invariant quantity, and has a complicated expression in QCD factorization (Manohar, 1991) • It does not look anything like gluon spin or helicity! Not in any textbook!

  7. Light-cone gauge • In light-cone gauge A+=0, the above expression reduces to a simple form which is the spin of the photon (gluon) (J. D. Jackson, CED), but is not gauge-symmetric: There is no gauge symmetry notion of the gluon spin! (J. D. Jackson, L. Landau & Lifshitz).

  8. Don’t know how to calculate • ΔG involves explicit light-cone correlation or real time. No one knows how to calculate this in lattice QCD (Models: RL Jaffe, Chen & Ji) • One can consider A+=0 gauge, but no one knows how to fix this gauge in lattice QCD • Thus there is no way to confront theory with experiment: G = ʃdx g(x) Is there a large contribution from small x?

  9. ALL from RHIC 2009

  10. Electric field of a charge

  11. A moving charge

  12. Gauge potential

  13. Observations • Although the transverse part of the vector potential is gauge invariant, the separately E┴ does not transform properly, under Loretez transformation, and is not a physical observable (X. Chen et al, x. Ji, PRL) • E ┴ generated from E ║ from Lorentz boost. • A lorentz-transformed E has different decomposition E = E┴ + E║ in different frames. • There is no charge that separately responds to E┴ and E║

  14. Large momentum limit • As the charge velocity approaches the speed of light, E┴ >>E║, B ~ E┴, thus • E┴ become physically meaningul • The E┴ & B fields appear to be that of the free radiation • Weizsacker-William equivalent photon approximation (J. D. Jackson) • Thus gauge-invariant A┴appears to be now physical which generates the E ┴ & B.

  15. Gauge invariant photon helicity • X. Chen et al (PRL, 09’) proposed that a gauge invariant photon angular momentum can be defined as ExA┴ • This is not an observable when the system move at finite momentum because this is only a part of the contribution which cannot be measured separately. • However, it becomes an observable in the IMF when Weizsacker-William’s picture is true!

  16. Theorem • Thus, one would expect that the total gluon helicity ΔG must be the matrix element of ExA┴ in a large momentum nucleon. • X. Ji, J. Zhang, and Y. Zhao (arXiv:1304.6708) is just the IMF limit of the matrix element of ExA┴

  17. QCD case • A gauge potential can be decomposed into longitudinal and transverse parts (R.P. Treat,1972), • The transverse part is gauge covariant, • In the IMF, the gauge-invariant gluon spin becomes

  18. One-loop example • The result is frame-dependent, with log dependences on the external momentum • Anomalous dimension coincides with X. Chen et al.

  19. Taking large P limit • If one takes P-> ∞ first before the loop integral, one finds • This is exactly photon (gluon) helicity calculated in QCD factorization! Has the correct anomalous dimension.

  20. Subtlety of the limiting procedures • There are two possible limits, • Taking p->∞ before UV regularization (physical case, light-cone) • Taking UV regularization before p->∞ (practical calculation, time-independent) • Two limits get the same IR physics • One can get one limit from the other by a perturbative matching condition, Z.

  21. Lattice QCD • ExA┴ is perfectly fit for lattice QCD calculation of ΔG! • To get large momentum nucleon, one has to have a fine lattice in the z-direction: P ~ 1/a • To separate excited states of the moving nucleon, one also needs fine lattice spacing in the time direction. • 322X642

  22. What a lattice calculation of ΔG implies? • Settles if axial anomaly plays an important role in the quark helicity measurement, by determining how large is ΔG • Since the experimental data says, how much ΔG sits at very small x? • How much the gluon helicity contributes to the proton helicity at small scale.

  23. x-dependence? • X-dependence of a parton distribution has been very difficult to calculate in the past. The only approach is through the local moments. • However, it is very difficult to calculate higher moments numerically. • It will be nice to find a way to directly calculate the x-dependence on lattice

  24. arXiv1305.1539

  25. observation • Usual parton distribution • Consider instead

  26. Relationship • The matching condition is perturbative • The correction is power-suppressed. For practical calculation, a momentum of 5 GeV might be good enough.

  27. The extension of the approach • GPDs • TMDs

  28. The extension of the approach • Wigner distribution • Light-cone amplitudes • Light-cone wave functions • Higher-twists….

  29. Conclusions • We find the gluon helicity measured in high-energy scattering is just EXA┴ in the large momentum limit, • This gives the gauge invariant and physically manifest notion of the total gluon helicity • This gives practical way to calculate ΔG • We find a practical way how to calculate light-cone distributions: PDFs, TMDs, GPDs, HTs, LCWFs, LCDAs, etc… ten years from now there will be a lot of lattice result.

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