Talk:Helicity dependent parton distributions

Overall this review paper is good. My main reservation is the lack of discussion of the role of orbital angular momentum of partons when discussing the longitudinal spin sum rule. The role of orbital angular momentum of quarks should be mentioned, at least in the models of hadron structure. From Lattice calculation we have hints that the total orbital momentum of quarks in the valence region is not large but the contribution of the sea quarks and gluons orbital motion could be large.

1) Mentioning more recent work that includes information on global analyses where more than just DIS data are included in the fits could be useful (see D. De Florian et al. arXiv:0804.0422). In fact DIS, SIDIS and hadron-hadron collisions are used to extract the most constrained distributions to date.

2) The authors chose an unusual name for the "helicity dependent momentum distribution functions" therefore I would recommend that they adopt the existing nomenclature found widely in the literature of this subject to minimize confusion. In the description of partons we have particles that are almost massless ( current quarks) or massless (gluons) and therefore "helicity" is more adequate compared to "polarization".

In some cases throughout the text the use of "longitudinal spin structure function" (for example for g1)is more adequate when it refers to the target. In fact the authors in the paragraph below equation 2 define g1 as "polarized structure function" but then go on later to refer to the same function as "longitudinal polarization function."

3) In the introduction second paragraph ".....rather than with 1/2, the quark model value,.." Perhaps it would help to include "non-relativistic" before quark model as the author noted later in the discussion.

"...with respect to to the motion..." remove one "to"

4) It would be important to add a reference to the work of Gross, Wilczek and Politzer if one talks about the link between the parton model and QCD through pQCD.

5)Section: The Proton Spin Problem.

reference missing in the following sentence below the Bjorken sum rule formula "...was derived using only the parton model and isospin invariance (see ) ..."

"...agreement to the EMC value, as it will be show." "shown" instead of "show".

6) Figure 3 on the neutron is mislabeled. The E154 collaboration is a SLAC experiment collaboration not JLab. The Jlab data are not on the figure but they belong to JLab experiment E99-117.

Numbering of the figures is to be fixed. There are 3 figures labeled figure 2 and the rest of figures labeled figure 3. This is perhaps a wikipidia problem when it displays the numbering of figures.

7) In the last last paragraph of the Experimental Determination of the Longitudinal Polarization section. The sentence "Inclusive data alone, however, are not able to distinguish the contributions from quark, anti-quark and gluon densities." is too negative for the inclusive. With proton, deuteron and neutron and a pQCD analysis the different contributions are usually extracted to a precision that depends on the precision of the data and the Q^2 lever arm.

In fact if one takes SIDIS alone there are many issues that makes the extraction of the sea or glue helicity distributions not a direct measurement. What is, however, powerful in recent analyses is the strength of combining data from different reactions to constrain the extracted distributions as in ref. D. De Florian et al. arXiv:0804.0422.