# Peter J., Jan 4, 2007

Under:
Hi GPC and PAs,

Here are some first comments on the paper draft:

(i) physics intro, 1st para: I find the physics intro to be a bit confusing. I looked briefly at refs [6,7] (Brodsky et al and Collins). Within my distinctly limited understanding of them I don't think the last sentence of the first paragraph is accurate, and the main physics point of interest in this measurement is missed.

Evidently, the SSA arises due to interference of left- and right-handed quark polarization states, and thus is sensitive to chiral symmertry breaking. If correct, this is important and should be featured prominently in the intro.

On the other hand, I am confused by the various claims about factorization in this process. Brodsky et al claim that the process cannot be factored into PDF and FF, while Collins claims that factorization holds but then derives a pdf $f_{1T}^\perp$ whose sign is opposite for DIS and DY (eq 3), i.e. a pdf whose value is process-dependent, which doesn't sound to me like factorization. The only thing I know for sure is that I am confused on this point and could use some guidance. I suspect that most non-experts will be similarly confused. The physics intro should be precise and clear about what the theory says.

(ii) p 1 left col 2nd para bottom: what is the specific relevance to this measurement of the inclusive jet cross section being described by (factorized) pQCD? I guess if it didn't work for the inclusive yields one could stop immediately. Is that the only point to be made here? Can one say more about constraints on PDFs and FFs?

(iii) p 1 right col line 9: I know nothing about Siberian snakes. What are the limits on possible non-vertical polarization states?

(iv) End of that para: give errors on polarization: 59\pm{xx}\% (57\pm{yy}\%).

(v) jet reconstruction: nowhere do you actually describe what "jet reconstruction" you do. P 2 left col line 6 talks about "jet clusters at level 2" and the caption of Fig 1 talks about "full jet reconstrcution" but the reader is left hanging about how a jet is actually defined. Is there some peak-finding with a cut-off radius, or what? I know that you use EMC energy only but the non-expert reader will not know what this implies, i.e. all of the EMC energy plus perhaps 30% of the charged hadronic energy, with some charged-track dispersion in the magnetic field that is not corrected for. You need a couple of paragraphs defining the jet finding used for the analysis and giving its comparison to full jet reco, justifying why this technique is adequate for this measurement (there is currently some of that later in the text but it should be consolidated).

(vi) Fig 1d: why only 2% of the data? I think I know the answer: that's what was reconstructed at the time you were in the thick of this analysis, but evidently more has been done in the meantime. Not usable?

(vii) p 2 left col middle: I printed the paper in B&W and don't see the 6-fold L0 peaks in fig 1a. Am I missing them?

(viii) p 2 right col 2nd para: the "favoring" of qg vs gg at forward vs midrapidity is qualitative. Can this be made quantitative, e.g using PYTHIA? What is the magnitude of the variation of the two contributions?

(ix) p 2 right col middle: "while we away the time-consuming replay of the full dataset including TPC..." is a STAR detail of little interest to others, and has a limited shelf-life. I suggest simply describing what was done, saying that this rapid analysis technique (in contrast to full jet reco) is sufficient for present purposes.

(x) Fig 1b and discussion of tails in p2 right col bottom: "might reflect moderately hard gluon emission" is weak. Can this be studied with a model calculation? But I also find it confusing because I don't know how the jet finding was done. Hard gluon emission will generate an acoplanarity only if it pushes some of the energy flow out of the jet cone, otherwise momentum is conserved. So I suspect that this tail depends on how the jet is defined. Needs more discussion.

I also wonder about tails being generated by the combination of relatively low multiplicity in low energy jets and only partial jet reco (EM plus ~30% hadronic, with some funny spread in the latter due to the field). Could unfavorable, perhaps rare, fluctuations in charged vs neutral pions generate such apparent tails which are not present for full jet reco? Perhaps a model study would help here. Anyway, the toy model in which you just fit with a Gaussian with an exponential seems inadequate - can you do a more meaningful study based on PYTHIA or HERWIG?

(xi) p 3 left col top: is there a jet energy dependence to <kT^2>? More generally, the distribution shown in Fig 1d goes out to ~50 GeV if I jack it up by eye by a factor 50. Can you make a few coarse energy bins to look at the dependence of the asymmetry on jet energy? You say somewhere that you expect the ET dependence of the Sivers effect to be small, but surely it would be good to test this.

(xii) definitions of A_N and r_\pm (eq 1 and 2): it's late in the evening and I am a bit tired, but frankly these formulas are not speaking to me at the moment. A_N is defined as the ratio of ratios, which is OK, but I am not getting the purpose of the sqrt. There are too many +- and -+ subscripts and zeta>pi vs zeta<pi which are hard to distinguish. Can you find a more transparent notation, or explain the structure of the definitions a bit better?

That's all for now. I didn't read the last third as carefully, I'll do that next time.

Hope these are helpful, talk to you tomorrow.

Peter