http://cms.cern.ch/cds/HIN-14-008
Ernesto
Congratulations for this nice work. Overall the paper is well written. We have suggestions (Type A) which can make the understanding of the topic easier, especially for people who are not directly involved in the HI analyses. We have also two comments about the physics content (Type B).
Type B
In the Introduction there is a lot of emphasis on the potential of this analysis on discriminating about hydrodynamic vs CGC models. On the other hand along the paper the experimental results are compared only against the predictions of the hydrodynamic model. As you mention the current accuracy on the measurement does not allow to draw any conclusions. Anyway it would be nice to see on Figure 7 also the curve predicted by CGC. If the CGC prediction is what is called “inclusive pT” I would write it explicitly
I would swap the columns of Figure 3 in order to have the delta_eta=0 regions of Pb-side and p-side histograms close to each other. Is there any physical reason why the p-side yield and the Pb-side yield should be different at eta=0? If not, this means that the first term of Eq.4 should be fitted simultaneously between the p-side and the Pb-side. Have you tried to do that, thus reducing the number of fitted parameters and most likely giving a chi2/ndof closer to 1? Can you do this and include it in the study of the systematic uncertainties?
Type A
General comments about style:
- Notation: is the label “trigger” (introduced at L42) a literature standard for denoting the primary track? I have found it confusing, e.g. it seems to define a track on which the experiment is triggered. Can it be changed into “primary” or “seed” or “tag”?
- Figures: would it be possible to have the axis representing “delta_phi” in units of “pi” instead of integers?
Perhaps it is obvious for HI experts, but it not harm to mention that short/long range usually referes to the eta-separation (I have understood correctly).
L19-33: I suggest to remove this paragraph. Although it contains interesting references to previous literature, it definitively breaks the flow of the introduction, e.g. description of the models (L7-18) followed by the description of the predictions relevant to this measurement (L34-41).
L40: “decorreclation” → “decorrelation”
L42: is the label “trigger” a literature standard for denoting the primary track? I have found it a bit confusing, e.g. denoting a track on which the experiment is triggered. Can it be changed into “primary” or “seed” or “tag”?
L52: remove “dN/dDeltaEta ~ dN/deta”. At this point I have found it confusing. What it is done in the analysis is explained in detail later.
L53-54: consider to move it at the beginning of the paragraph (L42).
L70: add a one-sentence guideline about the meaning of v2 and v3, e.g. “A non-vanishing v2 indicates …”
Eq.4 “E” → in the text (L129) is “epsilon”. More in general, I would not call a correction factor “epsilon_trk” as this name rather denotes an effciency. Which is the typical size of the correction factor?
L132: I would not make “Quantifying jet contributions” a separate subsection
L133: “particle eta windows.” → “particle eta windows in low and high multiplicity events.”
L136: for my education: in which of the four panel you observe the “extensive feature on the away side”?
Figure 2: caption “Distributions” → “Example of distributions” The upper limit of the y-axis (“1”) of the lower plots is partly hidden.
L153: “1/N_trig d2N/d(delta_eta)d(delta_phi)” is the differential yield and not the near-side averaged. Either remove it or use the formula with the integral sign.
L165: “which stand” → “which stands”
Eq.8: use for the multiplication the same symbol of Eq.5 (either “\times” or “\cdot”).
Eq.9: does factorization mean that you assume no correlation? If so Eq.9 does not add anything and you should limit to write in the above sentence “v_n(eta_assoc)/v_n(0)”.
L205-207: Postpone the definition of “eta_lab” to Sec.5, e.g. around L238.
L225-226: is there any inconsistency between the value in the text (3.6%, 5.7%, 14%) and those in Table 2 (3.9%, 5.8%, 15%)? Same comment for the values quoted in the captions of Figures 4-6.
L228 “From the two-particle correlation … va Eq.7.”: already explained. Remove it?
L229, L231, L246, L247 “blue points”, “red points”, “closed points”, “open points”: I would not repeat in the text what is already written in the caption of the figure.
Figure 6: in the caption: “Bands show” → “Band shows”
L277: remove “perhaps”
L278: “vs” → “as a function of”
L280: remove “about” since the uncertainty is written explicitly “eta~1.5” → “eta=1.5” what is approximate is the size of the decrease (20%) not the value of eta where you evaluate it
Figure 7: caption: either write “average pT” or “<pT>”
Mario
Abstract: as function → as a function about eta = 0 → around eta = 0 within errors → within uncertainties
line 6: at the LHC → of LHC line 10: sizes → size line 17: is “equilibrated system” correct? Perhaps “system at equilibrium” line 29: in collisions between heavy ions → in heavy ions collisions line 32: to pp and pA → to pp or pA (unless you can exclude that you can apply hydrodynamics to one and not the other) line 39: origine → origin line 47: about → around line 51: … in such an analysis → In this analysis, … line 68: unless it is a common term in HI physics, we suggest to refer to the normalization of V_n as “normalization”, and not “self-normalization” line 74: remove “used are” line 92: o the beam → to the beam line 101: are based on → make use of line 116: remove “for the high-multiplicity event selection” (redundant) Figure 1 caption: in low-multiplicity → tracks in low-multiplicity line 133: displays → shows lines 135-136 extensive feature → structure line 142: magnitude → normalization line 155: jet → jets Figure 3 caption: Fit functions by Eq. 5 → Fit results using the Eq.5 function line 161 nor → and so line 169 Table. → Table line 176: is nonuniform; it is → it not constant but line 190: averaging → the averaging line 202: at a fixed pseudorapidity direction. → in a fixed pseudorapidity direction: lines 211-212: by restricting one → by restricting to one line 219: signals → signal line 221: between → among line 239: is same → is the same line 241: without subtraction → without the subtraction Figure 4 caption: for p-side → for the p-side line 253: large pseudorapidity gap → a large pseudorapidity gap line 256: related to decorrelation → connected to decorrelation line 263: In the pPb → In pPb Figure 5 and 6 captions: Error bars are statistical → Error bars indicate statistical uncertainties only line 276: falls more → falls more rapidly
Marco Monteno
The aim of this article is a careful study of the features of the long-range dihadron correlations observed in “small system”, as the hot matter produced in pPb collisions at the LHC. ……….
A few comments on specific parts of the writeup:
- Fourier coefficients of the two-particle correlations DeltaPhi distributions are used to infer the single-particle anisotropy parameters v2, v3 to quantify their dependence on eta, that can be compared to the predictions of hydro and CGC models. That is based on a factorization assumption: how much is it justified?
- The description of the systematic uncertainties affecting the measurement of the Fourier coefficient V_n (lines 208-213) is clear, but the sentence is somehow “heavy”, and it could be convenient to slightly rephrase it.
- Another “critical point”, where the non-expert reader may have some difficulties, is the second paragraph of the section “Results”, especially when introducing the Fig. 5, where results are shown with/without subtraction of low-multiplicity data, arising some confusion (also the phrasing of caption of fig.5, with curves and symbols, is quite heavy and this does not help).
- It is not well explained (L38 and L253) how a large eta gap may arise in presence of fluctuating initial conditions in a small system (pA), thus producing a decorrelation. How this decorrelation will manifest itself?
However, the obtained results are interesting, the analysis methods are thorougly described, and overall the paper is very well written.
Finally, we conclude our review by indicating the following typos in the text:
ABSTRACT-LAST LINE pseudorapity → pseudorapidity
L9 at RHIC and the LHC → at the RHIC and the LHC
L39 origine → origin
L40 decorreclation → decorrelation
L60 approriate → appropriate
L74-75 The “data used” are from pPb……. by the CMS detector in 2013 “are used” (remove the second “are used”)
L92 o the beam → of the beam
L127-8 The “correction factor” at L127 is then quoted as “efficiency correction” at L128. I would prefer the first naming.
Eq.4 the factor E should be replaced by \epsilon (the rec. efficiency quoted at L129)
L131 A citation is done for GEANT. No citation for PYTHIA?
Maybe you should ref to: JHEP 0605 (2006) 026
FIG.2 !!! Inconsistency between figure legenda and figure caption!
Red dots correspond to distributions of associated yields after
background subtraction for low-multiplicity pPb collisions
(according to second line of the figure caption), whereas the legenda
in the right-upper panel says the opposite!
The figure caption is wrong and must be corrected.
L144 radian → radians
L153 remove the misleading formula of the differential yield, since you are quoting here the near-side averaged (integrated of |DeltaPhi|<pi/3) correlated yield, shown in fig. 3
L224 propopagated → propagated
L237 Eq. 10 → Eq. 9
FIG.5 (end of the 4th line of the figure caption): and → the
L277 why “perhaps less”? To attenuate, rather use: “slightly less”
L282 Two-particle Correlations → correlations
L348 Central d+ Collisions → Central d+Au Collisions
L353 doi:doi → doi
L389 (end of line): PHYS.REV. C I suppose is a “submitted to” . No arXiv ref?