Re: [Bfys-physics] Comments on LHCb-PAPER-2014-022

Dear Tjeerd,

I insert a few comments inline.

On 15/05/14 09:22, Tjeerd Ketel wrote:

Dear all,

Below I have combined Marcel's and my comments. Please, remove irrelevant remarks, as I got rather irritated by the imprecision of the paper.

Tomorrow I will post them on CDS.

Tjeerd

Comments on LHCb-PAPER-2014-022 (Nikhef)

General comments:

1. The beta acceptance factor comes a bit "out of the blue". As a reader

once you understand what it is, it is clear. However, it would help to make the definition a bit more explicit in the text. Perhaps already at page 3-4, when the rapidity ranges are shown in figure 2. Also, the y(Z)-prime could be mentioned there.

2. As a result of selecting the common CM-rapidity range - with small

overlap - the value of the R_FB changes drastically from 0.94 to 0.094. An alternative could be to extrapolate both measurements to a larger range of 1.5 < y' < 4.5 to reduce the statistical dependence. Of course at the cost of introducing systematic uncertainty due to the extrapolation.

3. A confusion of forward and backward boson production and the forward

geometry is introduced in the text. The footnote which defines forward and backward contradicts with the text.

You should better define forward and backward early on in the text, referring to the forward acceptance of LHCb and the direction of the proton.

Detailed comments by line: Abstract:

• Replace "dimuon mass is restricted to 60 to 120 GeV/c2" by "dimuon mass selection is between 60 and 120 GeV/c2". (Two times "to" is not clear.)
• Replace "The Z production cross section" by "The partial Z production cross section" or "The fiducial Z production cross section". (It is not clear that the cross section is defined in a certain range.)
• Conceptually, the most interesting measurement is the R ratio. However, it is not mentioned in the abstract. Why?

Line 2-5:

• This is a superfluous first sentence. Remove it or write "The physics program at hadron colliders with nuclear beams include crucial measurements of proton-nucleon collisions".
• Reverse first "can ..." and second "can ..." in the second sentence: "Measurements of proton-nucleus collisions are used for the determination of nuclear parton distribution functions (nPFD) and serve as references for nucleus-nucleus collision analyses".

Line 10-14:

- You should say that from here on you write R_a^A to stand for R_a^A(x_A,Q^2) - You have several instances of "1.0" (why the .0?) and then comes "unity". We suggest to use the words one and unity throughout.

• Replace "xA < 10-2" by "xA < 3x10-2 (Ref.[3])" (or Ref.[1]).
• Replace "smaller than 1.0" and "below 1.0" by "smaller than unity".
• Replace "caused by the European Muon Collaboration (EMC) effect [2]" by "by the EMC effect [2]".
• Remove "while the prediction of RAa is due to the Fermi motion above 1.0 for values of close to xA=1.0 [3]". (No written information is given about values close to xA=1.0 and to Fermi motion in Ref.[3]. There is also no experimental evidence for it.) And remove Ref.[3] for this reason.
• Replace "is expected to be" and "is predicted to be" by "is". (This is also stated in this way in line 15.)

Line 17:

• Remove "only".

Line 19: - deuteron-Gold or dAu

Line 20-21:

• Add "of the kinematic range in these experiments" to "consequence".
• Move "for values of Q2 above 10 GeV2" to directly after "nPDFs". (It relates to xA<0.01 and not to xA~1.)
• Replace "close to one" by "close to unity".

and add a comma afterwards

• Replace "10 GeV2" to "100 GeV2". (See supplementary figure).

Line 23:

• Add "and in the backward direction" before "to xA-values close to unity".

or just "large x_A values"

• "involve small xA-values" is more precise than "are sensitive to low xA-values".

Line 28:

• Replace "its forward geometry" by "its acceptance close to the beam line".
• Replace "probe xA down to very low values" by
• "probe electroweak boson production at very small values of xA".
• Replace "points in the forward direction" by "points toward the detector".
• Replace "points in the backward direction" by "points away from the detector".

Footnote:

• The footnote should be placed at line 22.
• Replace "'Forward' refers to positive rapidity values defined relative to the direction of the proton beam, 'backward' to the negative rapidity values" by "'Forward' refers to detected muons in the direction of the proton beam and 'backward' refers to detected muons in the opposite direction". (Rapidity makes the footnote unnecessarily longer.)

Line 31-33:

• Replace "the sensitivity at" by "the parton momentum fraction xA at".
• Replace "ranges down to O(10-4) in the forward case and up to one in the backward case at an energy scale Q2=MZ2" by "ranges from 3x10-4 to 3x10-3 in the forward case and from 0.2 to unity in the backward case at the four-momentum square scale of Q2=MZ2".

Line 35:

• Replace "This paper presents a first measurement" by "In this paper we present a first measurement".

Line 38:

• Replace "their pseudo rapidities, eta (-ln(tan(theta/2)), which" by "their absolute pseudo rapidities, eta, which". (We compare to positive values.)

Theta is undefined in this context. So you should better leave out the definition of eta. It's common knowledge.

Line 41: Add "fiducial" to "The fiducial cross section is measured as".

Line 43: Add "in the fiducial region" after "epsilon the total efficiency".

Line 44: Replace "The results are compared" by "The resulting cross sections are compared".

Line 99-102: This is not clearly explained: How can reweighing of a pp candidate (by counting is as 0.1 event for instance) change its track multiplicity to match that of a pA candidate? Or do you mean scaling of the track multiplicity of a pp candidate (by multiplying the track multiplicity by 10 for instance) ? If this only concerns the efficiency determination, then this paragraph (after rephrasing) should move to line 135 and the following suggestion about the caption of figure 3 (below) is irrelevant.

Figure 2:

• The rapidity scale of the right plot should be negative.
• The vertical scale "Candidates/(0.20)" still reads as "Candidates*5.0". We propose "Candidates/bin". Also for Fig.3.
• In the caption add ", yZlab," before "in the labo frame"

and spell out "laboratory"

and add ", y'Z," before "in the center-of-mass frame". Also add "Z" to the axis label "y'Z" or we may write "Y^CM_Z" to contrast against y^lab_Z. Same changes in lines 196 and 220 and corresponding equations.

Figure 3:

• Add information about the "scaling" or "reweighing" of the multiplicity in either the pp or the pPb case with reference to the text.
• Move "in (a) ... direction" to after "J/psi candidates".
• Take black open circles of black squares for the Z candidates. (In b/w copies there are no colours.)

Line 110: Does 0.16% have an uncertainty that would be taken into account in the systematics? Same in line 120.

Line 116-117: Remove "cone, radius and R" that refer to a circular geometric constraint: "of the tracks that differ in eta and phi with respect to the muon track by less than sqrt(Delta eta2 and Delta phi2)=0.5".

Line 121-123:

• Add "above" before "estimations".
• Add "rho".
• Add "A low background can be expected for the Z signal." (This will explain the high purity.)

Line 125: Add "fiducial" to "total fiducial efficiency".

Line 126: You have defined GEC to stand for "... cuts" in L.66, s GEC is already a plural.

Line 130: "similar as described" does not sound English to us.

Line 135-36:

• Also here "track multiplicity reweighted Z candidates" is not clear.
• Replace "comes from the GECs, These cuts are based on" by "is related to the global event cuts, based on".

Line 136: Replace "as a function of the muon pseudorapidity" by "as a function of both muon pseudorapidities".

Line 142: Add a remark about where we loose these candidates from the fiducial region and if this is reasonable choice for the cuts. Maybe be also what a common cut in y^CM-Z would do.

Line 146-162: Why don't you use the description environment instead of this enumeration with highlighted words?

Line 160: why "contains"? Line 163: aren't your systematic uncertainties correlated?

Line 190: Why is the choice made for an FB-ratio instead of a FB-asymmetry? It is also confusing as we introduced R(xA,Q2) as a nuclear modification factor, which we do not use here at all.

Also, FB should be in roman.

Table 1: Add a final row with total systematic uncertainties.

Line 179: "considering" is a bit short. You should say you compute the probability of a Poisson distribution to fluctuate to your observed signal. Line 180: Are the \cdot really needed? Why don't you put this as an equation?

Table 2: "the FEWZ code" is jargon and inconsistent with "FEWZ" in lines 173 and 192.

Fig.4: Say in the caption that the uncertainties on the theoretical models are negligible.

Line 221:

• Replace "the results" by "the measured cross sections". (We seem to have a 6.8 sigma Z signal as a result too.)
• The statistical 1 sigma uncertainty of RFB is not small compared to unity. So the reader will be puzzled by the statement that no nuclear effect can be concluded. It is explained in line 211, but here only the statistical 1 sigma = +0.104 is given.

Line 223-226:

• Remove "Nevertheless" and "excellent". Reverse first and second part of the sentence as: "This first observation of Z production in proton-nucleus collisions demonstrate the potential ..."
• What differential measurements? Like in figure 2? Then add "as a function of Z rapidity". (Let us be specific.)

"expected in the near future". We don't expect any pA data in the "near" future. Especially we do not expect any more pA data at 5 TeV ever.

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Cheers,

Patrick