Dear Authors,
The measurement is important for the diffractive physics community 
and thus should be published.

General comments: 
=================
- There seems to be a confusion between H1 and HERA. 
  In almost all cases HERA should be replaced by H1.

- Do you also consider the decay Psi(2S)->J/psi pi pi? 
  If not, is it a background?

- A number of comments concern the systematic uncertainty in
  the lines 159, 174 and 214-217.

- Why do we extract the integrated cross-section from a separate fit, 
  where we assume a constant W? 
  The sum of fits in bins of rapidity seems more logical.
  See comment on Fig. 3 and lines 133-134.

- Naively, one would expect that if one would integrate the results in 
  Table 1, one would get the cross section in line 223. 
  However, performing the integral (for a rapidity bin size of 0.25), 
  one obtains from Table 1 for the cross-section in the interval [2,4.5] 
  a value of (11.6 +/- 0.4 + 0.7) nb. 
  Correcting for the J/psi->mumu branching fraction 
  (which should probably be quoted in the paper) 
  gives (689 +/- 24 +/- 42) pb, which is over a factor 2 larger 
  than the result reported in line 223. 
  Why are the two numbers so different?

- I would suggest the authors to strengthen the presentation of 
  the actual measurement of p+p -> p+V+p and de-emphasize 
  the transformation to gamma+p -> V+p.
  In practise this would mean:

  [1] Reduce to the minimum the last paragraph of the introduction, 
  "The LHC results can be compared to the H1 and ZEUS etc.". 
  The discussion of Eq. (1) in that paragraph is not complete.
  The suggestion is to move the whole discussion to Sec. 4, Results.

  [2] In Sec. 4, single out the actual measurements and separate it from  
  the model-dependent comparison to HERA data. 
  In particular, I suggest a sub-section heading 4.1 with "transformation 
  to photon-proton exclusive production cross-section" or 
  whatever singles out that you are now using a model to transform your data. 
  The text in Appendix C of the note is by the way clearer and more orderly 
  than the paper, I would adopt that.

  [3] In Sec. 2, I would spell out how we "tag" exclusive events, by
  including the section 3.5 of the note "summary of selection requirements".

  [4] The discussion of the inelastic background in Sec. 2 is not clear  
  enough. However, Sec. 3.2 of the note is quite better. 
  Given its importance (30% of the events we tag as "exclusive" are 
  actually not exclusive), I would single out this shortcoming of the 
  rapidity gap definition as a separate subsection of Sec.2 
  and copy Sec. 3.2 of the note for this.

  [5] The extraction of the signal from the fit to the dimuon PT  
  distribution can be clarified. In particular, the statement that  
  "suggests" a b parameter value is too vague. Again, appendix A  
  makes a better job at explaining how this value is obtained and 3.2  
  states explicitly this is consistent with a fit that left b as a free  
  parameter. I would adopt some of this text for the sake of clarity.

  [6] The paper makes poor justice to the differential cross section 
  in Table 1.
  In particular it does not say how the fit in Fig.3 was done for  
  the various rapidity bins. What b was used?


Comments by line:
=================
Abstract:
- The present abstract gives not much more information than the title
and what one would expect from a measurement. More information on
the result and the interpretation is required:
- Add "the excellent agreement with ep-collision data and the confirmation
of the power-law behaviour up to $m_{\gamma p} = 2$ TeV/c$^{-2}$".
And "no evidence for effects, due to a saturation of the gluon density 
increase towards small x, is found".
- Replace the "random choice" suggesting "a number of" by "different", 
if this part of the abstract survives.

Line 3:
- Add comma before "providing".

Lines 6-9:
- Replace "which" by "that".
- Start a new sentence after "pomeron exchange".
- Insert dash in "photon-proton". Also in lines 51-52.

Line 13:
This is an ugly sentence with: "Describing ... challenges".
Who is describing? It seems that QCD is challenging itself.
"The consistent description of the transition regime between perturbative 
and non-perturbative QCD is a challenge". 

Line 17:
- What is "g(x,m_V^2)"?
- Why "as a function of m_V^2/4" instead of "m_V^2"?
- Define the introduced variables, as we do in other places.

Line 28:
- Move reference [7] after H1 to the end of the sentence.

Line 31:
- Remove "a" before "x=", or change to "a gluon momentum fraction of".

Lines 38-39:
- Add [8] [9] to [8,9] or place them as Superchic [8] and Startlight [9]. 
  Same comment applies for line 48 with [11] [12].

Lines 40-44:
- "using a number of PDFs" carries no information (as in the abstract). 
  Add here names and references.
- Replace "This is found " by "These are all found to ...".
- Remove ": a scale factor ...". If this is prescribed by [5] then
  we do not need to write it. If not, it is not well motivated here.

Line 47:
- Replace "overpredicts the HERA data" by 
  "predicts larger cross sections than those measured by H1". 

Line 54:
- Add the expressions for k and W in the equations; they are essential.
  In the line they are easy to miss.
- Explain when we have positive and when we have a negative sign.
- Define rapidity y of J/psi, as we do later for pseudo-rapidity too.
  It will then become clear that e is not the electron charge.

Line 56:
- Express the undetected protons in terms of y.
- Replace "go down the beam-pipe and remain undetected" by 
  "remain undetected inside the beam pipe".

Line 58: 
- Change "corresponding to almost... from 2010" to 
  "collected in 2010".

Line 60: 
- Remove ", $\mu$,". This definition is also given in line 213
  where it is further used.
- Replace "~" by "about".
- Replace "with larger values corresponding to larger luminosities" by
  "with most luminosity accumulated at larger values".

Figure 1:
- Move "in the backward region" after "which".

Line 116: 
- Start a new paragraph.

Line 130:
- Remove ":"
- Replace "*" by "\times" or "\cdot", or remove it.

Figure 3:
- How do you account for the non-Jpsi background in this fit?
- Make the fits recognisably in black-and-white print.

Lines 133-134:
- Add "Ref. " before [11]. Also in lines 201, 208, and 264.
- The unit of b is GeV^-2c^2.
- I don't understand the strategy: 
  You fit the distribution in Fig. 3 with a single value for b. 
  However, you measure the rapidity in each event so you don't need 
  to take an effective 'b', but could use formula 2. 
  It seems to make more sense to split the fit in 10 bins of rapidity 
  (or W), as you finally report in table 1, and then, afterward, 
  add up the distributions and the fitted function. 
  Or just show the 10 different fits! 
  You could then fit for 'b_0' in a simultaneous fit to the 10 bins, 
  rather than fit for an 'effective' b in line 158.

Line 139:
- Move the footnote to the references with its reference.
- Put all functions in roman style: \exp, \ln, \sinh...

Line 142: 
- Given figure 3, any reader will think that the result depends 
  crucially on how you estimate the inelastic contribution. 
  Therefore, I think that it is mandatory to show the fit 
  from which you extrapolate to two tracks in the event.

Line 152:
- Replace "LPAIR" by "J. Vermaseren" or "Ref. ".

Line 154:
- A reference is needed.

Line 164:
- Add brackets around the numbers before the unit in
  "($70\pm 4\pm 6)$\%". Also in lines 173, 182, and 185.

Line 173:
- Give the feed-down as percentage "of total decay" or "of gamma decay".

Line 175:
- Replace "has been determined" by "was estimated".

Line 181:
- The word "purity" is used without a definition in this context.

Line 187:
- Replace "$\frac{pN}{\epsilon L_{eff}}$" by
  "${pN}/(\epsilon L_\rm{eff}$}}$".
- Also replace mathematical "eff" by Roman "eff" in 188 and 214.
- Similar for following "trigger", "track", "muon", and "se".

Lines 206-208: 
- Something sounds odd here. 
  If we measure the trigger efficiency in the data, then why do we need 
  the simulation and a systematic error from the data-MC difference?

Line 223:
- Add a space before "pb".
- Replace " 1 " by " 1.0 ".

Line 226-240.
- Add estimated errors to 292, 396, 1124, 334, 1670, 710, 6.1, 17 pb.

Line 232:
- Explain what is meant by "a gap survival".

Table 1:
- Make one column with rapidity's and one with cross-sections.
  Split the table if vertical space is a problem.
- Replace "Summary of cross-section calculations" by 
  "Differential cross-section results".

Line 239:
- Replace "from WS and AS it is 17 pb" by "WS and AS predict 17 pb".
- Add the corresponding references.

Line 249: 
- "The purity within each bin is assumed to be the same as in the
  integrated sample". 
  How is this motivated? 
  Why don't we just float the background in each bin?

Lines 246-253: 
- The efficiency depends on rapidity. 
  However, you did not discuss this for the 'integrated' measurement 
  reported in line 223. 
  How do you take the rapidity dependence of the efficiency into account 
  for the integrated cross-section? 
  Do you rely on the simulation? 
  If so, shouldn't you add plots that compare the simulated and 
  measured rapidity distribution? 
  Or better, derive the integrated result as a sum of the rapidity bins, 
  as advocated above?

Eq. (3):
- Remove the nested fractions by replacing it with a "/".
  Or use {\em dfrac} instead of {\em frac} for enhanced readability.
- Make it one equation with $W^\pm$ and $k^\pm$ in stead of two 
  equations for $W^-$ and $W^+$ separately.

Fig. 4:
- Replace "points" by "dots, squares and triangles" in the caption. 
  This is also necessary for a black and white print.
- Replace "ZEUS" by "Zeus" to be consistent with the figure.
- Specify if the fit is to H1, Zeus, or both H1 and Zeus data.
- A word why we have no overlap with the Zeus and H1 data, but
  cover only lower and higher W, would be instructive in the caption.

Line 302:
- Rewrite "ppbar".

Line 307:
- Maybe Refs [15] from 1999 and [9] from 2004 can be taken together.

Lines 309-311:
- Replace "W. Schafer" by "W. Schaefer" and everywhere else in the text.
- Replace "conference" by ", Workshop at ECT* Trento" after 
  "Diffractive and electromagnetic processes at the LHC".
- Add the year "2010".

Lines 324, 335:
- Use the LHCb template. That will give you "Sj\ostrand" and ArXiv:".