Dear authors, 

First we would like to congratulate you with this fine  work. 
Please find our comments below. 

On behalve of the Nikhef group, 

Hella Snoek


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General comments
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> Introduction/Conclusions:
- You write the this paper brings the first experimental evidence of
Zc while the rest of the introduction and the conclusion is in fact
about a first measurement of a cross section in the forward
region. What is more important? Please check that the conclusion
section 'answers' the information in the introduction.  
- There is also some information in the introduction that is just
repeated later. Especially the last paragraph can maybe be skipped.

> Pile up contribution: 
We have a concern about the DTF chi2 that you use to estimate the
background and the systematic uncertainty due to pile-up. If we
understand correctly  you add the chi2s of the D and Z vertex fits,
obtained in a single DTF. The defined chi2 is not proper as the two
are in fact correlated.  
The only proper way to this with DTF is to create an intermediate 
'mother' particle for the Z0 and D. The vertex chi2 of this mother 
particle tells you something about whether the Z0 and D come from the
same vertex. Once you add a PV constraint to this mother particle, the  
chi2 also tells you something about the consistency with a PV. (I do 
wonder if you really want to require a PV if you actually do not need it.)

> Condensed:
The paper is quite short and there are some reference to the 
previous papers (p.e. for the efficiency calculation). Instead, is it 
not possible to summarize the method in a sentence and refer to 
the papers for more details?


> Supplementary material
I hope this will stay in the CDS version even after the review. 
About Fig.3: have you searched for Ds and Lc and failed to find them, 
or is that just a placeholder for future searches? 


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Other comments
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Abstract: 
- open charm -> charmed

- ". This corresponds to a" -> ", with" (The events do not correspond
  to anything if you do not give the expected background). 

- Replace "4.5" by "4.6" consistent with line 128.

- Replace "fiducial volume" by "kinematic acceptance"
  (It is a volume nor a plane.)

- Preference "The measured cross-sections are" over
  "The cross-sections are measured to be".


Main text:

L2: Footnote 1: That doesn't mean anything without the mass
window. Move this comment closer to the 60<m<120 mass window, when 
the/a Z is mentioned.  
Footnote 3: looks like mass cubed. Put it in parentheses

L6: The latter? You mean ref 5? If so why is that relevant here?

L2-10: The first paragraph misses a concluding sentence to why these
measurement are important. P.e. for background estimations of other
analysis.  

L11: This sentence reads quite strange here. It holds no connection to
the predictions that you are discussing in this paragraph. Suggestion:
put it as the first sentence of the Introduction section. See also the
comment earlier. 

L17:  See earlier remark on "fiducual volume".
 Add "for the selected kinematic region" after "written".
 (If I understand the meaning of factorization here correctly)

L22: I am puzzled. Can I conclude that Eq.(1) is not valid in our
case? Please state this more clear in your text.

L24: See earlier remark on "fiducial volume".

L56: is a repeat of line 23 

L58-68: are a repitition of L24-28

L62-65: You use meson and hadron alternatively, which makes the
sentence a bit hard to read 

L69: You don't really combine the D and the Z you constrain them to
the same PV. 

L79:  "the Z is " -> "the Z boson is"

L83: "feed down of charmed mesons" should be "feed down to charmed
mesons" or "feed down from beauty mesons" 

L84: "B->D" -> "b->c"

L90: "Since the contribution" -> "Since the relative contribution" 

L94: Do you correct for the 1.7 (1.3)% feed down? Is this a part of
the purity. If not the systematic should be much larger. 

L105: from Ref. [1]

L113: Usually one writes 95.3\pm3.8\% (pedantic people would then put
parentheses) 

L113: It is not exactly clear from the text how you reach the 95.3%
purity level. What do you add to the 2.8% bkg? 

Fig.1: Axis labels are too small and lack units

L118: why "Refs [2,4,5] and [1]"? Refs[1-2,4-5].  Could you maybe just
summarize the method and refer to the older papers for more details? 

L134: Same comment as before:  Could you maybe just summarize the
method and refer to the older papers for more details? 

Table 2: "associative" -> "associate"

L149-155:
- I suggest to replace "Results" by "Results and discussion".
- Replace "The cross-sections ... presented in Table 2" by  "The
measured cross-sections together with three theoretical predictions
are presented in Table 2"  
(The equations with the results are also given in the conclusionsand
the selected kinematic acceptance is already decribed in lines 24-28

L174-175: Replace "events in total with ... observed" by "events
observed with ... decay".

L178-181:
- Replace "The results  ... will be possible" by "The
considered theoretical predictions are all in agreement with our
results. With more statistics in the coming analysis of our 8 TeV data
we may also be able to discriminate between SPS and DPS-based
predictions." 
(There is also quantative agreement. What do we mean with "more data"?
- If the 8 TeV results will be exactly the DPS values with half our
present statistical uncertainty, then we will discriminate.) 
- The conclusion section does not answer the introduction. How
would you distinguish the SPS from the DPS?  Shouldn't both processes
be existent at the same time?