=== Questions ===
=================

Concerning the BDT: you include several kinematic variables, but you do not
discuss or show any study of possibile correlation with the invariant mass. 
Did you check this effect? The BDT is usually safe from this biases but is
important to check. 

Possible peaking backgrounds: did you consider the possibility of the
following decay: D-> rho0 (-> pi pi) mu nu? It has a high branching fraction
and needs only one misidentification.

How was the binning chosen? The fact that you see leakage from the 
resonances into your central bin seems to defeat the point of binning. 
Couldn't you rebin in a more appropriate way. More generally, if you do 
a search without any apriori knowledge, why do you bin at all? Cannot 
you publish a background-subtracted dimuon spectrum or Dalitz plane?

The usual question about multiple candidates : Can you prove that your 
choice of best candidate does not bias the result. i.e. are BDT and 
vertex chi2 uncorrelated? See LHCb-INT-2011-009.

Line 99: is there any kinematic requirement that aligns the momentum 
distribution of D->3pi to D->pimumu? The muon-ID strongly biases 
momenta. That affects your statement in line 132 that the shape is "well 
defined"

Line 120-121: The fits are split into a low and high q^2 region because you
are worried about the effects of the additional trigger coming in at 1 GeV.  
However, the lower q^2 fit also uses the phi region, which runs up to 1.2
GeV. Would the effect of this additional trigger thus nonetheless affect 
your lower q^2 fit?

Systematics: the systematic uncertainty due to TrackSmearing seems quite high, 
does that value refer to the single efficiency or to the ratio with respect to
the nomalisation channel? 

The systematics at the 4th,5th and 6th bullets appear to be all, at least
partially, correlated to the q^2 dependence model. 
  a) are they treated as correlated when calculating the final sum? 
  b) the model-dependent part of these systematics should be dropped when
  calculating the value of the model-dependent branching ratio. Is it so in
  your calculation?



=== Textual Comments ===
========================
General comment: 
We think that the paper needs some re-writing. It is rather detailed in some
technical aspects while not discussing enough some relevant aspects of the
analysis, like the binning and the results. 

Line by line comments: 
Line 7: remove the spaces inu mu- mu+.
Line 12: best -> most stringent 
Line 12: "Limits" should be singular
Line 15: "Throughout... implied": why this sentence here?
Line 22: the subsentence lacks a verb
Line 23: "subject of great interest at LHCb". That may be true but is of 
no interest to the outside world. What about a sentence saying LHCb is 
very competitive in RD charm?
Line 25: add a comma at end of line.
Line 26: Could you improve the wording?
Line 26: samples -> bins. Remove the 2nd for.
Line 47: The L0 trigger
Line 48: I have nothing a priori against ">" in text, but here some of 
the operators could easily be replaced by words, like > 1.68 -> exceeds. 
Why the parenthesis around GeV/c? You could write "exceeds (1.3 GeV/c)^2".
Line 48: Could you rephrase the sentence "transverse momentum multiplied > 1.68"?
Line 49: have a momentum ... and an impact
Line 52: "be," -> is
Line 55-56: The last sentence is obvious. Remove.
Line 60: $\chi^2_{NDF}$ -> $\chi^2/{\rm NDF}$. $\chi^2<30$ of what? Is 
that a chi^2 or a chi^2/dof?
Line 62, 75, 78: Please do not use Loki functors in publications. You 
could replace DIRA by some angle (e.g. $\theta_d$) or just repeat the 
definition in line 75. ("and known as the DIRA" would be "known as DIRA" 
anyway).
Line 69: so -> and thus 
Line 72: the BDT trains -> the BDT is trained
Line 78: MAXDOCA is undefined.
Line 80: "The variable" ... and then the equation
Line 81: "sum of over the n tracks": remove either "of" or "over"
Line 93: are forbidden -> must not
Line 98: rewrite as "... except for the requirement that two of the 
tracks have hits in the muon system."
Line 108: in the m(...) -> in m(...)
Line 113: know -> known
Line 129: Please say how the tails are fixed. MC?
Line 141: "rho/omega region shown in Fig. 3(b)" There is a mismatch between the text and the reference to the figure.
Caption Fig. 3:  Should it not be: "The excess seen in c) is due to a feed-down from the rho/omega->mumu resonance."?
Line 142: the large width. Rewrite as "The statistical significance of 
the two observed peaks are 6.1 and 6.2 for D->eta(pi) and Ds->eta(pi), 
respectively." (assuming that's what it means).
Line 147: As no ... windows upper limits.
Line 163-166: Could you improve the wording?
Line 165: "a number" what number?
Line 196: "imperfect emulation". The online reconstruction is not 
emulated but re-run. What do you mean?
Table 3: Effect -> Systematic error
Line 208: add a space between "eachm"