Dear Yanxi,

Congratulations with this very nice result!
On behalf of the Nikhef group we have some questions and suggestions.

Hope it helps, best regards, Niels on behalf of the Nikhef group

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Main comments
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1) We would like a stronger first paragraph, where the motivation is 
optimally presented for the PRL reader.
(a) We suggest to move L.12 - L.17 to the 2nd sentence; 
"The baryonic decays of B mesons are important, ... in the low mass region."
(b) We suggest to move the 'prediction' of L.190 to the first paragraph.
(c) Q: does a similar measurement also exist in Bs or Bu decays?

2) L.127 Here you correct the measured mass by +0.20 MeV due to FSR.
(a) Q: How is this usually done within LHCb? Did we always have such a correction
to the mass measurement?
(b) Q: The uncertainty to this correction is 0.03 MeV. Does that purely come 
from MC statistics, or does the uncertainty include theoretical uncertainties as well? 

3) L.143 You discuss a new selection for the BR(Bc) measurement compared
to the M(Bc) measurement. Q: To reduce the systematics, you decrease the yield from
23.9 to 19.3 events. Since the statistical unc is dominant, is this reduction
in statistics indeed compensated by a smaller systematic unc?

4) L.151-154 and Tab.2. We could not follow how you determined the single-track efficiency.
(a) Q: How did you use Lc->pKpi or D*->D0pi events? 
(b) Q: Does the single-track efficiency only refer to tracking efficiency on reconstructable effects?
(c) Q: Wat do you mean with "Reco. of p,pbar" in Tab.2 ? Is that the same single-track efficiency?
(d) Q: How is the unc due to the acceptance of 0.7% determined, in Tab.2 ?

5) L.135 The systematic uncertainty of the combined result is larger than the
systematic unc of your measured value, that is one of the inputs.
In App. A you explain a bit more: in the combination you weight the inputs using their
total uncorrelated unc. Then, presumably, you add the correlated sys uncertainty to the final result.
But how do you determine the total systematic uncertainty on the final result?


Textual suggestions
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Abstract
--------
* [Shorten a little bit:]
"corresponding to an integrated luminosity of 1 fb-1 taken ... at 8 TeV." ->
"corresponding to an integrated luminosity of 3 fb-1."
* [Add what is the stat/sys unc. easiest maybe to include "(stat)" and "(sys)" within the result itself. ]

p.1
---
L.4 "with the other one as a spectator" -> "with the other quark acting as a spectator"
L.11 "in the Bc->JpsimunuX" -> "using the Bc->JpsimunuX"
L.14 [We don't like the use of "etc". Better to skip it, and instead use "and" earlier in the sentence;]
"... production, and to search for..."

L.29 [Since you include references to most subdetectors, here you could add the OT reference?]
"... placed downstream~\cite{LHCb-DP-2013-003}."

L.53 " based on boosted decision tree (BDT)" -> "based on _A_ boosted decision tree (BDT) algorithm"
L.55 [skip "(ndf)"; it's obvious from the text]
L.58/59 and L.108/109 [It would be better to swap the order of JpsippPi and JpsippPi: 
the main decay should not be the one in brackets.]

L.65 [skip: "shown below" ]
L.65 "To have a better PID" -> "To improve the PID"
L.66 "asking" -> "requiring"

L.96 "Fig." -> "Figure"
L.99 "with tail parameters on both sides are fixed from simulation." ->
"with the tail parameters on both sides determined from simulation."
L.104 "times" -> "multiplied by"

L.108 "scaled with their ratio in simulation." ->
"scaled with the relative width obtained from simulation."

L.121 [Q: We didn't understand this sentence; perhaps try to rephrase?
"The average value of the one-to-one change of reconstructed mass"  ]

L.131 "In total the systematic" -> "The total systematic"

L.144 "decay is 19.3 +- ... " -> "decay decreases to 19.3 +- ... "

L.161 [Q: We believe that the value quoted, is the value for 1/re?  I.e. 1/re = 4.76% ?
Perhaps it is best to write at L.145+ :]
"BR(Bc->JpsippPi)/BR(Bc->JpsiPi) = rN/re" 
[ie., that it becomes clear that you _correct_ for the ratio of efficiencies? ]

Refs
----
[14]-[16] consistent use of BELLE

Suppl.
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Fig.3 [Would it be nice to add the mass measurements by CDF?]