Re: LC16542
    Measurement of the B 0 s rightarrow  mu  + mu  - branching fraction
    and effective lifetime and search for B 0 rightarrow  mu  + mu  -
    decays
    by R. Aaij, B. Adeva, M. Adinolfi, et al.

Dear Dr. Rama,

The above manuscript has been reviewed by our referees.  Acceptance
of your paper for publication is likely, but we first ask you to
consider carefully the appended comments.

With your resubmittal, please include a summary of changes made            
and a brief response to all recommendations and criticisms.

Yours sincerely,

Robert Garisto
Editor
Physical Review Letters
Email: prl@aps.org
http://journals.aps.org/prl/

IMPORTANT: Editorial "Review Changes"
http://journals.aps.org/prl/edannounce/PhysRevLett.111.180001

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Report of Referee A -- LC16542/Aaij
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For more than 20 years, the elusive decay of the Bs meson into a pair
of muons has been searched for in earnest by a number of high energy
physics experiments. From the beginning, this was seen as an important
channel to search for signs of "new physics", be it supersymmetry or
something else. Limits, and ultimately measurements, of this channel
put strong constraints on theories beyond the standard model. Along
with the failure to find new physics in other searches so far, the
measurement of the Bs to mu+mu- branching fraction became even more
important.

Just two years ago, the CMS and LHCb collaborations reported the first
observation of this decay in a tour de force analysis of their
combined data. With more than 250 citations in just two years, this is
one of the most cited publications by the experiments. An observation
by a single experiment is now reported for the first time with the
present measurement. In addition, an interesting excess in the search
for B0 to dimuons is not confirmed in the present paper, and the first
measurement of the Bs effective lifetime is shown. Both of these
results constrain new physics in a relevant way and are important
results on their own. There is no doubt that the paper proposed here
is one of the currently most important result by the LHCb experiment,
and that the result has wide implications on high energy physics in
particular, but that it will also attract attention among
non-specialists since the result touches upon some of the most
fundamental questions of physics.

The paper is well written and at an appropriate level for the subject.
There are sections in the middle part which are somewhat difficult to
grasp for the non-expert, e.g. regarding the discussion of the various
background contributions, but I believe the level of detail is about
right, and sufficient references are given for further reading on the
technical details. In the end, this is a rather complex measurement of
a phenomenon at the 10^-9 level, so one cannot expect all questions to
be answered in a short letter. The relevant details however are given,
such that the reader can have confidence in the scientific correctness
of the results.

I only have a few, relatively minor specific comments:

1. This is just about clarity: About the two normalization channels,
the text states "The normalization factors combining the two
normalization channels are...". For the reader not familiar with such
analyses, shouldn't one specify how this "combination" is
statistically performed?

2. Regarding the B0 -> mu+mu- limit, the text explains that "an upper
limit... is set using the CLs method". There's a vast variety of
"methods" that people call "CLs method" (there's "LEP style CLs" and
"LHC style CLs" and so on), which for example differ in the way
nuisance parameters are treated etc. I assume the authors are not
exactly using what has been described in the given reference from
2002, so I think at least half a sentence of additional explanation is
necessary.

3. Just after that, the model dependence of the Bs branching fraction
measurement is discussed, leading to a non-negligible additional
uncertainty. However, this additional uncertainty seems to be neither
included in the final result (one could write it as B +- stat. +-
exp.syst. +- model dep.), nor is it mentioned in the abstract or
summary. Because it is important for the interpretation of the result
in other contexts, why not at least mention it in the abstract,
summary, or result?

4. The discussion of the lifetime fit uncertainties should be improved
a bit. The text states that "The accuracy of the fit... is estimated
using a large number of simulated experiments with properties similar
to those found in the data". This is fuzzy and unclear - what are
"similar properties"? One has to guess that some unknown properties
were varied in some unknown way by some unknown amount. It would help
to be more explicit here.

5. The uncertainties on the data points in Figs. 1 and 2 should
probably be mentioned in the captions. How are they calculated? The
uncertainties are asymmetric in Fig. 1 and Fig. 2 (top), and appear to
follow what one would expect from poisson variations of the data. In
that case, given that there are a number of empty bins, shouldn't one
also include upper error bars for the bins where N=0? Then, in Fig. 2
(bottom), the uncertainties are symmetric, so what do the
uncertainties represent here?

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Report of Referee B -- LC16542/Aaij
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See Attachment: lc16542_report_1_b.pdf