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VUB-RECOIL  February 2008

VUB-RECOIL February 2008


reply to referees


Roberto Sacco <[log in to unmask]>


21 Feb 2008 12:05:48 -0800Thu, 21 Feb 2008 12:05:48 -0800





text/plain (404 lines)

Hi All,

you will find attached a first draft of our reply to the referees.  
Please have a look and let us know how they can be improved.


Concezio and Roberto


Re: LH11701
    Measurements of partial branching fractions for $\bar{B}
    \rightarrow X_u l \bar{\nu}$ and determination of $|V_{ub}|$
    by B. Aubert, M. Bona, D. Boutigny, Y. Karyotakis, J.P. Lees, et al.

Dr. R. Sacco
Stanford Linear Accelerator Center
Mail Stop 35
2575 Sand Hill Road
Menlo Park, CA 94025

Dear Dr. Sacco,

The above manuscript has been reviewed by our referees.  We ask you
to consider the enclosed comments from the reports.

While we cannot make a definite commitment, the probable course of
action if you choose to resubmit is indicated below.

( ) Acceptance, if the editors can judge that all or most of the
    criticism has been met.

(X) Return to the previous referee(s) for review if available.

( ) Submittal to new referee(s) for review.

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

Yours sincerely,

Robert Garisto
Associate Editor
Physical Review Letters
Email: [log in to unmask]
Fax: 631-591-4141

Report of Referee A -- LH11701/Aubert

The precise determination of |V_{ub}| is crucial to testing the CKM
sector of the standard model. The length of the side of the unitary
triangle opposite the angle beta is proportional to the ratio |V_ub|
/ |V_cb|, making its determination a high priority. Given the rapid
theoretical and experimental progress in this area, it is very
interesting to see how the determination of |V_{ub}| develops. In
this context, the paper uses 4.3 times the data previously used by
BaBar for the |V_{ub}| determination and significantly improves on
the precision of the earlier measurement. For the substantial advance
in its subfield and implications, I regard this work suitable for
publication in PRL.

The letter is well written and clear. Please consider the following,
minor corrections:

R: we have taken into account all the following remarks.

Title:  V_{ub} --> |V_{ub}|

Abstract, L7: (and everywhere else) replace "sys" with "syst"

p7, L4: "....measure branching fractions for such decays." please
add reference 20 (PRL92_071802) at the end of this sentence, as it
places this paper in the context of the previous measurement.

p7, L9: "..we present measurements of partial branching
fractions.." --> "..we present a measurement of partial branching

p7, L22:  change "fb^1" into roman: {\rm fb}

p8, FIG1 caption: "(points with statistical error)". One can't see
on the PRL size whether they have a statistical error, perhaps use
"(full circles)" to refer to them ?

p8, L5: "..estimated on Monte Carlo (MC) as.." --> "..estimated
using Monte Carlo (MC) simulation and is defined as...

p8, L11: I would perhaps use a capital letter for "threshold function
[15]" --> "Threshold function [15]"

p8, L15: "p*_l >1 GeV/c" what does the * stand for? Do you label a
particle's momentum with p* instead of simple p?

p9, L2:  use a long dash, with "--", between 0.473 and 0.523

p10, L6: "..and $N^{out}_u$ refers.." --> "..and N^{\rm out}_u$
refers..." that is, don't italicize "out"

p10, L19: "We estimate the error due to the signal..." --> "We
estimate the uncertainty due to the signal..."

p10, L19: "The signal modeling uncertainties..." -> "The uncertainty
on the signal modeling are due..."

p10, L21: "We also calculate the errors associated with the
uncertainties in the non-perturbative..." -> "We also calculate the
uncertainties due to the non-perturbative..." Here and in several
other places later on (e.g. in caption of Tab I) , you should
change "error" with "uncertainty". By systematic "error" one
means a constant shift to the central value of the measurement. By
systematic "uncertainty" it is intended a distribution of the error
with a certain width, which is the quoted number. So the +/- numbers
in Tab I should be referred to as "uncertainties" and not "errors".

p11, Tab I, caption: "Summary of the measurements of the fitted
numbers of events..."  --> "Summary of the fitted number of

p11, Tab I: Place a label for the M_X, P_+ and M_X,q^2 column. For
example "Method", or "Cuts"

p11, Tab I: Last column, add a "\times" before "10^{-3}"

p11, Tab 2, caption:  Add ":" after "..kinematic cuts, from" in
order to brake the long sentence.

p11, Tab2:  the formatting of the first row is not easily readable.
Remove the vertical space between "Shape" and "function" and on
all the other two-word column headings.

p11, Tab2: add "\pm" in front of numbers in the table

p12, L2: "...reducing the relative error by..." --> "..reducing
the relative uncertainty by..."

p12, L3:  Add "between the two measurements" after "...highly

Report of Referee B -- LH11701/Aubert

This Letter provides inclusive partial branching fractions for
charmless semileptonic B meson decay in kinematic regions in which
the dominant semileptonic decay to charm is suppressed. From these
regions, seven different values of Vub are extracted using several
different theoretical calculations of the corresponding partial

This paper summarizes the work for three challenging measurements.
The measurements themselves are key to improving our knowledge
of Vub, as they represent the kinematic regions that hold best
theoretical promise for control of the nonperturbative effects.
It's crucial to have a unified analysis such as this so that results
from these different region can be compared with clear control of the
experimental systematic uncertainties so that we can learn how well
we can understand the theoretical uncertainties associated with Vub.
Improved extraction of Vub is crucial to our understanding of the
flavor sector, and particularly to CP violation within that sector.
Therefore the paper is of interest both to the high energy physics
community -- experimental and theoretical -- and to the broader
physics community.

I should note that the analysis itself is quite complex, and that
much work has clearly gone into the measurements. However, with
the terseness forced by the PRL length, it's difficult to judge the
analysis and the completeness. The basic techniques are certainly
sound -- but the devil for an analysis like this is in control
of the background and exploration of how well that control is
understood. I strongly encourage the authors to submit a companion
PRD in which the details of the analysis are fully described.

R: indeed, we plan to provide a full description of the techniques
we employ in a PRD. The plan is to include further studies on a
timescale of a few months.

Overall the writing is clear.

I recommend publication after the items below have been considered
or addressed.

1) Since this is intended for PRL, the introduction and conclusion
must be reworked to be more accessible to a general audience.
For the introduction, this should be straightforward: the context
alluded to in the first sentence could be expanded. The second
sentence should be clarified (B->pipi is also proportional to |Vub|^2
and doesn't have a pesky neutrino -- sharpen the argument). The
jargon (eg, X_u, X_c) will be familiar to the specialist but are
undefined and, in a related fashion, the basic mass difference
explanation for why X_clnu is suppressed is not given. Etc...

R: we rewrote the introduction accordingly.

More serious, though, is the conclusion. Table 1 presents 7
different values of Vub, for which the uncertainties are highly
correlated. To me that implies that for any given comparison, the
largest th'y error dominates the level of uncertainty. Comparing
results for Mx vs P+ vs Mx,q2, there are differences that are over
two "standard deviations" apart (or however one should interpret
those theory errors). The situation is not even acknowledged
in the paper. At the very least some statement *must* be made.
Preferably, some more interesting questions should be addressed,
which would truly make the analysis high quality for PRL: What
value of Vub should the reader take away (for comparison to which
value in the PDG, for example)? What are potential pitfalls in the
different regions that may bias on region or another (in theory
or exp't)? Could the pattern of differences tell us anything?
In short, what can we learn from these 7 numbers either directly or
relative to other measurements? If there's nothing that we can do
immediately to learn something from these numbers, then shouldn't
the theory errors at the very least be inflated?

Also in the conclusion, but more for spirit of clarification, are
the Vub's being compared to the inclusive/exclusive average in the
PDG Vub+Vcb review? Are these results appreciably correlated with
that average given the correlation with the previous Mx analysis?
Perhaps more direct comparison with other uncorrelated experimental
determinations with similar kinematic regions, and of "the" Vub from
this paper with exclusive measurements, would be more beneficial
to the general reader.

The shortcomings of the conclusion for PRL are the sole reason for
not, at this point, recommending "The paper should be published
in PRL after minor revisions are made, without further review."

R: we have reworked the conclusions in order to evaluate the
compatibility of our different determinations of Vub. We have done
that, in particular, in the BLNP framework after determining the
statistical and systematic correlations between the experimental
determination of the partial branching fractions. We see an agreement
between the Mx and Mx-q^2 analyses, while the Pplus measurement
differs at 2.5 sigma level (this disagreement is also seen by Belle).
We also state that BGE and BLNP give consistent results and that the
values we measure are in good agreement with other
inclusive $V_{ub}$ determinations, and compatible, although
systematically larger, with measurements from charmless
exclusive semileptonic decays.

2) The definition of P+ refers to a jet direction -- can this
direction be clarified?  Is  it the jet axis calculated from the
hadronic X system?

R: It is indeed calculated from the hadronic system, as its definition
suggests. We decided to drop the reference to the jet direction though,
and just give the definition of P+, along with a reference to
theoretical papers.

3) For the Breco purity selection, does "Breco decay" simply refer
rejection of final states with a particular particle content (ie,
a particular decay mode) such as "5 charged pi's + 2 pi0's", or is
it more finely structured: "5 charged pi's + 2 pi0's in particular
kinematic regions"? As written, it sounds like the latter imposed
on an event-by-event basis. Have I misunderstood, and it's really
just a final state selection?

R: it is not a rejection, rather a selection of decays of the other
B in the event into a specific state. The number of final states that
we fully reconstruct is rather large (of the order of 1000); for
some combinations of particles we may have specific invariant mass
requirements. However, there is not enough space inn this paper to
embark in a detailed description of our selection; we plan to include
a thorough description the upcoming PRD.

4) Does "photon energy loss" in the m_ES fit description refer to
initial state radiation, final state radiation or both?  The phrase
"caused by photon energy loss" refers only to the m_ES tail,
correct? For either ISR or FSR, does the systematic parameterization
ansatz include uncertainty in the level of radiation (or is it not
important on the few % scale)?

R: "photon energy loss" refers to energy loss in the detector

5) The paper mentions Xclnu decays entering because of undetected
KL's. Do D semileptonic decays also pose a problem since these
will also give an additional undetected particle? The charged
leptons tend to be rather soft, so depending on the lepton id
criteria might not get vetoed... If these are important, have the
D s.l. branching fractions and the B->D momentum spectrum been
considered in the systematics?

R: The requirements on the minimum lepton momentum (1 GeV) and
the lepton charge reduce the background from D s.l. decays to
a level of a few percent.
We take into account the residual contribution by varying the D s.l.
branching ratios in our assessment of systematic uncertainties.

6) Some idea of the effectiveness of the D*lnu veto based on
m_nu^2 would be useful to include -- both rejection factor and
signal efficiency. The shape in signal that one would expect is
not so decay. For clarity in the paper, I suggest changing the
variable name to avoid confusion with the p_nu introduced in the
previous paragraph, whose mass this variable is not...

R: the requirement  m_veto<-3 reduces the
D*lnu background by about 36% while keeping more than 90% of signal  

7) Minor typo: In the paragraph and sentence beginning "To extract
the distribution  in the variables...", I think "subsequently
separating" should be "subsequently separate"

R: We changed the sentence.

8) Somewhat more detail on the spectral fits is warranted, since
these are a key component of the method. For example, presumably
N_u and N_u^out are normalizations for two different fit components
corresponding to decays within and without the final kinematic
region at the generator level. How are these treated in the fit?
Are they tied together by the particular theoretical model used to
evaluate the efficiency? Or, do the float independently? Nice to
know, since that couples to how one needs to consider the models
in the systematics analysis.

R: N_u is, in fact, the number of data events in the signal region after
all cuts. N_u^out is an estimate of the contribution to N_u of signal
events reconstructed in the signal kinematic region but coming from
outside the said region. In the fit, the MC shapes of signal and N_U^out
components are forced to be the same, according to the theoretical  

9) I couldn't tell how Nsl and BGsl are determined. Are these
obtained from fits to the Breco mES distribution where an additional
lepton has been required? If I haven't simply missed something,
the method should be stated in the paper.

R: the number of background events is determined from MC simulation.
The corresponding systematic uncertainty due to our knowledge of
the background composition has been taken into account.

10) Minor typo: Nsl= ... and BGsl=... are the measured *numbers of*
semileptonic  events and... "numbers of" is missing.

R: We changed the sentence.

11) In systematics, at what level does modeling of hadronic showers
(beyond KL's) in the calculation of the X system matter? Eg.,
fluctuations or splashback could result in showers that are
relatively isolated from tracks in the calorimeter, so will bias
the X calculation and therefore any efficiency or spectral shape
for Mx and P+.

R: modeling of hadronic showers, including fluctuations and splashback
is taken into account in the GEANT 4 MC simulation. The associated
systematic uncertainties are included in what we call "detector
systematics". The combined effect due to the reconstruction of
neutral clusters in the calorimeter actually dominates the total
detector systematic uncertainty, ranging from 1.4% for Mx,
to 2.9% for Pplus, to 2.5% for Mx-q2.

12) In the systematics table, does {\cal B}(D) refer to D branching
fractions? Can the paper clarify how these branching fractions,
or what subset, is varied?

R: we varied the branching fractions within their experimental errors
around the central values. We have rewritten the description
in the paper.

13) There's no mention of final state radiation, which I would assume
to be sizable for electrons. What effect does this radiation have
on efficiency and background smearing, and how well is it known?
What effect does radiation have on generator level quantities
(spectral shape biases), and at the 1-2% level of systematics
considered in the paper, do the kinematic regions with radiation
map well enough onto the regions the theorists have calculated
rates for without radiation? This question can be both absolute
in nature or refer simply to how the MC is treated -- for example,
one could get an inappropriate generator level q2 boundary if by
looking at the generator level (p_e + p_nu)^2 after radiation.

R: in our analysis, FSR is simulated using PHOTOS.
We checked that the effect of FSR is negligible by studying
kinematic variables in MC samples produced with and without PHOTOS.

14) The paper notes specifically notes that the mix of charged
and neutral B pairs is not 50-50. Is there any systematic effect
associated with this in terms of the mass spectrum of the hadronic
system being, in reality, somewhat different, etc., particularly
when average B lifetimes are used in the end? From figure 2 and
the roughly 60/40 B+B-/B0B0bar split, it looks like there should
be enough statistics to get Vub independently from these two
samples (and the tagging, of course, allows the separation into
those subsamples). Are the Vub results from the two subsamples
consistent, and do the patterns in the different kinematic regions
manifest themselves in the same way?

R: we checked that PBR obtained separating the samples by
the charge of the reconstructed B  meson are
consistent with each other within the uncertainties and
indeed the patterns in the different kinematic regions
manifest themselves in the same way.
A measurement of PBR separately for charged and neutral
Bs is important to determine weak annihilation effects that
are currently taken into account in the theoretical models. A more
detailed study of experimental cross-feed between charged and neutral
Bs is required and will be included in the upcoming PRD.

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