Hadronic Mass Moments from Semileptonic B Meson Decays at BABAR Henning Fl

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Hadronic Mass Moments from Semileptonic B Meson
Decays at BABARHenning Flächer

• OUTLINE

• Mass Moment Measurement
• Interpretation in context of HQET
• Conclusions

• Motivation
• Fully reconstructed B mesons
• Event selection

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Motivation
CKM matrix describes quark mixing by relating the
weak- to the mass- eigenstates and accommodates
CP violation.
Semileptonic B decays give direct access to Vub
and Vcb Allow for factorization of leptonic and
hadronic currents.

• Advantages of inclusive B decays large rates
• OPE provide expansions for inclusive observables
  like
• semileptonic decay width
• mean values of hadronic mass distributions
• mean values of lepton momentum spectra

Precise theoretical calculations if OPE provides
consistent framework
GSL?Vcb2 (1corrections)
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Vcb and Vub from semileptonic B decays

• When measuring Vcb we look for b ? c
  transitions
• Due to the strong interaction it is only possible
  to measure B?Xc l?

In semileptonic B decays final state interaction
is reduced to a minimum Only then it is possible
to relate b?c to B?Xc
Xc can be D, D, D D() (n)p Invariant mass
of Xc is our observable!
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Theory The way towards Vcb
Operator Product Expansions within the context of
Heavy Quark Effective Theory provide a powerful
tool to predict decay rates with very high
precision (2)
Similar expansions in other variables
Consistency needs to be tested and verified!!!
Inclusive B decays invariant mass of X-system
ltMXgt In b? s ? gamma decays photon energy ltE?gt
Important assumptionQuark-Hadron Duality
Non-perturbative parameters appear in the
expansion which need to be measured by experiment
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Analysis Method
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Fully Reconstructed B-Mesons
A big step forward!
Reconstruct one B candidate in the event
Reconstruction efficiency rather low, only 0.4
? only possible at a B-Factory like BaBar!
With a data set of 89 M BB pairs (Run12 only) we
obtain 350 000 fully reconstructed B mesons
Considering the semileptonic branching fraction
we finally obtain 350000 x 2 x 10.8 70 000
semileptonic decays.
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Semiexclusive B reconstruction
Consider the following decays

• Advantages
• Breco momentum
• Breco Flavour
• Background reduction
• (combinatorics)
• mES sideband

Add p, p0, K, K0s iteratively until

• Disadvantage
• limited statistics

Lepton required on recoil side
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What is Xc?
Decays B ? Xc l v
D
D
Four resonances above D
Four non-resonant decays
Broad mass spectrum above the two narrow
resonances (D,D)
Mass spectrum on detector level
Moment
with
(weighted sum)
MXdet (GeV)
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Event Topology
BB -gt Breco (X,l,Pmiss)

• Apply Energy and Momentum conservation
• EBreco EX El E? - EPEPII 0
• PBreco PX Pl P? - PPEPII 0
• 4 Constraints
• Mass Constraints
• M(Breco)M(X,l,?)
• ? 1 Constraints

Kinematically closed environment reconstruction
of all kinematic quantities (energies, momenta,
masses) in the B meson rest frame can be achieved!
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Selection
Event Selection
Identification of exactly one lepton with P gt
0.9 GeV
Event quality cuts Emiss-pmiss lt 0.5
GeV Breco Quality P gt 40
Furthermore Emiss gt 0.5 GeV
pmiss gt 0.5 GeV Total charge
?Q 1
7100 signal and 2100 background events
Lepton charge consistent with prompt B decay
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Analysis Strategy
P0.9GeV
To obtain the true mean value of the hadronic
mass distribution we need to

• Subtract Background from
• misreconstructed B mesons
• Calibrate measured masses
• event-by-event
• Subtract remaining B
• Background
• Apply efficiency and
• acceptance corrections

P1.5GeV
Extract ltMXgt as a function of the minimal lepton
momentum
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Effect of kinematic Fit Resolution Functions
Kinematic Fit improves resolution of the
invariant mass MX
Almost unbiased measurement irrespective of final
state
Reduction of branching fraction dependence
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Calibration Curve
Define calibration curve independent of
underlying model!

• binning in bins of Mxtrue

• True modes
• D
• D
• D (two narrow two broad)
• XH (4 spin dependent D()PI)
• Large variety of different models
• and different final states

Mxtrue binning (example)
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Calibration Curves

• Relate measured hadronic mass to true mass of
  X-system
• linear relation as function of true mass
• irrespective of decay and underlying model

4 XHnreso
4 XHreso
D
D
Application of calibration results in true mass
irrespective of decay mode and model
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Extraction Method Full Formula
The full formula has not only to take into
account the mass bias but it must also account
for lepton acceptance and efficiency differences
Ri .
Calibrate the measured mass event-by-event
Subtract remaining background
correction factor
If correction factor ? 1 and RMS small for PDF
variations (i.e. dropping contributions to
XHresoand XHnreso) only a small systematic error
for model and branching fraction dependence is
achieved!
? Since we measure Mx we of course also can get
Mxn
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Distribution of Bias Factors
Pgt1.1 GeV
RMS(Pgt1.1) 0.01 GeV2
RMS(Pgt0.9) 0.01 GeV2
Pgt0.9GeV
ltMX2gt (GeV2)
ltMX2gt (GeV2)
Vary assumptions for X model Change X
composition disregard single decay modes and
combinations, i.e. drop them in pairs, triplets
etc. and observe how MC correction changes
Spread is small compared to other error
sources stat. error 0.05 GeV2 det. sys. error
0.03 GeV2 background 0.04 GeV2
Small model dependence!
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Result First and Second Moment of MX Distribution
ltMXgt
ltMX2gt
Clear Pmin dependence, reflecting increasing
contribution from high mass final states.
Points are highly correlated
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Crosscheck on MC
Moment ltMX2gt
Derive calibration and background subtraction
from MC and apply to independent MC data
sample.
Works both, for lower Pmin cut and
differentially in bins of P
Difference to true value
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Crosscheck on Data Partial Reconstruction of
D
Select a data control sample where the true
underlying mass is know
Use the decay B0 ? D l? Partially reconstruct
D decays by identifying a slow charged pion
MD2
Apply the complete extraction procedure to this
data sample and measure ltMXgt as a function of the
lepton momentum.
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Consistency of Results
We split the data into statistically independent
subsamples and repeated the measurement.
Statistical Errors only!
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Interpretation in Context of HQET
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Reminder of Expansion
as(2)
1/mB
One expansion for every P cut
1/mB2
1/mB3
perturbative expansion in as
non-perturbative expansion to first, second and
third order in 1/mB
Determine parameters ? and ?1
? mass difference between B meson and b quark
?1 negative of kinetic energy squared of b
quark ?2- chromo-magnetic coupling of b quark
spin to brown muck
?2 known from B-B and D-D mass splitting
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Interpretation in context of HQET
? Calculations from Falk and Luke
(Phys.Rev.D57424-430,1998)
Fit for ? and ?1 by minimizing ?2 taking
correlations between data points into account
OPE fit to the BABAR data
The extracted parameters describe all hadronic
mass moment measurements
Prediction of P dependence for ltMX2gt using ?
from b? s ? and ltMX2gt at Pmin 1.5 GeV results
in less consistent description.
OPE prediction using CLEO data only ltMx2gt and
ltE?gt from b?s?
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Results in ?1-? plane
This fit is performed in the MS scheme
for comparison with other experiments
Experimental Errors only!
We extract ?1 -0.36 0.09 GeV2 and ? 0.53
0.09 GeV
CLEO ?1 -0.24 0.07 GeV2 and ? 0.35 0.07
GeV
(experimental errors only)
Hadronic Mass Moments from all three
Experiments overlap in same region.
Band in ?1-? plane from b? s? slightly offset
??2 1 ellipse
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A more comprehensive Approach

• Two possibilities
• Check consistency of the HQE calculations by
• comparing hadron moments from BABAR
• with other moment measurements
  (external input)
• Use the BABAR hadron moments together with
• ?SL (BABAR) to obtained an improved
• determination of Vcb

External Input

• Based on improved OPE calculations in the
• ?(1S) mass scheme (Phys. Rev. D67. 05012, 2003)
• we can now include moment measurements
• in the fit as well as ?SL
• Simultaneous extraction of
• HQE parameters and Vcb !

BABAR Input
(development of fit code in close collaboration
with theorists)
Calculate ?SL from BABAR data only! ? life time
measurements and BR(B?Xl?) have by now
reached a precession that makes ?SL
(BABAR) very competitive!
MeV
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Consistency of the HQE Hadron Moments vs. Lepton
Moments
BABAR only
Simultaneous extraction of Vcb, mb1S, and ?11S
from a fit to the HQE in the 1S mass
scheme (O(1/mB3) parameters are fixed in the fit)
Vcb - mb1S plane
?11S - mb1S plane
Note ??21 contour include already part of the
theory errors. Only O(1/mB3) uncertainties are
not included!

• Good agreement between BABAR moments and other
  hadron moment measurements
• ??21 contour of hadron moments and lepton
  moments do not overlap
• ?indication for large O(1/mB3) corrections or
  maybe even more ?
• (bear in mind that ?SL is common in both fits!)

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Vcb extracted using BABAR data only
MeV
BABAR only
Contributed to EPS03 in Aachen

previous inclusive Vcb measurement from BABAR
Vcb 42.3?0.7(exp)?2.0(theo) 5 (Phys. Rev.
D67, 2002)
Precise measurement of Vcb with input from
BaBar data alone (life time, branching
fractions, moments) and very competitive (3
total error)!
Caveat We still have to establish the
consistency of the OPE to at least the same
level of accuracy we would like to achieve for
Vcb (lt1)!
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Summary and Conclusion

• We have measured the first and second moment of
  the MX distribution
• for different P cuts (0.9 to 1.6 GeV).
• With a completely new and unique extraction
  approach we were able to
• overcome model uncertainty which leads to a
  significant improvement of
• the hadronic mass moment measurement.
• Using a simultaneous extraction of Vcb, mb1S,
  and ?11S from a fit to the HQE
• calculations we obtain an improved measurement
  of Vcb which is based on BABAR
• data only!
• A comparison with other hadron moment
  measurements from CLEO and DELPHI
• demonstrates good agreement.
• A consistency test of hadron and lepton moments
  in the framework of the OPE leads
• to inconclusive results and demonstrates again
  the importance of the determination
• of all the O(1/mB3) parameters from data.
• ? More moment measurements from different
• physics processes will be
  needed to test HQETOPE to the level of lt1.

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Backup Slides
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The PEP-II B-Factory
Energy asymmetric e-e- collider
E GeV e / e- 9.0 / 3.1
I mA e / e- 1550 / 1175
Bunches 1034
L cm-2 s-1 6.582 x 1033
Lint pb-1/day 395.1
(peak performance)
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PEP-II and BaBar Performance
Continuous improvement in delivered and recorded
luminosity!
This analysis is based on Run12 only (82 fb-1)
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The BABAR - Detector
Electromagnetic Calorimeter 6580 CsI(Tl)
crystals (electrons/photons)
1.5 T Solenoid
e (3.1 GeV)
Cerenkov Detector (DIRC) 144 quartz bars 11000
PMTs
e- (9 GeV)
Drift Chamber 40 layers
Instrumented Flux Return iron / RPCs (muon /
neutral hadrons)
Silicon Vertex Tracker 5 layers, double sided
strips

• SVT 97 efficiency, 15 mm z hit
  resolution (inner layers, perp. tracks)
• SVTDCH ?(pT)/pT 0.13 ? pT 0.45
• DIRC K-? separation 4.2? _at_ 3.0 GeV/c ?
  3.0? _at_ 4.0 GeV/c
• EMC ?E/E 2.3 ?E-1/4 ? 1.9

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Checks on Background Subtraction
Charged Bs
Select events where lepton charge is inconsistent
with prompt B decay
Pgt1.4
0.8ltPlt1.0
1.0ltPlt1.4
Fit MC shapes to data to obtain normalization
Neutral Bs
MC scaling factor compatible with 1
_
0.8ltPlt1.0
1.0ltPlt1.4
Pgt1.4
Contribution from B0-B0 mixing