Presentation on my work at the University of Victoria.

1. Search for signal in
B->D(*)ππlν
channel in BaBar data
- Ruturaj Apte
IIT Bombay,
India
Working under
Prof. Robert Kowalewski
Thomas Lueck

2. Outline
● Motivation for studying semileptonic decays
● Motivation for our search for the D(*)ππlν channel
● Some details about the experiment
● Reconstruction of the Btag meson
● Introduction to “normalization modes” and how they
help in reducing the uncertainties in the final branching
fraction calculation
● Multi variate fisher analysis
● Fitting of components

3. Motivation for studying semileptonic
decays
● In the Standard Model, the CKM matrix helps describe CP violation
in weak interactions involving quarks of different flavors.
● Precise determination of these matrix elements is crucial for a
stringent test of the SM and for reducing theoretical uncertainties in
the many New Physics searches with flavor.
● Semileptonic B decays are used to measure the matrix element |Vcb|
● According to Babar measurements, the experimental estimate for the
quantity
BF(B->D(*)τν)/BF(B->D(*)lν) shows a deviation of 3.3σ from the
SM prediction. The channel we are looking for is a background
for the above search and depending on our result the above
3.3σ deviation can increase or decrease.

4. Motivation for our search
● The sum of the Branching Fractions for all the known exclusive
semileptonic decays of the B meson do not match the inclusive
B->Xclν Branching Fraction. The Branching fractions are :
B -> Dlν : 2.30 (+/-) 0.1
B -> D*lν : 5.34 (+/-) 0.12
B -> D**lν : 1.64 (+/-) 0.18
Inclusive B->Xclν : 10.91 (+/-) 0.14
● Gap between exclusive and inclusive : 1.63 (+/-) 0.3

5. Y(4S) resonance and need for a Btag
● The asymmetric e+e- B factories operate at a center of
momentum(CM) energy of E = 10.58 GeV
● This energy corresponds to the mass of the Y(4S) resonance, a
bound state and it then decays almost exclusively to approx equal
numbers of and B+B- pairs.
B0 ̄ B0
● The B mesons are almost at rest in the Y(4S) frame
● No other additional particles are produced.
b ̄b

6. Y(4S) resonance and need for a Btag
● The decay products of the B and antiB overlap completely in
the detector.
● The vertex resolution is not good enough to
unambiguously assign charged tracks
● Hence a kinematic reconstruction one of the B mesons in
a fully reconstructed decay mode is needed to assign
particles to the B and anti-B
● The characteristic semileptonic decay is searched by
identifying a reconstructed track left by an electron or a
muon with CM momentum > 0.6 GeV

7. Event Reconstruction Technique
● A total of 2968 separate decay chains are considered in the
reconstruction of the Btag.
● One of the independant variables used to study the Btag
reconstruction is “delta E” which is the energy difference
between the reconstructed and expected energy of the Bmeson.
Delta E = ErecB – (Ecms of the colliding beams)/2
● Energy substituted B mass (mESB) =
mESB = sqrt(E2/4 – PB
2)
E = CM energy of the colliding beams
PB = three vector momentum of the B meson.

8. Event Reconstruction Technique
● After that a D(*) meson is reconstructed using a subset of D meson
decay modes.
● While looking for decays with an added pion, we look for an extra
pion track which has not been used for any reconstruction.
● The percentage of Y(4S) decays with a lepton that give an
acceptable Btag reconstruction is only about 0.5%
● We do not want to rely on the MC to estimate our Btag reconstruction
efficiency

9. Delta E Plot for B->Dlν signal side

10. MESB Plot for B->Dlν

11. Normalization Modes
● The estimate of the systematic uncertainty related to differences
in the efficiency for reconstructing the Btag in data and MC is
non trivial.
● Thus we use a sample of B->D(*)lnu as normalization modes in
order to cancel out these uncertainties.

12. Efficiency calculation
● Signal Efficiency is defined as the number of signal events
reconstructed divided by the number of signal events actually
produced in the detector.
● We estimate this efficiency from the MC samples
● Efficiency = Nsig,mc / (NBBbar,MC*bf*2)
Nsig,mc = Signal yield from MC
NBBbar,MC = Total BBbar events generated in MC
bf = Branching Fraction of the signal decay mode put in during
MC generation of the events

13. Calculation of the Branching
Fraction
● Once we extract the signal yield from the fit, we can continue to
estimate the branching fraction.
● BF = Nsig,data / ( εtag*εsig * NBBbar,data )
εtag = efficiency of the Btag reconstruction
εsig = efficiency of reconstruction on signal side
Nsig,data = signal yield from the fit
NBBbar,data = Number of B,Bbar events from the data

14. How it will help in our final BF
calculation
● We need to cancel out the uncertainties that arise due to systematic
uncertainty related to differences in the efficiency for reconstructing
the Btag i.e. εtag
● BF(B->DPiPilnu)/BF(B->D(*)lnu) = Nsig * εsig,norm / εsig * Nnorm
● The BF(B->D(*)lnu) is taken from the well measured BFs of
these modes.
● We thus cancel out the εtag assuming it is the same for both,
the signal and normalization modes.

15. Variables of Interest
● (Missing mass)2 : missing mass square for a particular decay say
B->(D(*)lν + nπ) is defined as :
(mmiss)2 = (Py – PBtag - PD(*) – Pl
- Ppions )2
should peak at 0 for signal
● plep_cms : momentum of the lepton in the cms frame.
● mESB : sqrt(E2/4 - PB
2)
where E is the total CM energy and PB is the momentum of the B
meson.

16. Variables of Interest
● Delta E = ErecB – (Ecms of the colliding beams)/2
should peak around 0 for correctly reconstructed Btag.
● Extra Energy : The total energy of all the neutral particles and
photons that have not been used for reconstruction of any particle
● Unmatched neutral : Number of neutral particles that have not been
used in any reconstruction.
● Charged multiplicity : number of charged particles used in the
reconstruction of the Btag candidate.

17. Variables of Interest
● CosThrustB : the cosine of the thrust angle of the Btag candidate
with respect to the rest of the event.
● MoltNB : number of neutral particles that have been used to
reconstruct the Btag.
● Fox2CT : A variable that measures the event shape. It is close to 1
if the event is jetlike and close to 0 for a spherical event.

18. Why they are useful

19. Cuts applied
● Flavor correlation between D and B.
● Charge Flavor correlation between the D and the lepton
● 5.27GeV < mESB < 5.29GeV
● Momentum of lepton in cms frame > 0.6GeV
● -2GeV < mm2 < 2GeV
● Total charge of the event has to be 0.
● Total charge of the D meson and added pions has to be <=1

20. Multivariate Analysis
● The method of Fisher discriminants was used to reduce the
background levels
● The variables used for the fisher tuning were
ExtraEnergy,mESB,unmatched neutral multiplicity, multiplicity of
charged particles in Btag reconstruction, absolute value of cosine of
Thrust angle of Bmeson with the rest of the event ,
Fox2CT, multiplicity of neutral particles in Btag.
● A different Fisher cut expression was obtained for each decay
mode increasing the significance of the data.

21. (Missing mass)2 plot for B->Dlν

22. Improvement in signal to
background ratio
● We quantify the signal/background ratio using a quantity called
significance
Significance = S/ sqrt(S+B)
S : number of signal events
B : number of background events
● Significance of the mm2 plot without the fisher tuning = 164.432
● Significance after the fisher tuning = 171.106
● Improvement is not very drastic since these decay modes are
already quite clean

23. Similar analysis done for the
B->Dπlν

24. Improvement due to Fisher Tuning
● Initial significance with D** that is the D1, D1
' , D0 , D2 as the
signal and all other components as background is
= 15.80
● Significance after fisher tuning
= 21.38
● An increase of 35.31%
● This analysis was repeated for B -> D(*)πlν and
B -> D(*)ππlν for charged as well as neutral D mesons.

25. B ->D*+ππlν

26. Improvement due to Fisher
● Significance before the Fisher cuts :
19.6025
● Significance after the Fisher cuts :
22.76
● An increase by 16%

27. Fitting to the (missing mass)2
● Used the RooFit package for fitting
● Did an unbinned maximum likelihood fit for the data.
● The components that were fitted were:
1. Dlnu
2. D*lnu
3. D**lnu
4. Other BBbar decays
5. Continuum events ( e+e- -> qqbar ; q != b)
✔ Used 2 types of fit methods : one by extracting histogram pdfs from
the MC histograms and other using RooKeysPdf which gives a
smooth pdf

28. Fit Plots for B -> D+lν

29. Fit Plots for B -> D+lν

30. Final Fit for B -> D+lν

31. Results of the fit
● Signal Yields for the normalization mode calculated from the
fit are :
D0lν = 8365.84 +/- 135.485
D+lν = 3671.78 +/- 86.834
D*0lν = 12052.4 +/- 143.713
D*+lν = 5498.93 +/- 84.4648
● Signal reconstruction efficiencies :
εsig(D0lν) = 0.077 %
εsig(D+lν) = 0.014%
εsig(D*0lν)= 0.123%
εsig(D*+lν) = 0.022%

32. Conclusion
● The normalization modes have been measured to get their
signal yields and to get the signal efficiencies for the individual
decay modes.
● We now need to fit to the 2 pion cases using the MC generated
samples for DΠΠlν to get the signal yield and the
efficiencies for this channel.
● We then use the double ratio formula to get our final Branching
Fraction.