Measurement of the Branching fraction B( B D* l ) C. Borean, G. Della Ricca G. De Nardo, D. Monorchio M. Rotondo Riunione Gruppo I – Napoli 19 Dicembre

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Measurement of the Branching fraction B( B D* l )

C. Borean, G. Della Ricca

G. De Nardo, D. Monorchio

M. Rotondo

Riunione Gruppo I – Napoli 19 Dicembre 2002


Outline

• Introduction

• Analysis strategy

• Main aspects of the analysis

• Branching ratio results on 2000-2002 data set.


• The decay BD*l proceeds by the quark level decayb c. The decay rate therefore depends on |

Vcb|.

• The picture is complicated by the strong interaction

b W

cgVcb

B D*l and the CKM element Vcb

HLV

GiDB cb

Fqcqb 2

)( M

On general ground, the amplitude is

uuL )1( 5

exact leptonic current :

BbcDH |)1(| 5

hadronic term calculated by means of non perturbative methods:


Measurement of |Vcb| from B D*l

Strong interaction effects can be studied by means of theHeavy Quark Effective Theory (HQET).

differential decay rate

Kinematics function Form factor. theoretical

uncertainty of ~4% when

w1

Fitting dГ/dw, |Vcb|F(w) is obtained. Theory ( HQET) provides F(w) allowing the extraction of |Vcb|, by means of an extrapolation w 1 (zero recoil).

CLEO


Analysis strategy

Define selection criteria for the signal. Study the backgrounds directly from data as far as

possible. Estimate remaining backgrounds from Monte Carlo

simulation. Determine the selection efficiency. Correct the

Monte Carlo simulation evaluating the corresponding systematic uncertainty.

Fit signal and residual background from resonant and non-resonant semileptonic decays, exploiting available physical constraints (we studied both electron and muon channel and 4 D0 decay modes)


The data set analyzed

2000: 20.25 fb-1 on-resonance 2.60 fb-1 off-resonance



2000 MC 2001 MC 2002 MC


Event selection

• A Charged D* meson and a lepton (muon and electron) of opposite charge are reconstructed the D* meson is reconstructed from the D*D0soft D0 reconstructed in 4 modes

• The Neutrino is NOT reconstructed

e/

D0s

B

D0 is reconstructed in 4 decay modes

Constrained vertex fit on D0 decay products, soft pion and lepton

The signal is measured in 2 4 independent signal samples .


Backgrounds

• The event is not from BB (continuum)• Reconstructed D* is fake (combinatorial)• lepton is fake (fake lepton)• B D* X the other B l X (uncorrelated)

The amount of these backgrounds are be estimated directly from experimental data


Data samples definition


Combinatorial background

The mass difference m=(MD*-MD) discriminate real D* events

against combinatorial events.

Maximum likelihood fit, using a double gaussian for the peaking component and an empirical distribution for the background.The data set has been subdivided in several samples sharing the same peak and combinatorial parameters according to the different resolutions ( i.e. SVT only tracks w.r.t. DCH tracks) and background distributions.


Continuum background

D*

soft

D0

K

e,

e+

e–

c

–c

XThe peak yield fitted in off-resonance experimental data has been scaled according to the known ratio of data collected on-resoncance and off-resonance.

D*

soft

D0

K

B0

e,

Signal


Fake lepton background

• Signal leptons are tight leptons (maximum purity)• Fake lepton sample composed by lepton candidates which fail

the loose lepton identification criteria• The PID selection efficiencies and mis-id probabilities as a

function of momentum and direction are re-weighted according to our sample lepton momentum distribution

the fraction of fake leptons in all samples are extracted

measured yield efficiency matrix true number of events


Uncorrelated background

• Events with a D* and a lepton in the same side are enriched in uncorrelated

background.

D*

soft

D0

K

B0

e,

Signal

Opposite-side Same-side

signal

other B

D*

soft

D0

K

B0(+-)

X

B0(-+)

e,

Y


Background characterization

Fake D*Fake lepton

continuumuncorrelated

From data control samples


Fitted Yields

2000

2001

2002


Correlated background

several decay modes contributing

the overall amount is small

Estimated from Monte Carlo simulation

D* soft

D0

K

B0(+–)e,

X

D*

soft

D0

K

B0

e,

-

Signal


Extraction of the signal

• After the background subtraction, the signal sample is composed by B D*l and B D** l and non resonant semileptonic decays with a D*.

• To disentangle the signal the variable cos BY is used:

muonselectrons

signal

D**


Fit of signal and background components

• Least square fit to the 4 (D mode) 2 (lepton mode) data samples all parameters free or imposing equal ratio of D* / D**

fractions signal and backgrounds shapes taken from Monte Carlo


Examples of fit results

electron - D0 K - year 2000 muon - D0 K - year 2002


Monte Carlo correction and systematics uncertainties

• Correction for PID

• Correction for tracking and 0 reconstruction efficiencies. lepton, D0 decay products but most important soft tracking.


Soft pion tracking efficiency

• The soft momentum spectrum is really soft: 50 MeV <pT< 200 MeV

• From control sample D* decays the relative tracking efficiency between soft and ordinary tracks has been measured


Other systematics uncertainties

• D** background composition and shape modeling in Monte Carlo ( 2%).

• vertexing algorithm (1%)• stability in the binning (1%)• lepton momentum cut (1%)


Branching fraction (blind!)


Conclusions

• We have analyzed the largest D*l sample ever

• Systematics is dominant in the uncertainty.

• Some modes have large uncertainty. Most probably the K and K 3 mode will be used for the measurement leaving the other two for a cross check.

• Dominant uncertainties does not depend on us

• f+/f0 ratio and D0 PDG branching fractions!

• D** modeling is 2%.

• It may improve when D** measurements will be performed.

• soft pion efficiency is historically the main concern. We think we have it under control.

• Results are blind. BABAR internal review is started. They will give the OK for unblinding. Conference target: Moriond ( march 2003)


Backup slides


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Measurement of the Branching fraction B( B D* l ) C. Borean, G. Della Ricca G. De Nardo, D. Monorchio M. Rotondo Riunione Gruppo I – Napoli 19 Dicembre