1. 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

2. Outline Introduction Analysis strategy Main aspects of the analysis Branching ratio results on 2000-2002 data set.

3. Riunione Gruppo I – Napoli 19 Dicembre 2002 The decay B  D*l proceeds by the quark level decay b  c.  The decay rate therefore depends on |V cb |. The picture is complicated by the strong interaction b WW c gV cb B  D*l and the CKM element V cb On general ground, the amplitude is exact leptonic current : hadronic term calculated by means of non perturbative methods:

4. Riunione Gruppo I – Napoli 19 Dicembre 2002 Measurement of |V cb | from B  D*l Strong interaction effects can be studied by means of the Heavy Quark Effective Theory (HQET). differential decay rate Kinematics function Form factor. theoretical uncertainty of ~4% when w  1 Fitting dГ/dw, |V cb |F(w) is obtained. Theory ( HQET) provides F(w) allowing the extraction of |V cb |, by means of an extrapolation w  1 (zero recoil). CLEO

5. Riunione Gruppo I – Napoli 19 Dicembre 2002 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 D 0 decay modes)

6. Riunione Gruppo I – Napoli 19 Dicembre 2002 The data set analyzed 2000:20.25fb -1 on-resonance 2.60fb -1 off-resonance 2001:35.54 fb -1 on-resonance 3.79fb -1 off-resonance 2002: 24.30fb -1 on-resonance 3.19fb -1 off-resonance 2000 MC2001 MC2002 MC

7. Riunione Gruppo I – Napoli 19 Dicembre 2002 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 *  D 0  soft  D 0 reconstructed in 4 modes The Neutrino is NOT reconstructed e/e/ D0D0 ss B D 0 is reconstructed in 4 decay modes Constrained vertex fit on D 0 decay products, soft pion and lepton The signal is measured in 2  4 independent signal samples.

8. Riunione Gruppo I – Napoli 19 Dicembre 2002 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

9. Riunione Gruppo I – Napoli 19 Dicembre 2002 Data samples definition

10. Riunione Gruppo I – Napoli 19 Dicembre 2002 Combinatorial background The mass difference  m=(M D * -M D ) 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.

11. Riunione Gruppo I – Napoli 19 Dicembre 2002 Continuum background D*  soft D0D0 K  e,  e+e+ e–e– c – c X The 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 D0D0 K  B0B0 e,  Signal

12. Riunione Gruppo I – Napoli 19 Dicembre 2002 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 matrixtrue number of events

13. Riunione Gruppo I – Napoli 19 Dicembre 2002 Uncorrelated background Events with a D* and a lepton in the same side are enriched in uncorrelated background. D*  soft D0D0 K  B0B0 e,  Signal Opposite-side Same- side signal other B D*  soft D0D0 K  B 0(+-) X B 0(-+) e,  Y

14. Riunione Gruppo I – Napoli 19 Dicembre 2002 Background characterization Fake D* Fake lepton continuum uncorrelated From data control samples

15. Riunione Gruppo I – Napoli 19 Dicembre 2002 Fitted Yields 2000 2001 2002

16. Riunione Gruppo I – Napoli 19 Dicembre 2002 Correlated background  several decay modes contributing  the overall amount is small  Estimated from Monte Carlo simulation D*  soft D0D0 K  B 0(+–) e,  X D*  soft D0D0 K  B0B0 e,  - Signal

17. Riunione Gruppo I – Napoli 19 Dicembre 2002 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**

18. Riunione Gruppo I – Napoli 19 Dicembre 2002 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

19. Riunione Gruppo I – Napoli 19 Dicembre 2002 Examples of fit results electron - D 0  K  - year 2000muon - D 0  K  - year 2002

20. Riunione Gruppo I – Napoli 19 Dicembre 2002 Monte Carlo correction and systematics uncertainties Correction for PID Correction for tracking and  0 reconstruction efficiencies.  lepton, D 0 decay products but most important soft  tracking.

21. Riunione Gruppo I – Napoli 19 Dicembre 2002 Soft pion tracking efficiency The soft  momentum spectrum is really soft: 50 MeV <p T < 200 MeV From control sample D* decays the relative tracking efficiency between soft  and ordinary tracks has been measured

22. Riunione Gruppo I – Napoli 19 Dicembre 2002 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%)

23. Riunione Gruppo I – Napoli 19 Dicembre 2002 Branching fraction (blind!)

24. Riunione Gruppo I – Napoli 19 Dicembre 2002 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 D 0 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)

25. Riunione Gruppo I – Napoli 19 Dicembre 2002 Backup slides

26. Riunione Gruppo I – Napoli 19 Dicembre 2002