Inclusive Semileptonic B Decays at BABAR

1. Inclusive Semileptonic B Decays at BABAR Thorsten Brandt (TU Dresden) for the BABAR Collaboration Introduction The BABAR Detector Determination of |Vcb| Determination of |Vub| Conclusion and Outlook 19th International Workshop on Weak Interactions and Neutrinos

2. Introduction • Semileptonic B decays provide pure method for determination of |Vcb| and |Vub| • theoretical framework : OPE / HQE • Analysis of final states yields additional information : • b quark mass „L“ • nonperturbative QCD parameters • (-) kinetic energy squared of b-quark in B-meson „l1“ • consistency checks of OPE / HQE • Inclusive precision measurements • small statistical and systematic errors • high sensitivity to theoretical uncertainties

3. Qualitative Overview Inclusive El spectrum Rate Semileptonic B decay Shape  |Vcb|2,|Vub|2 El[GeV] Inclusive Mx spectrum shape Shape Rate for Mx < 1.5

4. Inclusive Spectra & HQE • Heavy Quark Expansion relates s.l. decay rate and |Vcb| : • related to b-quark mass • l1 : (-) kinetic energy squared of b-quark in B-meson • l2 : chromo-magnetic coupling of b-quark spin to gluon field • Similar expressions for shapes of Mx and El spectra • for different threshold momenta of the lepton • first moment of invariant mass-squared of hadronic system Combined fit to measurements of Mx2 for different Emin and to measurement of G(BX l n) 

5. The BABAR Detector

6. Y(4S) Identification of S.L. Decays • Problem 1: Two Bmesons decaying in the detector • which tracks belong to the semileptonically decayed B ? • Use events with fully reconstructed hadronic B decay: • look for decay modes using • Momentum and flavor of both B mesons determined

7. x 102All Events 2200 2000 1800 1600 1400 1200 1000 800 400 200 0 Events / (2.5 MeV/c2) 5.20 5.22 5.24 5.26 5.28 5.30 mES [GeV/c2] 1 lepton, pl > 1 GeV/c 12000 10000 8000 6000 4000 2000 0 Events / (2.5 MeV/c2) 5.20 5.22 5.24 5.26 5.28 5.30mES [GeV/c2] Identification of S.L. B decays • 4000 fully reconstructed B decays/fb-1 • purity 65% if lepton is present • Does the lepton originate from a semileptonic B decay ? • exclude s.l. D decays : • Is the XH - system complete, i.e. has no particle been missed ? • check missing 4-momentum • Pmiss must be compatible with neutrino hypothesis :

8.  Mx2  Analysis : Event Selection • 1 lepton with p*>0.9 GeV/c • Mx2 determined by 2C kinematic fit • energy conservation, p2miss = 0, mB high mass final states background 2250 2000 1750 1500 1250 1000 750 500 250 0 signal Mx2[GeV2/c4] Events / 1.8 MeV/c2 (p* >0.9 GeV/c) BABAR Y(4S) (p>0.5 GeV/c E>0.5 GeV) 5.22 5.23 5.24 5.25 5.26 5.27 5.28 5.29 mES[GeV/c2]

9. 12 11 10 9 8 7 6 5 4 3 verification withexcl. modes Mx2 [GeV2/c4] ~ calibration (D,D*, D**,D(*)p mix) 3 4 5 6 7 8 9 10 11 12 Mx2 [GeV2/c4] 4.5 4.4 4.3 4.2 4.1 4.0 Mx2 [GeV2/c4] 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 pmin[GeV/c] Determination of  Mx2  ~ • Measured Mx2 differs from true Mx2 • For each interval in Mx2, full detectorsimulation delivers Mx2 and Mx2 • calibration curve which is applied todata event-by-event • weak dependence on decay model • validated on data with decays • function of multiplicity and • Event selection efficiency • depends on decay mode • influence on Mx2 determined with MC,systematic error estimated by variationof individual exclusive decay modes ~

10.  Mx2  - Fits to BABAR Moments -0.1 -0.2 -0.3 -0.4 -0.5 • Extract OPE parameters from fit to  Mx2  vs. pl - curve • include correlations • based on work of Falk and Luke • Comparison with CLEO and DELPHI : • All hadronic moments consistent l1MS [GeV] 0 0.2 0.4 0.6 0.8

11. 44 43 42 41 40 39 38 |Vcb|[10-3] 4.4 4.6 4.8 5.0 mb1S [GeV/c2] Combined Fit to  Mx2  and GSL • Include measurement of inclusive semileptonic branching fraction into the fit • GSL = (4.37  0.18) x 10-11 MeV • Good agreement with hadronicmoments from CLEO, DELPHI • Moments of lepton spectrum • agreement on the 1s level when using CLEO/DELPHI lepton momentsinstead of BABAR hadron moments - expect higher precision after inclusion of BABAR measurements -

12. Measurement of Br(BXul n) • Use Mx spectrum of XH • good b c l n rejection • large acceptance (50- 80%) • Problems : • leading order dependence onshape function of Mx • low statistics of Breco sample • Additional event selection : • lepton with p* > 1 GeV/c • veto on K+, K0s anddecays • m2miss < 0.5 GeV2/c4 , Qtot = 0

13. Br(BXul n) : Signal Extraction • Extract number of signal events by fitting sum of 3 shapes to Mx distribution : • signal, b c l n background, other sources (<1%, misidentified leptons, t and secondary charm decays) • S / B comparable to excl. analyses : • B0 : S/B  3.6 , B+ : S/B 1.2

14. B Xul n: Results • Branching fraction : • Extraction of |Vub| : • formula from CKM 2002 workshop , tB=(1.608  0.016) ps: error due to extrapolation and efficiency error of transition G(BXu l n)  |Vub| (Uraltsev, Hoang)

15. Conclusions & Outlook • Precise measurements of and decays : • New model-independent determination of moments of hadronic mass distribution in semileptonic B decays • fully consistent with measurements from DELPHI and CLEO • BABAR measurement of moments of lepton energy spectrum in progress • measurements for different values of Emin, starting at 0.6 GeV • tighter constraints on OPE parameters and |Vcb|

16. Backup Slides

17. Systematic studies on <Mx2> • Cross-check on data : • sample ofdecays identified by reconstruction of slowpion from • analysis on this sampleyields D* - mass • Consistent measurements inindependent data sets • electrons / muons , years, B0 / B+ • variation of cut on • variation of Breco purity • add / remove decay channels

18. Fit to Moments