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B d Mixing Measurements with the BaBar Detector

B d Mixing Measurements with the BaBar Detector. Cecilia Voena Università “La Sapienza” & INFN Roma. on behalf of the BABAR Collaboration. ICHEP2002. B 0 B 0 Mixing. V * td. b. t. d. -Proceeds through box diagram. B 0. B 0. W. W. B 0. d. t. b.

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B d Mixing Measurements with the BaBar Detector

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  1. Bd Mixing Measurements with the BaBar Detector Cecilia Voena Università “La Sapienza” & INFN Roma on behalf of the BABAR Collaboration ICHEP2002

  2. B0B0 Mixing V*td b t d -Proceeds through box diagram B0 B0 W W B0 d t b if q/p  1 => CP violation in mixing (assuming CPT invariance ) md constraint in (,) plane Mass difference between Hamiltonian eigenstates F.Parodi - mdsensitive to Vtd CKM matrix element - Extraction of Vtd limited by theoretical hadronic uncertainties 23533+00-23 MeV (D. Becirevic and L. Lellouch, at this conference)

  3. B0B0 Mixing at Asymmetric B Factories Before B factories: md =0.472 ± 0.017 ps-1 -can perform time dependent measurements At BaBar, asymmetricB factory, we measure: -high statistics sample of fully reconstructed Bd,no Bs new result -Mixing and lifetime with B0D*-l+l decays -Mixing with hadronic decays (PRL 88:221802[2002]) -Mixing with dilepton events (PRL 88:221803[2002]) 23 x 106 BB (B0D*-l+ and dilepton) 32 x 106 BB (hadronic)

  4. B0B0 Mixing with Fully Reconstructed B0 p- K+ K K+ Tag B sz ~ 160 mm + Reco B sz ~ 80 mm - e- e+ D0 - (4s) l+ Dz bg = 0.55 <|Dz|> @ 250 m 3. Reconstruct inclusively the vertex of the “other” B meson (BTAG) 4. Determine the flavor of BTAG 1. Reconstruct one B meson in the flavor eigenstates D*-l+l or hadronic (e.g.D*-+ ) 2. Reconstruct the vertex position 5. Compute the proper time difference Dt @Dz/gbc 6. Fit the Dt spectra of mixed (B0B0 or B0B0) and unmixed (B0B0)events

  5. t Distribution of Mixed andUnmixed Events realistic mis-tagging & finite time resolution perfect flavor tagging & time resolution B0B0 B0B0 ,B0B0 Decay time distribution for signal events + - B0 lifetime oscillation frequency Mistag probability

  6. B0D*-l+ Selection • Reconstruct candidate D* using full decay tree D*+ D0+,D0K-+,K-+0,K-+-+,Ks+- • Combine with lepton candidate (p* > 1.2 GeV) • Require consistency with D*-l kinematics • (angles, missing ) signal • Cannot reconstruct B0 mass and • energy because of the missing  • => use m(D*)-m(D0) distribution background

  7. Sample Composition Peaking background: real D*non from semileptonic decays signal  sample -uncorrelated lepton from other B -fake lepton -continuum events Combinatoric: fake D* m(D*)-m(D0) (MeV) • Select several control samples to characterize (fraction,t shape) • the main sources of backgrounds

  8. Lepton Tag l- n W- c b D, D* B0 d B0 d d d Flavor Tagging Hierarchical, Mutually Exclusive, Tagging Categories For electrons, muons and Kaons use the charge correlation W+ W- b c s K* 0  K- + Kaon Tag Neural network exploits information carried by non-identified leptons and kaons, soft pions from D* decays

  9. B0D*-l+Mixing Likelihood Fit Results • Simultaneous fit to all signal and background samples float m, B0, mistag probabilities, signal and background t resolution parameters, fraction of charged B Preliminary - Dmd=0.492±0.018±0.013ps-1 + B0 =1.523+0.024-0.023±0.022ps correlation coefficient (m, B0) = -0.22 Floating parameters: Innermost m, B0 m, B0, mistag m, B0, fB+ m, B0, fB+,mistag All signal t par Default fit error ellipse Mixing asymmetry -6B0 6B0

  10. Dmd=0.516±0.016±0.010 ps-1 Mixing with Hadronic Events • Fully reconstructed B0D(*)-+/+/a1+,J/K*0 • All Dt and mistag parameters simultaneously • extracted from data 6347 B0 (purity ~86%) used for sin2 measurement • Largest syst. is B0 lifetime • lifetime measurements • with hadronic events 1-2w PRL 87 [2001] t0 = 1.546  0.032  0.022 ps t = 1.673  0.032  0.023 ps t/t0 = 1.082  0.026  0.012 /2md PRL 88[2002]

  11. Mixing with Inclusive Dilepton Events Very precise mixing measurement • Select events with 2 high momentum leptons (sample contains ~50% B+) • Largest syst. is B0 lifetime and resolution function parameterization Dmd= 0.493±0.012±0.009 ps-1 99010 dileptons (B0B0 purity ~40%) PRL 88[2002] with the same sample: T and CP violation in mixing |q/p|=0.998±0.006±0.007 PRL 88[2002]

  12. B0 Lifetime with Partially Reconstructed B0D*-l+ reconstruct lepton and only Kinematic constraints at the (4S): in a small cone around D* direction,  and  from simulation -pD* = p+ missing neutrino -M2= (pB0- pD* - pl)20 B0 = 1.529 ±0.012 ±0.029 ps PRL 89[2002]

  13. Summary • New simultaneous measurement of Dmd andB0with the exclusive B0D*-l+l sample Preliminary Dmd= 0.492±0.018±0.013 ps-1 B0 = 1.523+0.024-0.023±0.022 ps • Combined BaBar result for • Bd mixing frequency: Dmd= 0.500±0.008±0.006 ps-1 2% error

  14. Backup slide Signal t Model

  15. Backup slide Background t Model

  16. Backup slide Systematic error

  17. Backup slide Systematic error

  18. Backup slide Mistag fraction determined from simultaneous fit to Bflav sample

  19. “l not from Brec” Enriched : D* with flipped l Peaking Bkgs Fake l Enriched : l candidate fails e,m ID Continuum D* Enriched : Off-resonance Data Combinatoric Enriched : Sideband Backup slide Signal m Sample

  20. Backup slide

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