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B d Mixing Measurements with the BaBar Detector

This study presents results on B0-B0 mixing and CP violation, utilizing data from the BaBar detector at the asymmetric B factory. Critical measurements include the mass difference between Hamiltonian eigenstates and the determination of the CKM matrix element Vtd. The analysis encompasses fully reconstructed B0 decays, hadronic events, and dilepton events, leading to precise values for the mixing frequency (Δmd) and B0 lifetime (τB0). The findings contribute to unraveling the dynamics of B meson behavior and testing the principles of CPT invariance in particle physics.

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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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