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S. Bolognesi & M.A. Borgia for the CP-violation exam

A measurement of the B 0 B 0 oscillation frequency and determination of flavor-tagging efficiency using semileptonic and hadronic B 0 decays. S. Bolognesi & M.A. Borgia for the CP-violation exam. Introduction. The strategy The experimental environment. S. Bolognesi & M.A. Borgia.

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S. Bolognesi & M.A. Borgia for the CP-violation exam

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  1. A measurement of the B0B0 oscillation frequency and determination of flavor-tagging efficiency using semileptonic and hadronic B0 decays S. Bolognesi & M.A. Borgia for the CP-violation exam

  2. Introduction The strategy The experimental environment S. Bolognesi & M.A. Borgia CP-violation exam

  3. B0B0 mixing through NLO aEW diagrams involving exchange of up-type quarks if perfect flavour tagging ≈ “dilution factor” due to mistag rate (w) distance between Brec and Btag decay (≈ tB = 1.548±0.032 ps) time resolution function with parameters The measurement • top contribute is dominant sensible to Vtd element of CKM matrix • one B reconstructed in a flavour eigenstate (Brec) one B only tagged as B0 or B0 from its decay products (Btag) • mixed if same flavor /unmixedif opposite flavor • PDF for the two categories (mixed + / unmixed -) 3 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  4. Likelihood and time independent analysis • Likelihood = sum over all events (mix. & unmix.) and over different tag types (with its own Di) minimized to extract simultaneously Dmd, Di (and some ai) • Time-independent analysis = neglecting background and assuming Brec correct identification, the observed time-integrated fraction of mixed events χobs as a function of B0B0 mixing probability χd can be expressed as: where ωis the mistag rate, and χ d = ½ xd2/(1+xd2) = 0.174 ± 0.009 and xd2=Δmd/Γ 4 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  5. BaBar detector • DCH + SVT 8.9 fb-1 @ Y(4s) + detection and momentum measurement for charged particles 0.8 fb-1 @ 40 MeV below Y(4s) (10.1 ± 0.4) × 106 BB pairs • DCH particle identification (dE/dx) • SVT vertex information Dz ≈ 50 mm for Brec Dz ≈ 100-150 mm for Btag • DIRC particle identification (charged hadrons) • EMC photons, electrons and neutral hadrons • IFR (RPC) muons and neutral hadrons 5 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  6. Particle identification • Electrons • track + EMC (shower shape, E/p) efficiency 92% • dE/dx in DCH mistag (p) 0.3% • Cherenkov angle in DIRC • Muons efficiency 75% • interaction lenghts and # hits in IFR mistag (p) 2.5% • MIP in EMC • Kaons where • Cherencov angle dE/dx • # photons efficiency 85% mistag (p) 5% 6 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  7. Brec Dz Beam Spot pseudo track (Btag) Time resolution s(Dt) dominated by s(Dz) s(Dz)dominated by s(DzBtag) Dz ≈ 260 mm s(Dz) ≈ 180 mm • reconstruct Brec • compute the Btag direction from the energy conservation pseudo-track extrapolated from the interaction point in the Btag direction • Btag vertex = intersection of pseudo-track with all the other tracks • Resolution function is the sum of three gaussians (3 parameters from MC 3 parameters from fit) 7 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  8. Flavor tagging 4 tagging cathegories • 4 strategies to define if Btag is B0 or B0 (pCM>1.1 GeV against charm semileptonic decay) • Lepton tag: presence of a prompt lepton • Kaon tag: total kaons charge not 0 • 2 neural network cathegories: 5 neural network algorithm 4 based on tracks 1 exploits the charge of high momentum particles whose outputs are combained in a single full neural network tagger (xNT [-1,1]) NT1 NT2 8 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  9. Analysis Hadronic decay channels Leptonic decay channels S. Bolognesi & M.A. Borgia CP-violation exam

  10. B0rec D*-p+ ( / r+ / a1+ ) D0p- K+ p- K+ p- p0 K+ p+p- p- K0sp+p- B0rec D-p+ ( / r+ / a1+ ) K+ p- p- K0sp- B0rec J/y K*0 • Cuts against continuum (e+e-→ qq) e+ e- / m+ m- Hadronic decays • Usual cutson intermediate/final particles: • resonances invariant mass (±2s) • vertex c2 • threshold on momenta • opening angle between decay products • B0 candidates characterized by • beam energy sobstituted mass: 5.270 < mES < 5.290 (GeV) • EB0 – Ebeam in Y(4s) CM: |DE| < 3sDE where sDE = E resolution (19 -40 MeV) • normalized second Fox-Wolfram moment (R2=H2/H0) < 0.5 • large angle between thrust axis of B0 and of the remaining tracks (K0s→ p+ p-, p0→ gg) 10 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  11. Backgrounds (HD*) Data extracted from fit to the mES distrbution * HD = Hadronic decays 11 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  12. Usual cutson intermediate/final particles: • invariant mass (±2s) around nominal D0 mass • vertex c2 > 1% • threshold on momenta • D0 candidates: • combination with all charged tracks (pTmin 50 MeV/c and charge opposite to that of the candidate K) => D* candidates • Mass difference: • m(D*-)-m(D0) (± 2.5σ) of the nominal value Semileptonic decays • D* candidates: • D*- , pl > 1.2 GeV back-to-back => cosθ(D*- ) < 0 • Neutrino existence consistency: (solving in the Υ(4s) system frame) 12 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  13. After cuts, 7517±104 B→D*lν events • 3101±64 in the mode • 1986±51 in the mode • 2430±56 in the mode Sample composition • Mass difference distributions for each flavor tagging category Backgrounds are larger for semileptonic modes than for hadronic modes 13 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  14. Background • Three types of background to B0→D*lν: • Combinatorial background • Wrong-lepton background • B+ background • Combinatorial • Due to falsely reconstructed D* candidates • Estimated by fitting Δm(D*-D0) distributions • Gaussian + threshold function with a sharp rise followed by exponential tailoff • Signal region within ±2.5σ of the peak in Δm(D*-D0) • Combinatorial background control sample provided by the sidebands region • 0.150 < Δm(D*-D0) < 0.160 GeV/c2 14 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  15. Wrong-lepton • Wrong-lepton:D* combination with wrong lepton Four potential sources: • ”Fake lepton” (estimated selecting events in which a track candidate has failed very loose lepton criteria is substituted for the lepton candidate) • Real D* from one B + real lepton from the other B (“uncorrelated lepton” bg) (estimated by parity-inversion of the lepton momentum in the Y(4s) frame => control sample) • Events of the type B0→D*DX in which the D decays semi-leptonically produce a non-primary lepton(estimated with Monte Carlo, less than 1% => neglected) • ccevents producing real D* and lepton in back-to-back configuration. (estimated using combinatorial-subtracted off-resonance data) 15 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  16. B+ background Due to B-decays which involve additional final state particles (B→D*(nπ)lν) • B0→D*(nπ)lνthat pass selection criteria are considered as signal (they contribute to the measurement of Δmd and the additional low momentum π does not affect the tagging algorithm) • B-→D*+(nπ)l-νconsidered as bg: they do not oscillate and must be corrected for in extracting Δmd and their mistag rate may differ from that of B0 decays as well 16 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  17. Results Likelihood fit results Time integrated method results Combined results S. Bolognesi & M.A. Borgia CP-violation exam

  18. Backgr. treatment (LM*) • PDF must be extended withbackground contributions (where b = background sources i = tagging cathegories) • f = fraction of signal or background events • B =empirical description of Dt distribution in background events • Fit to the background control samples (mES sidebands) to determine time dependence, dilution factor, resolution function: three components • zero lifetime: • non zero lifetime, no mixing: • non zero lifetime with mixing: for each background source * LM = Likelihood method 18 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  19. Dt distribution (LM*) HADRONIC SAMPLE LEPTONIC SAMPLE * LM = Likelihood method 19 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  20. Time dependent asymmetry a(Dt) (LM*) HADRONIC SAMPLE LEPTONIC SAMPLE * LM = Likelihood method 20 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  21. Fit results (LM*) • Identical analysis procedure on MC data with detailed detector simulation: • fit results consistent with a priori insterted value and MC truth information • observed differences applied as a correction to the measured values * LM = Likelihood method 21 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  22. Systematic errors (LM*) HADRONIC SAMPLE • Statistical error dominant, followed by MC correction uncertainties (Dt for Dmd) LEPTONIC SAMPLE • Systematic error dominant due to big uncertanties in background characteristic (Dt for Dmd) * LM = Likelihood method 22 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  23. Time integrate (single bin) method • First aim: measurement of themistag rate • Main feature: restriction of the sample to events in a single optimized Δt interval (| Δt | < 2.5 ps because of Babar vertex resolution) Events with | Δt | > 2.5 ps have on average equal numbers of mixed and unmixed events => contribute nothing to the determination of the mistag rate Considering the different background contribution: => fs, fβ = fraction of signal and background source χβ = fraction of mixed events in each background source χobs = Observed fraction of mixed events χ d = ½ xd2/(1+xd2) and xd2=Δmd/Γ, while χ’d takes into account the sample restriction 23 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  24. Results • Hadronic Signal region defined as events with mES > 5.27 GeV/c2 Fraction of mixed events in the background determined by tag category using the sideband control sample, mES< 5.27 GeV/c2 • Semileptonic - bg evaluated for each tag category and for each D0 decay - mistag fractions calculated individually by tag category and decay mode using the Eq. shown - combination of the different decay modes, using the statistical errors to weight the individual results 24 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  25. Systematic errors Sources of systematic error for the mistag measurement on the hadronic and semileptonic samples • Hadronic • Semileptonic 25 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  26. Comparison between the two methods Combining results for the hadronic and semileptonic B samples for the likelihood fit method and for the single-bin method and taking into account the systematic errors Preliminary mistag rate Single bin fit uses a subset of the sample used for the other method The two sets of results are uncorrelated Good agreement between the two methods. Final result: Q ≈ 0.28 26 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  27. Final result • Hadronic sample: • Leptonic sample: 27 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

  28. Back-up slides S. Bolognesi & M.A. Borgia CP-violation exam

  29. Fox-Wolfram moments The Fox-Wolfram moments   ,                , are defined by    is the opening angle between hadrons and      the total visible energy of the event        are the Legendre polynomials To the extent that particle masses may be neglected,         . It is customary to normalize the results to    , i.e. to give                . 2-jet events tend to give          for even and      for odd. 29 S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

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