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New Results from Babar: Evidence for D 0 -D 0 Mixing In D 0 → K - p +

New Results from Babar: Evidence for D 0 -D 0 Mixing In D 0 → K - p +. Kevin Flood University of Wisconsin for The Babar Collaboration. Moriond EW March 13, 2007. Outline. Charm mixing formalism SM and New Physics predictions Mixing analysis methodology Results and systematics

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New Results from Babar: Evidence for D 0 -D 0 Mixing In D 0 → K - p +

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  1. New Results from Babar:Evidence for D0-D0 Mixing In D0→ K-p+ Kevin Flood University of Wisconsin for The Babar Collaboration Moriond EW March 13, 2007

  2. Outline • Charm mixing formalism • SM and New Physics predictions • Mixing analysis methodology • Results and systematics • Summary Kevin Flood Moriond EW March 13, 2007

  3. Time Dependence of Mixed Final States: No CP Violation • Mixing implies that the physical states are not pure flavor states , • Charm mixing values typically quoted using scaled parameters x, y • For |x|, |y| << 1, time-dependence of a hadronic final state with mixing and DCS amplitudes in the limit of no CP violation, and where , with dKp being the relative strong phase between DCS and mixing amplitudes • Time-integrated mixing rate Moriond EW March 13, 2007

  4. Time Dependence of Mixed Final States: CP Violation • If CP is not conserved, the time distribution for D0 and D0 differ • Define CP violating observables • Direct CP violation in DCS Decay • CP violation in mixing • CP violation in interference between decay and mixing: • Rewrite time dependence to explictly include asymmetries Moriond EW March 13, 2007 Kevin Flood

  5. Charm Mixing Predictions Standard Model New Physics • Box diagram SM charm mixing rate naively expected to be very low (RM~10-10) (Datta & Kumbhakar) • Z.Phys. C27, 515 (1985) • CKM suppression → |VubV*cb|2 • GIM suppression → (m2s-m2d)/m2W • Di-penguin mixing, RM~10-10(Petrov) • Phys. Rev. D 56, 1685 (1997) • Enhanced rate SM calculations generally due to long-distance y contributions: • Recent SM predictions can accom-modate high mixing rate (Falk et al.) • x,y≈ sin2 qC x [SU(3) breaking]2 ~1% • y: Phys.Rev. D 65, 054034 (2002) • x: Phys.Rev. D 69, 114021 (2004) • Possible enhancements to mixing due to new particles and interactions in new physics models • Most new physics predictions for x • Extended Higgs, tree-level FCNC • Fourth generation down-type quarks • Supersymmetry: gluinos, squarks • Lepto-quarks • Large possible SM contributions to mixing require observation of either a CP-violating signal or | x | >> | y | to establish presence of NP ~ g u u c c ~ H0 ~ q q ~ g u c u c supersymmetry FCNC Moriond EW March 13, 2007 Kevin Flood

  6. Babar Detector and Dataset • Babar integrated luminosity ~390 fb-1 (Runs 1-5) • Peak instantaneous luminosity ~1.2 x 1034 cm-1 s-1 Run 5 Runs 1-4 Babar Detector Kevin Flood Moriond EW March 13, 2007

  7. Mixing Analysis Strategy • Blind analysis of D*+→ D0(→Kp) p+tag • All event selection, fitting methodology determined before looking at the data • Four-dimensional unbinned maximum LH fit • First fit M(Kp), DM = M(Kpptag) – M(Kp) [correlated fit] • Fix results of first fit, then fit decay time and errors • High-statistics RS dataset gives WS signal PDFs • No MC dependence, all PDFs obtained from data • Fit WS proper time distribution to distinguish DCS and mixing contributions • Use M(Kp) and DM to separate backgrounds from signal • Several WS proper time fits • no mixing • mixing, no CP violation • mixing, CP violation Kevin Flood Moriond EW March 13, 2007

  8. Event Selection • Beam-constrained simul-taneous fit of K, p, ptag tracks • fit probability > 0.001 • decay time error < 0.5 ps • -2 < decay time < 4 ps • D0 selection • CMS p* > 2.5 GeV/c • K, p particle identification • DCH hits > 12 • 1.81 < M(Kp) < 1.92 GeV/c2 • ptag • CMS p* < 0.45 GeV/c • lab p > 0.1 GeV/c • SVT hits > 5 y beam spot interaction point x • 0.14 < DM < 0.16 GeV/c2 • Select candidate with greatest fit probability for multiple D*+ candidates sharing tracks Kevin Flood Moriond EW March 13, 2007

  9. RS/WS Datasets After Event Selection x103 1,229,000 RS candidates events/1 MeV/c2 events/0.1 MeV/c2 64,000 WS candidates Kevin Flood Moriond EW March 13, 2007

  10. RS/WS M(Kp), DM Distributions Correlation between m and m in signal events taken into account in PDF Kevin Flood Moriond EW March 13, 2007

  11. Signal and Background Kinematic Fit Categories • Fit RS/WS M(Kp), DM distributions with signal and three background PDFs • RS categories • Signal: peaks in M(Kp), DM • True D0 combined with random ptag: peaks in M(Kp) • Mis-reconstructed D0: peaks in DM • Semileptonic D0 decays; singly mis-identified D0→ p+p-, K+K- • Purely combinatoric, non-peaking • WS categories • Signal: peaks in M(Kp), DM • True D0 combined with random ptag: peaks in M(Kp) • Mis-reconstructed D0: peaks in DM • Doubly mis-identified D0→ K-p+ • Singly mis-identified D0→ p+p-, K+K- • Purely combinatoric, non-peaking Kevin Flood Moriond EW March 13, 2007

  12. Simultaneous Fit to RS/WS Data RS signal: 1,141,500±1200 combinations WS signal: 4030±90 combinations Kevin Flood Moriond EW March 13, 2007

  13. Proper Time Analysis • Fix M(Kp) and DM PDF shapes • Fit RS decay time, error distribution to determine signal lifetime and resolution model • Signal, background D0 PDF: exponential with sum of three gaussians resolution model fit using per-event lifetime errors • Random combinatoric PDF: gaussian + Crystal Ball function • Fix WS resolution and DCS lifetime from RS fit • Signal PDF: theoretical mixed lifetime convoluted with resolution model from RS fit • Background D0 PDF: shares RS PDF • Random combinatoric PDF: gaussian + Crystal Ball function separate from RS fit Kevin Flood Moriond EW March 13, 2007

  14. RS Decay Time Fit • Varied fit parameters • Fit class normalizations • D0 lifetime • Resolution model • Combinatoric shape • D0 lifetime is con-sistent within total (stat+sys) error with PDG value plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2 Kevin Flood Moriond EW March 13, 2007

  15. WS Mixing Fit: No CP Violation • Varied fit parameters • Mixing parameters • Fit class normalizations • Combinatoric shape Mixing – No mixing PDF Data – No mixing PDF plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2 Kevin Flood Moriond EW March 13, 2007

  16. Systematics • Sources • Variations in functional form of signal and background PDFs • Variations in the fit parameters • Variations in the event selection • Single parameter systematic estimates from difference between parameter value from fits with and without variation, expressed in units of the statistical error Kevin Flood Moriond EW March 13, 2007

  17. best fit best fit, x’2 ≥ 0 X (0,0) Mixing Contours: No CP Violation • y’, x’2 contours computed by change in log likelihood • Best-fit point is in non-physical region x’2 < 0, but one-sigma contour is in physical region • correlation: -0.94 • Accounting for sys-tematic errors, the no-mixing point is at the 4-sigma contour RD: (3.030.160.06) x 10-3 x’2: (-0.220.300.20) x 10-3 y’: (9.74.42.9) x 10-3 Kevin Flood Moriond EW March 13, 2007

  18. Mixing Contours: CP Violation • Fit D0 and D0 samples for mixing separately x'2– = -0.020+-0.041% y’– = 0.963+0.614% x'2+ = -0.024+-0.043% y'+ = 0.982+0.637% No evidence for CP violation Kevin Flood Moriond EW March 13, 2007

  19. M(Kp), DM Fits in Decay Time Bins • Kinematic fit done independently in five decay time bins • RWS independent of any assumptions on resolution model Kevin Flood Moriond EW March 13, 2007

  20. Validation: RS Mixing Fit • Perform mixing fit to RS data • No mixing signal expected • y’ = (2.6 ± 2.4) x 10-4 • x’2 = (9.2 ± 10.6) x 10-6 • No mixing signal found • -2 (logLHmix – logLHno-mix) = 1.4 Kevin Flood Moriond EW March 13, 2007

  21. Summary • Assuming CP conservation and including systematic effects, we find a charm mixing signal at the 4 sigma confidence level • y’ = (9.7 ± 4.4 ± 2.7) x 10-3 • x’2 = (-0.22 ± 0.30 ± 0.20) x 10-3 • No evidence for CP violation • Results consistent with previous analyses • Babar Kp, 2003: (-56 < y’ < 39) x 10-3 , x’ < 11 x 10-3 (95% CL) • Belle Kp, 2006: (-28 < y’ < 21) x 10-3 , x’ < 3.6 x 10-3 (95% CL) • Assuming dKp ~ 0, can also compare with Babar and Belle measurements of y using decays to two-body CP eigenstates • Belle, 2003: y = (11.5 ± 6.9 ± 3.8) x 10-3 • Babar, 2003: y = (9 ± 4 ± 5) x 10-3 • New Babar charm mixing result in semileptonic mixing will be discussed in charm mixing review by Marco Staric • complementary technique to Kp, but not as sensitive and no significant signal observed Kevin Flood Moriond EW March 13, 2007

  22. Additional Slides Kevin Flood Moriond EW March 13, 2007

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