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Selected Charm Physics Results from BaBar

Selected Charm Physics Results from BaBar. D 0 mixing: wrong-sign decays D 0 ! K + p ¾ D 0 mixing: lifetime ratios 3-body D 0 decays Conclusions. D 0 mixing: wrong-sign decays - motivation. D 0 mixing small in Standard Model GIM suppression: Momentum transfer to light quarks:

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Selected Charm Physics Results from BaBar

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  1. Selected Charm Physics Results from BaBar D0 mixing: wrong-sign decays D0!K+p¾ D0 mixing: lifetime ratios 3-body D0 decays Conclusions

  2. D0 mixing: wrong-sign decays - motivation • D0 mixing small in Standard Model • GIM suppression: • Momentum transfer to light quarks: • b-quark contribution CKM suppressed: • SM estimates: • Observation of mixing, esp. jxj > jyj possible sign of new physics • Mixing with CP violation unambiguous sign of new physics

  3. D0 mixing: wrong-sign decays • Decay-time distribution of wrong-sign decays (jxj,jyj¿1, CP conservation assumed): • d:unknown strong phase difference between CF and DCS amplitudes • Fit parameters: where

  4. D0 mixing: wrong-sign decays • When searching for CP violation, we measure TWS(t) for D0 and samples separately • Using + and – to denote D0 and respectively, CP violation may be observed as • an asymmetry in the direct decay rates • an asymmetry in the mixing rates • or as a difference in the interference between the direct and mixing amplitudes (most likely sign of new physics) • Fit parameters: (sign of x0 is not determined)

  5. BaBar detector (EMC) (DIRC) (SVT) (DCH) (IFR)

  6. Characteristics of signal, background

  7. D0 mixing: wrong-sign decays Data samples: • Projections with fit. • Luminosity: 57 fb-1(75 million events) • »120,000 RS decays • »440 WS decays

  8. RS RS WS WS D0 mixing: wrong-sign decays Unbinned extended log-likelihood fit to data: • 4 Categories: • Fit right-sign and wrong-sign samples simultaneously in 4-d variable space: • The right sign sample fixes the D0 lifetime and resolution model parameters for unmixed decays • Deviations in the wrong-sign sample indicate mixing • Background lifetime distributions are determined from sidebands in data

  9. D0 mixing: Wrong-sign decays BaBar preliminary results, CP violation and mixing allowed: • samples fitted separately • Generate Toy MC experiments to evaluate confidence level limits • statistical (dashed) • Statistical+systematic (solid) • 95% CL limits are for the case where x02 is free

  10. D0 mixing: Wrong-sign decays BaBar preliminary results, mixing but no CP violation: • samples fitted together • Comparison with CLEO limits (statistical only)determined using delta log likelihood from fit • Preliminary BaBar results consistent with no mixing

  11. D0 mixing: Wrong-sign decays BaBar preliminary results, no mixing but direct CP violation allowed: • DCS decays only: exponential WS time distribution • Direct CP asymmetry: • Results consistent with no CP violation: • Comparison with other experiments:

  12. D0 mixing: Lifetime ratios • Measuring y: • D0!K-K+ and D0!p-p+ : CP=+ • D0!K-p+: mix of CP=+ and CP=- • All have nearly exponential lifetime distributions • The ratios of these lifetimes depend on y: • Advantage: many systematic uncertainties cancel in the ratio.

  13. Channel Events Signal purity 158,000 99.5% 16,500 97.1% 8,350 92.4% D0 mixing: Lifetime ratios • D0 data samples • From decays • Luminosity: 57 fb-1 • Channel yields and purity:

  14. D0 mixing: Lifetime ratios Decay time fits: Unbinned maximum likelihood PDF: • D0 signal: • exponential Resolution function • Backgrounds: • prompt and exponentialcomponents Resolution function • Tail component • Backgrounds separated from signalusing sidebands • Lifetime statistical error: st=1.3 fsec • PDG 2002 average: 411.7§ 2.7 fsec st=1.3 fsec

  15. D0 mixing: Lifetime ratios Decay time fits st=4.4 fsec st=6.5 fsec

  16. D0 mixing: Lifetime ratios BaBar preliminary results: • From ratio of (blinded) lifetimes: • Comparison: • Systematic errors:

  17. 3-body D0 decays Preliminary analysis of 3-body D0 decays based on 22 fb-1 data set • Ratio of branchingfractions: • Many systematics cancel in taking the ratio, making these measurements very precise • Amplitude analysis of Dalitz plots: • Most D0 three-body decays dominated by intermediate resonances • A systematic study of three-body D0 decays can shed new light on light meson spectroscopy, especially in the scalar meson sector • In this first analysis, include only known resonances • Inclusion of the charge conjugated states is always implied

  18. 3-body D0 decays BaBar preliminary reconstruction efficiencies and yields Efficiency over Dalitz plot determined from phase space MC: Yields determined from fits to D0 mass distributions:

  19. 3-body D0 decays BaBar preliminary ratios of branching fractions (from 22 fb-1 dataset):

  20. 3-body D0 decays • Dominated by K*+(892)K- • Other modes insignificant BaBar preliminary results:

  21. 3-body D0 decays • Several modes interfering: • Shoulder near threshold due to • Significant non-resonant term needed BaBar preliminary results:

  22. 3-body D0 decays • Strong f(1020) signal interfering with a threshold scalar in K+K- • also seen BaBar preliminary results:

  23. Conclusions • Time evolution study of wrong-sign and decays based on 57 fb-1 are consistent with no mixing and no CP violation • Assuming CP conservation, 95% CL limits: • Assuming no mixing, • From lifetime ratio analysis, measurement of the mixing parameter y based on 58 fb-1: • First Dalitz amplitude analyses of and updated branching ratios based on 22 fb-1 • More sophisticated analyses based on (1999-2002) 91 fb-1 dataset are in progress

  24. Backup slides

  25. Overview • Time evolution analysis of the wrong-sign decay : • Limits on mixing and CP-violating rates • Measurement of the doubly Cabibbo-suppressed decay, RD assuming no mixing • Measurements of mixing parameter y=(G1-G2)/(G1+G2) from lifetime ratios: • Preliminary analysis of 3-Body D0 decays: • Ratios of branching fractions: • Amplitude analysis of Dalitz plots

  26. Predicted SM and NP mixing rates • Predicted mixing amplitudes and rates forx(SM)y(SM)x(NP): • the x(SM),y(SM) points average aroundjxj, jyj¼ 10-3, but the range of values spansseveral decades • The x(NP) predictions appear to be somewhathigher, with several predictions above 10-2 • These are amplitudes; the SM rates are 10-6and below

  27. D0 mixing: wrong sign decays • Incorporate CP violation in (1) through the substitutions • where + (-) refers to D0 (D0). CP violation will be manifested as • in the direct decay rates • in the mixing rates • or through the interference between the two: • 6 fit Parameters:

  28. Mixing phase convention • flavor states ¹ mass eigenstates will mixwhere • Define and • Ambiguity: • Define the variable and restrict its range

  29. D0 mixing: wrong sign decays • Ambiguities: • equivalent to or the label change • To avoid this ambiguity, require: • Sign of x0§ not determined leads to a 4-fold ambiguity in determining (x0,y0): 1 4 3 2

  30. D0 mixing: wrong sign decays • Wrong sign decays: D0!K+p-(D0!K-p+) occur: • Via doubly Cabibbo-suppressed (DCS) diagram: • Or if D0 mixes to a D0: which then decays through a Cabibbo-favored (CF) diagram:

  31. D0 mixing: wrong sign decays Selecting charm decays: • Tag D0 flavor using • Cut on D* CM momentum to reject • Strict particle ID on the D0 decay products • from DIRC • dE/dx from tracking system • Average K, p PID efficiency: »85% • Average K (p) mis-ID rate: 3% (2%) • Improve resolution: vertexing, beam constraints • Track quality, K helicity Å, pt, t, stcuts • Define signal, sideband regionsin

  32. 95% confidence limits contours For each test point , generate an ensemble of toy MC experiments Each toy MC sample is generated from the PDF whose parameters other than are fixed to those from the fit to data Fit each toy MC sample in the parameter subspace Evaluate the log-likelihood difference The confidence limit is determined by counting the fraction of those MC experiments which are more consistent with the point than the data point is: D0 mixing: wrong sign decays fit to data

  33. D0 mixing: wrong sign decays • Systematic effects (ordered) • Event selection criteria • Fit model systematics • Fixing all but the lifetime parameters of the wrong sign signal • Detector systematics (alignment errors, charge asymmetry)

  34. D0 mixing: wrong sign decays • Including systematic effects in contours • Each systematic effect produces a small change in the fitted mixing parameters • Estimate the significance SI of the ith systematic uncertainty relative to the statistical error by using toy MC experiments to calculate the fraction of experiments consistent with the shifted parameters due to statistical fluctuations alone • Uniformly scale the statistics only contour by the scale factor with respect to the most likely fit point

  35. The Fit

  36. K0 K0 K0 u a) c) K+ p+ s s K0 d c s K- K- D0 D0 u u d) b) u K+ p+ K+ d s c d D0 D0 d p- p- u u

  37. 3-body D0 decays • Dalitz plot (Isobar) analysis: • Unbinned maximum likelihood fit • where • x = signal fraction • G(mD) is the reconstructed D0 mass distribution • h = acceptance • Ai = quasi two-body amplitudes • BWi: relativistic Breit Wigner with mass-dependent width • Ti(W): angular distribution described by Zemach tensors • : the complex coefficients being fitted

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