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Recent Results on D 0 - D 0 Mixing from BaBar

Recent Results on D 0 - D 0 Mixing from BaBar. William Lockman for the BaBar Collaboration Lepton-Photon 2007, Daegu, S. Korea. Topics from BaBar Experiment. Introduction D 0 -D 0 Mixing in Lifetime Ratio of D 0 K + K  ,  +   vs D 0 K   +

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Recent Results on D 0 - D 0 Mixing from BaBar

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  1. Recent Results on D0-D0 Mixing from BaBar William Lockman for the BaBar Collaboration Lepton-Photon 2007, Daegu, S. Korea

  2. Topics from BaBar Experiment • Introduction • D0-D0Mixing in Lifetime Ratio of D0K+K, +vsD0K+ • Search for CP Violation in D0K+Kand D0 to be submitted to PRL • D0-D0 Mixing in the Decay D0K • Conclusion LP07 - William Lockman

  3. Charm meson mixing Why would observation of charm mixing be interesting? It would complete the picture of quark mixing already seen in the K, B, and Bs systems. K — 1956 Bd— 1987 Bs — 2006 It would provide new information about processes with down-type quarks in the mixing loop diagram. It would be a significant step toward observation of CP violation in the charm sector. It could indicate new physics. LP07 - William Lockman

  4. BELLE PRL 98, 211803 3.2σ signal Current Evidence for D0-D0 mixing D0K+- Combined PRL 98,211802 5.7σ signal 3.9σ signal BELLE y (%) 2.4σ signal arXiv:0704.1000 D0KK, - D0Ks- x (%) LP07 - William Lockman

  5. Flavor States Mixing • Flavor eigenstates can mix through weak interaction: • Mass eigenstates: • Flavor state time evolution: • Mixing if either or nonzero LP07 - William Lockman

  6. A. Petrov, HEP-PH/0611361 Standard Model Predictions • Short-distance contributions from mixing box diagrams in the Standard Model are expected to be small : • b quark is CKM-suppressed • s and d quarks are GIM suppressed • mainly contributes to the mass difference x≈ O(10-5) or less • Long-distance contributions dominate but hard to estimate precisely • expect |y| ≤ 0.01 • |x| ~ 0.1 - 1|y| LP07 - William Lockman

  7. BABAR Charm Factory: 1.3 million Charm events per fb-1 Integrated luminosity ~384 fb-1 used for mixing results presented here ~500M cc events BaBar is a large acceptance general purpose detector providing excellent tracking, vertexing, particle ID and neutrals detection LP07 - William Lockman

  8. BaBar Generic Mixing Analysis Identify the D0 flavor at production using the decays • select events around the expected • The charge of the soft pion determines the flavor of the D0 Identify the D0 flavor at decay using the charge of the Kaon Vertexing with beam spot constraint determines decay time, and decay time error, D0 decay vertex Beam spot: x¼ 100 m, y¼ 6 m right-sign (RS) wrong-sign (WS) D0 production vertex LP07 - William Lockman

  9. D0-D0Mixing in Lifetime Ratio of D0K+K, +vsD0K+ D0K: CP-mixed D0(t) KK, : CPeven Determine the quantities If CP is conserved in mixing and decay, but violated in the interference between them, these quantities are related to the mixing parameters CPV in interference of mixing and decay: LP07 - William Lockman

  10. BaBar, PRL 91, 162001(2002) 91 fb-1 Previous lifetime ratio results BELLE, PRL 98, 211803 (2007) 540 fb-1 LP07 - William Lockman

  11. sideband D0K signal box BaBar (yCP, Y) analysis overview • Select D*D0D0KKKdecays from 384 fb-1 • Event selection chosen to minimize backgrounds • systematics affecting signal mostly cancel • background systematics don’t cancel between modes • Unbinned likelihood fit to (t,t) to obtain hh • signal resolution determined from fitting data • Backgrounds taken from MC and sidebands • Determine ycp and Y from lifetimes LP07 - William Lockman

  12. Decay time fits to determine (yCP, Y) =409.3±0.7 fs =401.3±2.5 fs =404.5±2.5 fs =407.6±3.7 fs =407.3±3.8 fs K and KK lifetimes differ! LP07 - William Lockman

  13.  BaBar (yCP, Y) results Tagged results from 384 fb-1: Result in good agreement with BELLE measurement LP07 - William Lockman

  14. BaBar (yCP, Y) systematics Systematic uncertainties (%): Variations: • Signal: PDF shape, polar angle dependent resolution offset, signal interval • Charm backgrounds: yields and charm lifetime • Combinatorial backgrounds: yields, shape and sideband region • Selection: t criterion, treatment of multiple candidates • Detector: Alignment and energy loss LP07 - William Lockman

  15. e.g., D0 → K+K-: u W+ K+ c s D0 u s K- u u W+ K+ c s s D0 u s K- Only SCS decays probe penguins u Search for CPV in D0 KK,  Two amplitudes with different strong & weak phases needed to observe CPV (in SM from tree and penguins) 2 weak amplitudes with phase difference strong phase difference Standard model predictions for direct CPV asymmetries in these modes: O(0.001% - 0.01%) F. Bucella et al., Phys. Rev. D51, 3478 (1995) S. Bianco et al., Riv. Nuovo Cim. 26N7, 1(2003) LP07 - William Lockman

  16. Search for CPV in D0 KK,  Measure the time integrated CP asymmetries Experimental procedure: • fit m,m distributions to determine raw signal weights • Determine relative D0/D0 soft pion tagging efficiency using D0K data • greatly reduces systematic uncertainties • correct for forward-backward asymmetries in eecc production • extract aCP LP07 - William Lockman

  17. Search for CPV in D0 KK,   KK No evidence for CPV in either mode LP07 - William Lockman

  18. DCS Interference Mixing Mixing in D0K Time dependent WS rate : where and • Two types of WS Decays: • Doubly Cabbibo-supressed (DCS) • Mixing followed by Cabibbo-Favored (CF) decay Two ways to reach same final state  interference! mix K : strong phase difference between CF and DCS decay amplitudes LP07 - William Lockman

  19. signal mis-tagged D0 mis-reconstructed D0 combinatoric signal box yields: RS and WS (mK, m) fits Determine signal and background yields in subsequent Dalitz analyses. signal and sideband regions m m m m LP07 - William Lockman

  20. D0K RS Dalitz fit Time-integrated analysis to determine CF amplitudes, LP07 - William Lockman

  21. signal mis-tagged D0 mis-reconstructed D0 +combinatoric D0(t)K WS Dalitz fit results Through t-dependence, distinguish DCS amplitudes from the CF amplitudes arising from mixing. LP07 - William Lockman

  22. y’’ stat.+syst. 68% 95% 99% 99.9% x’’ Mixing parameter contours and results + no-mix x best fit Results are consistent with no mixing at 0.8%, including systematics LP07 - William Lockman

  23. BaBar D0-D0 Mixing Summary Presented more evidence for D0-D0 mixing from BaBar experiment: • D0 Kto D0 KK, lifetimes: • D0K time-dependent Dalitz analysis: In D0 KK, decays, • no evidence for CP violation • no evidence for CP violation in mixing: No mixing excluded at  LP07 - William Lockman

  24. Backup Slides LP07 - William Lockman

  25. Kbackup LP07 - William Lockman

  26. Time evolution of WS D0Kdecays • Two types of WS Decays: • Doubly Cabbibo-supressed (DCS) • Mixing followed by Cabibbo-Favored (CF) decay Two ways to reach same final state interference! mix Discriminate between DCS and Mixing decays by their proper time evolution (assuming CP-conservation and|x|«1, |y|«1) : DCS decay Mixing Interference between DCS and mixing K : strong phase difference between CF and DCS decay amplitudes LP07 - William Lockman

  27. D0KFit Procedure Unbinned maximum likelihood fit performed in stages Fit m(K) and m distribution: Separate signal from background in subsequent decay time fits Fit RS decay time distribution: Determine D0 lifetime and decay time resolution function R(t) Fit WS decay time distribution: Use D0 lifetime and decay time resolution function from RS fit Fit WS signal to Compare fits with and without mixing to determine significance Fit D0 and D0 samples separately to search for CP violation In this analysis, all parameters are determined by fitting data, not MC LP07 - William Lockman

  28. RS and WSmK ,mDistributions Separate signal from background by fitting over the full range shown in the plots 1.81 GeV/c2 < mK < 1.92 GeV/c2and 0.14 GeV/c2 < m < 0.16 GeV/c2 For displaying decay time fits, integrate over a signal box 1.843 GeV/c2 < mK < 1.883 GeV/c2 and 0.1445 GeV/c2 < m < 0.1465 GeV/c2 Selected RS data Selected WS data LP07 - William Lockman

  29. RS Proper Time Fit D0 lifetime and resolution functionfitted in RS sample Consistent with PDG Systematics dominated by signal resolution function LP07 - William Lockman

  30. Δm - m(Kπ) Fit Results LP07 - William Lockman

  31. Wrong-sign mK , mfit The mK , mfit determines the WS branching ratio RWS 4,030 § 90 WS signal events BABAR (384 fb-1): RWS = (0.353 § 0.008 § 0.004)% (PRL 98,211802 (2007)) BELLE (400 fb-1): RWS = (0.377 § 0.008 § 0.005)% (PRL 96, 151801 (2006)) LP07 - William Lockman

  32. data - no mix PDF mix - no mix PDF WS Fit with Mixing • Fit results allowing mixing: RD: (3.03±0.16±0.10)x10-3 x’2: (-0.22±0.30±0.21)x10-3 y’: (9.7±4.4±3.1)x10-3 WS mixing fit projection in signal region 1.843 GeV/c2 < m < 1.883 GeV/c2 0.1445 GeV/c2 < m < 0.1465 GeV/c2 LP07 - William Lockman

  33. Evidence for D0-D0 mixing! Mixing contours • Fit D0 and D0 samples together assuming no CP violation • y, x’2 contours computed bychange in log likelihood • Best fit point in non-physical region •  contour extends into physical region • correlation: -0.95 • Accounting for systematicerrors, no-mixing point is atthe 3.9 contour Best fit Best fit, x’2 ≥ 0 + No mixing: (0,0) 1 – CL = 3.17 x 10-1 (1s) 4.55 x 10-2 (2s) 2.70 x 10-3 (3s) 6.33 x 10-5 (4s) 5.73 x 10-7 (5s) RD: (3.03  0.16  0.10) x 10-3 x’2: (-0.22  0.30  0.21) x 10-3 y’: (9.7  4.4  3.1) x 10-3 LP07 - William Lockman

  34. Allowing for CP Violation Fit D0 (+) and D0 (-) samples separately CP violation if any (+) parameter differs from corresponding (-) x’+2: (-0.24±0.43±0.30)x10-3 y’+: (9.8±6.4±4.5)x10-3 RD=(0.303±0.016±0.010)% AD=(-2.1±5.2±1.5)% x’-2: (-0.20±0.41±0.29)x10-3 y’-: (9.6±6.1±4.3)x10-3 No evidence for CP violation LP07 - William Lockman

  35. BABAR 1 BABAR 2 BABAR3 BaBaR/BELLED0!K comparison Results consistent within 2 PRL 96,151801 400 fb-1 stat. only no-mixing excluded at 2s BELLE 2 statistical LP07 - William Lockman

  36. Systematics: variations in Functional forms of PDFs Fit parameters Event selection Computed usingfull difference with original value Results are expressed in units of the statistical error Systematics, Validations Validations and cross-checks Alternate fit (RWS in time bins) Fit RS data for mixing x’2 = (−0.01±0.01)x10-3 y’ = (0.26±0.24)x10-3 Fit generic MC for mixing x’2 = (−0.02±0.18)x10-3 y’ = (2.2±3.0)x10-3 Fit toy MCs generated with various values of mixing Reproduces generated values Validation of proper frequentist coverage in contour construction Uses 100,000 MC toy simulations LP07 - William Lockman

  37. Lifetime ratio backup LP07 - William Lockman

  38. D0(t) KK,  • Using the D*D0D0KKKdecays from 384 fb-determine the quantitiesand • CP violating quantities: • Lifetimes with CP violation: • If CP is conserved in mixing: where CPV in mixing: CPV in interference of mixing and decay: LP07 - William Lockman

  39. Event Categories LP07 - William Lockman

  40. Also evidence for D0-D0 mixing BELLE Ratio Measurement BELLE PRL 98, 211803 3.2σ signal LP07 - William Lockman

  41. Mass Projections • Mass Projections (mGeV/c): • Signal Purities (1.8495 < m < 1.8795 GeV/c2): LP07 - William Lockman

  42. Lifetime difference Cross Checks Performed several cross checks to ensure unbiased fit results • Fits to generic and signal MC • Fits with independent resolution functions • Subdivided fit results into different running periods, D0 lab angles (cos, phi, psi=angle between D0 decay plane and bending plane) • use high statistics Kpi untagged data sample Conclusions • No hidden differences between the modes observed which could bias the mixing parameters, except in the polar angle variation where a small difference in mixing parameters was observed. This is accounted for in the Signal systematic. LP07 - William Lockman

  43. Direct CPV backup LP07 - William Lockman

  44. Direct CPV Results: aFB KK  There is a significant FB asymmetry LP07 - William Lockman

  45. Direct CPV cross Validations LP07 - William Lockman

  46. D0 KK, : CPV in Decay Soft pion tagging efficiency determined using CF decay Yields: • no-tag D0Ksample: determines the efficiency D0K relative to D0K • tagged Ksample: determines the slow pion efficiency D0K relative to D0K • Slow pion efficiency correction is then applied to D0and D0KK LP07 - William Lockman

  47. Production asymmetries and CPV • Forward-backward asymmetries in cc production • Interference in e−e  cc as mediated by either a virtual photon or a virtual Z0. • Higher-order QED box- and Bremsstrahlung-diagram interference effects • Both effects are antisymmetric in cos, the polar angle of the D0 CMS momentum • Direct CPV is symmetric in this variable • Construct symmetric (aCP) and antisymmetric (aFB) combinations of the yield asymmetries versus cos LP07 - William Lockman

  48. Direct CPV Systematic Variations LP07 - William Lockman

  49. D0(t)K WS Dalitz fit • The WS signal contains both DCS and CF amplitudes. • The CF amplitudes are determined in the RS fit and fixed in the WS fit • The total time dependent WS PDF iswhere yields are determined from the (m,m) fit • The Dalitz and time distributions for mis-tag events are taken from the RS Dalitz model and RS time distributions • The term is determined by a (m,m,t) interpolation to the signal box from the sideband regions LP07 - William Lockman

  50. HFAG Rmix world average LP07 - William Lockman

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