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Mixing and CP Violation in neutral D meson system

This presentation discusses the formalism and experimental view of mixing and CP violation in the neutral D meson system. It provides an overview of the experimental apparatus and analysis strategy, measurement status, and future perspectives. The presentation also includes a history of CP violation and an introduction to the Standard Model and CKM matrix. The mixing in the neutral meson system and the different categories of CP violation are explained. The requirements for measurements and the colliders used for the analysis are also discussed.

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Mixing and CP Violation in neutral D meson system

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  1. Mixing and CP Violation in neutral D meson system BESIII 物理分析讲习班 2008年6月26日 郑阳恒 中国科学院研究生院

  2. Some notations • CPV: CP violation • DCS: Doubly Cabbibo Suppressed • CF: Cabbibo Favored ( CA: Cabbibo Allowed ) • SM: Standard Model • MC: Monte Carlo • RS: Right Sign D0→K-π+ • WS: Wrong Sign D0→K+π- Y. Zheng (GUCAS)

  3. Outline • Introduction (formalism and experimental view) • Experimental Apparatus & Analysis Strategy • Measurement status • Summary and Future perspective Y. Zheng (GUCAS)

  4. Motivation • Universe in 15 billion years ago: high degree of symmetry between matter and antimatter • Matter encounters antimatter  annihilated • Present-day Universe: matter >> antimatter, Why? • Standard Model: CP violation is the KEY • Neutral meson mixing CPV Y. Zheng (GUCAS)

  5. A little history • Before 1956, discrete space-time symmetries were considered to be exact • 1956, T.D. Lee and C.N. Yang suggested P violation. (Nobel prize) • 1957, C.S. Wu et al., discovered P violation experimentally. • 1964, Christenson et al., discovered CP violation in the neutral K system. (Nobel prize) • 1973, Kobayashi and Maskawa: KM model with  6 quark flavors  CP violation • 1980, Carter, Sanda and Bigi: Large CP asymmetries in B decays. • 2001, B-factories observed CP asymmetries in B decays. • How about D decays? Y. Zheng (GUCAS)

  6. Starting point: Standard Model • Described the elementary building blocks of matter and interactions. • Account for all experimental phenomena to a high degree of precision. • Many predictions verified experimentally. • A successful theory. • CP violation is one of the least constrained sector. • measurements of Mixingand CP violation in neutral D meson system provide a sensitive testing method. Y. Zheng (GUCAS)

  7. u, c, t d’, s’, b’ CKM Picture of CP Violation In Standard Model: Cabibbo, Kobayashi & Maskawa: CKM matrix SU(2) doublets in standard electroweak model: Y. Zheng (GUCAS)

  8. W q q’ Lagrangian • In a physics system: Lagrangian  the dynamics • Quark flavor transitions: q  Wq’ • Lagrangian of charged weak current via W Under the CP transformation • Vij  Vij* (at least 1 non-real) CP-violation (only source in SM) Y. Zheng (GUCAS)

  9. Parameterization of CKM matrix complex phase  CPV • Wolfenstein parameterization(A,ρ,η,: Cabibbo angle ~ 0.22): Y. Zheng (GUCAS)

  10. Mixing in neutral meson system d,s,b u c __ D0 W W D0 _ _ c u d,s,b u,c,t b(s) d(s) __ __ B0(s)(K0) B0(s)(K0) W W _ _ _ _ d(s) b(s) u,c,t Mass and flavor eigenstates: “Box diagrams” for second order weak processes: Time evolution: Y. Zheng (GUCAS)

  11. Time dependent decay amplitudes • For any decay final state f, |f CP|f Y. Zheng (GUCAS)

  12. Time dependent decay rates • For any decay final state f, |f CP|f • IfCPis conserved: Y. Zheng (GUCAS)

  13. CP violation categories • CP violation in decay (direct CP violation) : observed in B decays • CP violation in mixing (indirect CP violation) : observed in neutral K system • CP violation in interferencebetween decay and w/o mixing (mixing-induced CP violation) : observed in neutral B system Y. Zheng (GUCAS)

  14. D0phys(t) f D0phys(t) f D0 D0 neutral D decays (to common states) For convenient: Y. Zheng (GUCAS)

  15. D→Kπ (non-CP eigenstates) p+ u d D0K-p+ is a Cabibbo-allowed favored decay mode (Br=3.8%) W c s K- u u There are 2 ways that a D0can decay to the opposite combination K+p-: 2) D0 D0; D0K+p- decays • Doubly Cabibbo-suppressed decays (DCS) K+ u p- d s u W c d W c u s p- D0 u u K+ D0 D0 c u u mixing Cabibbo-allowed D0 decay (CA) Br ~ 0.014% Y. Zheng (GUCAS)

  16. RS WS D→Kπ (non-CP eigenstates) • f = K-π+ , f = K+π- δ: strong phase diff. (assume CPV via weak phase only), φ: weak phase Y. Zheng (GUCAS)

  17. Mixing and CPV: D→Kπ • If CP is conserved: fit WS distribution → Mixing • For CPV case: fit two WS distribution separately • Direct CP violation in DCS Decay • CP violation in mixing • CP violation in interference between decay and mixing: Y. Zheng (GUCAS)

  18. CP eigenstates: K+K-,π+π- • CP eigenstates, a simpler case: |f CP|f = ±|f • CP Asymmetries: (Integrated time-dependant decay rates over time, assume no CPV in decay, AD=0) Y. Zheng (GUCAS)

  19. Requirements for Measurements • LargeD meson source ( Br(Df) ~ 10-2 - 10-3) • very high luminosity e+e- collider B-factories, BEPCII • B meson reconstruction • high quality ~4 detetor Belle, BaBar, BESIII • Tag flavor of the D meson • good particle id  dE/dx, Cherenkov, TOF, EMC • Measure proper-decay-time difference (Belle, BaBar) • high precision vertexing (Δz)  silicon strip vertex detector • Likelihood fit to the t distributions • Measure Branching Ratios (BESIII) Y. Zheng (GUCAS)

  20. Colliders: KEKB and PEP-II B B production threshold Asymmetric e-e+ Colliders@ U(4S) Coherent BB production KEKB: 8x 3.5 GeV, bg=0.43 PEP-II 9 x 3.1 GeV, bg=0.55 s(BB)/shad=0.28 Y. Zheng (GUCAS)

  21. e+ 3.1 GeV e- 9 GeV Detectors: BaBar and Belle ECL KLM ECL KLM DIRC: Quartz bar + water tank ACC: aerogel Cherenkov counters SVT: 5-layer SVD: 3-layer. (4-layer soon) … … high performance vertexing and PID Y. Zheng (GUCAS)

  22. A typical collision event • Signals: • D*+  D0+ K-+ K+K- -+ • Variables used to separate signal and background • D invariant mass: M(Kp, KK, pp) • Dm = M(Kpptag) – M(Kp) Y. Zheng (GUCAS)

  23. Signals: D*+→ D0(→Kp, KK, pp) p+tag t=(ldec/p)(m/c) Proper decay time Y. Zheng (GUCAS)

  24. Analysis Strategy in B-factories • Signals: D*+→ D0(→Kp, K K, pp) p+tag • Backgrounds: shapes from MC, fractions from data • Fitting method: Unbinned maximum likelihood fit. • Proper-time distribution: • Resolution function: from RS data fitting • Fit WS data event-by-event • Several proper time fits are performed. • lifetime (no mixing) • mixing, no CP violation • mixing, CP violation • Monte Carlo: search for systematics and validate statistical significance of results. Y. Zheng (GUCAS)

  25. Signal selection criteria • Beam-constrained vertex fits of K, p, ptag tracks. • ptag charge gives D flavor at production. • Require fit probability > 0.001 • D0 selection • CMS p(D*) > 2.5 GeV/c to eliminate D0’s from B decays • K, p particle identification • DCH hits > 11 • 1.81 < M(Kp) < 1.92 GeV/c2 • decay time error < 0.5 ps • -2 < decay time < 4 ps • ptag • CMS p* < 0.45 GeV/c • lab p > 0.1 GeV/c • SVT hits > 5 Y. Zheng (GUCAS)

  26. Background shapes • True D0combined with a random πs • peaked at M(Kp) • does not peak in Dm • Mis-recon D0 • peaked at Dm • does not peak in M(Kp) • Combinatorial background • does not peak in both M(Kp) and Dm • Shapes: from MC • Yields: from 2-D fit Y. Zheng (GUCAS)

  27. RS(top)/WS(bottom) Datasets After Event Selection Integrated Luminosity Approximately 384 fb-1 x103 BaBar Data BaBar Data 1,229,000 RS candidates events/1 MeV/c2 events/0.1 MeV/c2 BaBar Data BaBar Data 64,000 WS candidates Y. Zheng (GUCAS)

  28. Resolution function • There is no perfect measurement! • Likelihood for proper decay time measurement: • Resolution function models: (from RS signal fit) • Signal: triple-Gaussian model • random πs and Mis-recon D0: triple-Gaussian model (same as signal) • Combinatorial background model: double-Gaussian (from sideband) Y. Zheng (GUCAS)

  29. Validation: RS lifetime BaBar Data The D0 lifetime is consistent with the Particle Data Group value, within the statistical and systematic errors of the measurement. Plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2 Y. Zheng (GUCAS)

  30. WS Mixing Fit: No CP Violation • Varied fit parameters • Mixing parameters • Fit class normalizations • Combinatoric shape BaBar Data Mixing minus No mixing PDF Data minus No mixing PDF BaBar Data Plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2 Y. Zheng (GUCAS)

  31. 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: -0.95 BaBar Data • Accounting for systematic errors, the no-mixing point is at the 3.9-sigma contour RD: (3.030.160.06) x 10-3 x’2: (-0.220.300.21) x 10-3 y’: (9.74.43.1) x 10-3 Y. Zheng (GUCAS)

  32. Results for Kp Analysis Y. Zheng (GUCAS)

  33. D0 reconstruction and lifetime fit Y. Zheng (GUCAS)

  34. ycp , AΓresults Y. Zheng (GUCAS)

  35. Charm events at threshold are very clean Ratio of signal to background is optimum Lots of systematic uncertainties cancellation while applying double tag method Mixing at threshold Bad news: no time-dependent information Good news: Quantum coherence, CP tags The coherence of two initial D allows simple methods to measure DDbar mixing, strong phase and CP violation What about Charm factory? Y. Zheng (GUCAS)

  36. BEPCII/BESIII experiment • Will collect collision data in July! • Will operate at Y. Zheng (GUCAS)

  37. Coherent D0 – D0 states • (3770)  D0D0 / (3770)  D+D-  50/50 (3770) : spin=1, cc bound state, Mass: 3.771 GeV D0 : spin=0, Mass: 1.864 GeV  D mesons created  at rest in CM  DD orbit angular momentum L=1 • Bose statistics  D0D0 state anti-symmetric  D0D0 and D0D0 are prohibited • At any time : one D0 one D0 until one D decays Y. Zheng (GUCAS)

  38. D Pairs at Different Experiments 128 M is expected at BES-III with 4 years’ luminosity. 5 M is expected at CLEO-c until 2008. 700M 500M 128M 5 M 0.2 M (3770) peak (4S) Peak Higher statistics Background free Y. Zheng (GUCAS)

  39. Charm tags Y. Zheng (GUCAS) Single tags • reconstruct one D meson Double tags • Both D and Dbar are reconstructed Flavor tags in Mixing language • Semileptonic modes: K(p)en CP tags • CP even • CP odd

  40. K,  Identification at BESIII Y. Zheng (GUCAS)

  41. Golden channel D0Kp Semileptonic channel Ken, Kmn, etc RM measurements @3.773 GeV experiment theory Double tag measurements. Number of R.S. tags at BESIII are expected to be 104-105, the sensitivities of Rmix will @ 10-4—10-5 2-body identical final states are Required in both D hadronic decays Y. Zheng (GUCAS)

  42. CP eigenstate Tags CP + K+K- (3.89X10-3 ) p+p-(1.38X10-3 ) Ks p0p0 p0p0(8.4X10-4) KSKS (7.1X10-4) r0p0(3.2x10-3) CP – KSp0(0.012) Ksh (3.9X10-3) KSh' (0.0094) KSr0 (0.0078) Ksw (0.012) KSf (4.7X10-3) Dalitz Analysis For Ks modes: CPV effect of Ks need to be considered! (Prof. Xing/Zhizhong’s suggestion) In 20fb-1y(3770) data, we can get > 4.5x105 CP+ tags and > 3.6x105 CP- tags With large sample of CP tags, we may improve the measurements of strong phase, probe the direct CP, and other mixing parameters Y. Zheng (GUCAS)

  43. CP Violation 1. Direct CP Violation (in decay) 2. Indirect CP Violation (in mixing) 3. CP violation in the interference between decays with/without mixing Y. Zheng (GUCAS)

  44. Quantum Coherence Suppose Both D0 decay to CP eigenstate f1 and f2 . Thus if a final state such as (KK)(pp) observed, we immediately have evidence of CP violation In 20 fb-1y(3770) data, > 1000 double CP+ and CP- tags can be obtained. if 100%CPV, it lead to ACP~10-3 level Y. Zheng (GUCAS)

  45. - - (r,h) Unitarity Triangle B0 pp B0rp Vub*Vud+Vcb*Vcd+Vtb*Vtd = 0 (B system) B0 D(*)p B+DK B0 J/yKs B0fKs B0D(*)D(*) Y. Zheng (GUCAS)

  46. 3from B- D0 K- • No hadronic uncertainty • Methods • Gronau-Wyler original method • Atwood-Dunietz-Soni Method • Dalitz method • Problem: statistics Y. Zheng (GUCAS)

  47. 3 3 δB Gronau-Wyler original method • Theoretically clean • Experimentally challenging • Hadronic D decay modes: hard for D flavor tagging • Semi-leptonic D decays : Background too high • CP eigenstate decays of D: small Branching ratio Y. Zheng (GUCAS)

  48. Atwood-Dunietz-Soni Method • Use interference between • B+ DK+and B+ DK+follows by D (D)  f • To get a common final state f, we need • Double Cabibbo Suppression (DCS): f = K+ -, K+ K- • K- K mixing: f = KS0 , KS+- • D hadronic parameters: • Decay rates: • rD, δD :measured from Charm factory (see next slides) • (rB, δB, 3) 3 unknowns, 4 measurements  3 Y. Zheng (GUCAS)

  49. δDfrom Charm-factory • Get rD from the large tagged D decay samples (B-factory or Charm factory (CLEO-c sensitivity: ~0.05 from 3fb-1)) • δD Charm factory on(3770) accurately measured (Soffer hep-ex/9801018) • Reconstruct Double Tags: CP and f • CP+: K+ K-, +  -,Ks00 • CP- : Ks0 ,Ks ,Ks • Asymmetry in CP+ and CP- of D decays: • Input RD= rD2 from PDG • BESIII sensitivity: <0.06 from 20fb-1 for cosD Y. Zheng (GUCAS)

  50. Dalitz method • Three body D decays: KS+-,+-0,KSK+K-… • Effect of D – D interference Y. Zheng (GUCAS)

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