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D 0 - D 0 Mixing at BaBar

D 0 - D 0 Mixing at BaBar. D 0 - D 0 Mixing at BaBar. Jon Coleman SLAC Representing the BaBar Collaboration. B-factories are charm factories. B-factories are excellent laboratories for charm physics: at a lumi 1.2x10 34 cm -2 s -1 produce ~ 16 evts/sec.

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D 0 - D 0 Mixing at BaBar

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  1. D0-D0 Mixing at BaBar D0-D0 Mixing at BaBar Jon Coleman SLAC Representing the BaBar Collaboration

  2. B-factories are charm factories B-factories are excellent laboratories for charm physics: at a lumi 1.2x1034 cm-2s-1 produce ~ 16 evts/sec • Main topics in charm physics at B-factories: • Charm meson and baryon spectroscopy • Dalitz plot analyses • Rare charm decays • D0 mixing and CP violation searches SLAC Annual Program Review

  3. D0-D0 Mixing Formalism • Mixing occurs if the weak interaction splits the masses or widths of the mass eigenstates: well established phenomenon in neutral K, Bd, Bs systems. Mixing parametersx & y are expressed in terms of the mass and width differences of the mass eigenstates SLAC Annual Program Review

  4. Introduction • Mixing among the neutral meson flavor eigenstates provides important information about • electroweak interactions, including CP violation • the CKM matrix • mixing loop virtual constituents • D0 system exhibits the smallest mixing • Short distance Standard Model (SM) suppression: • D mixing loop involves down-type quarks • b quark loop suppressed: • s and d quark loops GIM suppressed • mass difference amplitude O(10-5) or less • long distance mixing amplitudespredominant but hard to quantify A. Petrov, HEP-PH/0611361 SLAC Annual Program Review

  5. Right-sign (RS) decay D0 decay vertex Beam spot: x ~ 100 m y ~ 7 m, D0 production vertex 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 improves mKp , Dm, flight length and hence proper decay time t, and its uncertainty st D0 K-p+- right-sign (RS) D0 K+p- -wrong-sign (WS) + SLAC Annual Program Review

  6. Time dependent WS decay rate • Two types of WS Decays: • Doubly Cabibbo-supressed (DCS) • Mixing followed by Cabibbo-Favored (CF) decay Two ways to reach same final state interference possible! • Use time dependence to separate DCS and mixing: mix (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 SLAC Annual Program Review

  7. 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 • All parameters are determined by fits to data, not from MC SLAC Annual Program Review

  8. m(Kp) & DmFit Results SLAC Annual Program Review

  9. 2007 D0Kresults • 2007 D0Kdecay timeanalysis: 384fb-1 • First evidence for mixing: 3.9(including systematic uncertainties) • x'2, y' consistent with previous BaBar result PRL 91 171801 (2003) • Mainly y' (consistent with ycp if K ~ 0) • No CP violation • Results confirmed by CDF D0K+- PRL 98,211802 (2007) 3.9σ signal SLAC Annual Program Review

  10. D0KCDF Measurement Evidence for mixing at 3.8s PRL 100, 121802 (2008) Fitted signal (12.7  0.3)K • Different Analysis • Different Production Environment • Confirmation of BaBar mixing result • Nearly identical results! SLAC Annual Program Review

  11. 2008 WS data D0Kdecay time fit • fit results still blinded • more data: 470 fb-1 • reprocessed to improve tracking efficiency • eliminate s backgrounds from e- sources • improved signal PDF • improved systematics • prior to unblinding: • investigate systematics • perform cross checks • planned time table: • initial results ICHEP 2008 • published results end 2008 • Can we attain a 5 mixing signal in a single measurement? UPDATE is BLINDED SLAC Annual Program Review

  12. D0-D0Mixing in Lifetime Ratio of D0K+K, +vsD0K+ • D0 mixing and CP violation alter decay time distribution of CP eigenstates with effective lifetimes thh: Measured quantities Mixing and CP observables • yCP=y andDY=0 if CP conserved. • (yCP=0 andDY=0 if no mixing) SLAC Annual Program Review

  13. Mass Projections • Mass Projections (mGeV/c): • high signal purities (1.8495 < m < 1.8795 GeV/c2) • minimize effect of broken charm/combinatorial backgrounds in signal box SLAC Annual Program Review

  14. Decay time fits to determine yCP, Y =409.3±0.7 fs =404.5±2.5 fs =401.3±2.5 fs =407.6±3.7 fs =407.3±3.8 fs K and KK lifetimes differ SLAC Annual Program Review

  15. BELLE, PRL 98, 211803 (2007) 540 fb-1 yCP, Y results • Tagged results accepted for publication in PRD-RC (2008) • Tagged results from 384 fb-1: • combining with previous BaBar measurement of yCP obtained from an untagged sample of D0KKdecays: • results agree with those from BELLE • No evidence for CPV 3 SLAC Annual Program Review

  16. The Untagged Analysis: from 384 fb-1 • No D* flavor tag used: • Higher background to signal than in the tagged analysis, but 4x the statistics • compared to tagged analysis, expect smaller statistical, larger systematic errors • No measurement of CP violating quantity, Y • Construct the untagged sample to be disjoint from the tagged sample • allows tagged and untagged results to be “trivially” combined K KK UPDATE is BLINDED SLAC Annual Program Review

  17. Summary No-mixing point excluded at > 6.7σ No-CPV point still allowed at 1σ • BaBar was first to show evidence for D0 mixing • Result (y`~1%, x`2 ~0 ) • independently confirmed by CDF in the same channel • Striving to improve our sensitivity to mixing through a new measurement of ycp employing an untagged analysis on a disjoint dataset and an updated measurement of x`2, y` in D0Kp • No evidence for CPV at current experimental sensitivity SLAC Annual Program Review

  18. The scorecard • Mixing analyses • D0K+p- • D0K+K-, p+p- • D0K(*)-l+n • D0K+p- p0 • D0Ksp+p- • D0p+p-p0 • D0 K+p- p+p- • Search for CP violation: time integrated • D0K+K-, p+p- • D0p+p-p0, K+K-p0 • D+ K+K-p+ = results soon! SLAC Annual Program Review

  19. Extra material SLAC Annual Program Review

  20. Mixing between Flavor States • Flavor eigenstates can mix through weak interaction: • Physical eigenstates D1 and D2 ≠ flavor eigenstates • If weak interaction splits the masses or widths of physical eigenstates, flavor state mixing will occur,as seen from the time evolution: • mixing parameters: Schroedinger eqn governs time evolution (off diagonal M and  elements determine mixing) D1 = CP D2 = CP In the limit of CP conservation: SLAC Annual Program Review

  21. RWS vs. decay-time slices If mixing is present, it should be evident in an RWS rate that increases with decay-time. Perform the RWS fit in five time bins with similar RS statistics. Cross-over occurs at t ¼ 0.5 psec as in residuals plot. No-mixing fit RWS fits Dashed line: standard RWS fit (2=24). Solid, red line: independent RWS fits to each time bin (2 = 1.5). SLAC Annual Program Review

  22. Systematics: variations in Functional forms of PDFs Fit parameters Event selection Computed using full difference w.r.t. original value Results are expressed in units of the statistical error 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 2007 Kpi systematics, validations SLAC Annual Program Review

  23. 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 • Tagged results limited by statistical errors (not systematics) SLAC Annual Program Review

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