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B S Mixing Measurements at the Tevatron

B S Mixing Measurements at the Tevatron. Michael Kirby Radboud University, Nijmegen NIKHEF. New Trends in High Energy Physics, CRIMEA 2006. B Meson Flavor Oscillations. Neutral B mesons can spontaneously transform in the corresponding antiparticle. Mixing involves CKM elements,

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B S Mixing Measurements at the Tevatron

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  1. BS Mixing Measurementsat the Tevatron Michael Kirby Radboud University, Nijmegen NIKHEF New Trends in High Energy Physics, CRIMEA 2006

  2. B Meson Flavor Oscillations Neutral B mesons can spontaneously transform in the corresponding antiparticle Mixing involves CKM elements, measuring mq constraints the unitarity triangle New exotic particles may run in the loop  mixing sensitive to NP Form factors and B-parameters from Lattice calculations are known at ~15% level

  3. ms and the side of UT md  f2BBB [(1-r)2+h2] circle centered in (r,h)=(1,0) f2BBB known at 15% from LQCD • many theoretical uncertainties cancel in the ratio • |Vts|/|Vtd| can be determined at ~4% (hep-lat/0510113) Experimental challenge: |Vts| >> |Vtd|  ms >> md  needs to resolve > 2.3 THz oscillations Previous Dms measurements: LEP/SLD/CDF-I: ms > 14.4 ps-1 @ 95% CL HFAG Average for PDG 2006

  4. Unitarity Triangle Fit • just for illustration, other fits exist • CKM Fit :Dms: 18.3+6.5 (1s) : +11.4(2) ps-1 -1.5 -2.7 from Dmd Lower limit on Dms from Dmd/Dms

  5. Neutral B Meson system mixture of two mass eigenstates (No CP violation case): BH and BL may have different mass and decay width Dm = MH– ML (>0 by definition) DG = GH - GL The case of DG = 0 B Mixing

  6. Measurement .. In a Perfect World “Right Sign” “Wrong Sign” what about detector effects?

  7. Realistic Effects displacement resolution flavor tagging power, background momentum resolution (L) ~ 50 m mis-tag rate 40% (p)/p = 5%

  8. Real Measurement Layout Data momentum resolution displacement resolution flavor tagging power scan for signal: A(ms=15 ps-1)= ? Unbinned Likelihood Fitter measure frequency: p ~ e-t/[1±AD cosmt]  R(t)  ms = ?

  9. vertexing (same) side e,,Jet 4 2 4 1 “opposite” side 3 Road map to ms measurement 5 • Collect as many Bs as possible • Tevatron, Trigger, Reconstruction • Extract Signal • Bs flavor at decay inferred from decay products • Measure proper decay time of the Bs meson • Si trackers, per event primary vertex, candidate specific decay time resolution • Determine Bs flavor at production (flavor tagging) • PID (TOF, dE/dx) • Flavor tag quantified by Dilution: D=1-2w, w = mistag probability • Measure asymmetry between unmixed and mixed events • In practice: perform likelihood fit to expected unmixed and mixed distributions

  10. This analysis: Feb 2002 – Jan 2006  1 fb-1 Tevatron Luminosity

  11. SMT H-disks SMT F-disks SMT barrels DZero Detector • Spectrometer : Fiber and Silicon Trackers in 2 T Solenoid • Energy Flow : Fine segmentation liquid Ar Calorimeter and Preshower • Muons : 3 layer system & absorber in Toroidal field • Hermetic : Excellent coverage of Tracking, Calorimeter and Muon Systems

  12. X  (e)+ B (e) π- D-S  K-  K+ Signal Selection Muons were selected by triggers without lifetime bias = no online/offline Impact Parameter cuts Trigger muon can be used as tag muon : gives access to eDs sample with enhanced tagging purity

  13. Signal Selection X Eff=30% + B (e) PV D-S π-  LT(DS) K-  K+ • Ds lifetime is used to have non-zero selection efficiency at Interaction Point • Bs can decay at IP and be reconstructed

  14. Flavor Tagging and dilution calibration • Identify flavor of reconstructed BS candidate using information from B decay in opposite hemisphere. Ds a)Lepton Tag : Use semileptonic b decay : Charge of electron/muon identifies b flavor n Bs e / m b)Secondary Vertex Tag : Search for secondary vertex on opposite Side and loop over tracks assoc. to SV. m cos f (l, Bs) < 0.8 c) Event charge Tag: All tracks opposide to rec. B Secondary Vertex

  15. Dilution in md measurement • Combine all tagging variables using likelihood ratios • Bd oscillation measurement with combined tagger Dmd= 0.5010.030±0.016ps-1 Input for Bs measurement Combined dilution:D2=2.48±0.21±0.08 %

  16. Cross-check on BdXD±() Amplitude Scan • EXACTLY the same sample & tagger • Amplitude Scan shows Bd oscillations • at correct place  no lifetime bias • with correct amplitude  correct dilution calibration • Same results for two other modes DØ Run II Preliminary

  17. + J/ vertex PV - L±σL Measure Resolution Using Data • Ultimately Dms sensitivity is limited by decay length resolution – very important issue • Use J/ sample • Fit pull distribution for J/ Proper Decay Length with 2 Gaussians • Resolution Scale Factor is 1.0 for 72% of the events and 1.8 for the rest • Cross-checked by several other methods DØ Run II Preliminary

  18. Results of the Lifetime Fit • From a fit to signal and background region: BsDs mn X BsDs e n X Ds  K*K Ds fp

  19. Bs decay samples after flavor tagging • NBs( fp + m) = 5601 102 • NBs(fp + e) = 1012  62 (Muon tagged) • NBs(K*K + m) = 2997  146 BsDs mn X Ds fp BsDs mn X Ds  K*K BsDs e n X Ds fp

  20. Amplitude Scan of BsXDs() • Deviation of the amplitude at 19 ps-1 • 2.5σ from 0  1% probability • 1.6σ from 1  10% probability

  21. Log Likelihood Scan • Resolution • K-factor variation • BR (BsDsX) • VPDL model • BR (BsDsDs) Systematic Have no sensitivity above 22 ps-1 17 < Dms < 21 ps-1 @ 90% CL assuming Gaussian errors Most probable value of Dms = 19 ps-1 “Direct Limits on the Bs Oscillation Frequency” hep-ex/0603029 – published by Physical Review Letters

  22. More Amplitude Scans • New results : Amplitude scans from two additional modes BsDs (fp) e n X BsDs mn X Ds fp Ds  K*K

  23. Combined D0 Result • Amplitude is centred at 1 now, smaller errors • Likelihood scan confirms 90% CL Dms limits: 17-21 ps-1 • Data with randomized tagger : 8% probability to have a fluctuation (5% before for mfp mode) • Detailed ensemble tests in progress

  24. CKM Fit Before D0

  25. CKM Fit With D0 Limit

  26. The CDFII Detector • multi-purpose detector • excellent momentum resolution (p)/p<0.1% • Yield: • SVT based triggers • Tagging power: • TOF, dE/dX in COT • Proper time resolution: • SVXII, L00

  27. Event Selection: Fully Hadronic Bs used in this analysis • Bs momentum completely reconstructed • Excellent decay time resolution, good S/N • Small BR  low statistic • Good sensitivity at high values of ms Cleanest decay mode: BsDs[] [KK] 

  28. Event Selection: Semileptonic Bs Ds Mass • Missing momentum () • Poorer decay time resolution • Large BR  high statistic • Good sensitivity at low values of ms l+Ds Mass 48000 l+Ds candidates, 75% are from Bs decay • Minv(l+Ds) helps reject BG • BG Sources: • Ds + fake lepton from PV • Bs,dDsDX (DslnX) • cc

  29. Flavor Tagging Performances Two types of flavor tags used in CDF • OST: produce bb pairs: find 2nd b, determine flavor, infer flavor of 1st b • calibrated on large samples of B0 ad B+ decays • SST: use charge correlation between the b flavor and the leading product of b hadronization • performances (D) evaluated in MC, after extensive comparison data VS MC Same-side kaon tag increases effective statistics  ~4

  30. Amplitude Scans • example: B0 Mixing signal in hadronic decays • points: A§(A) from likelihood fit for different m • yellow band: A § 1.645 (A) • m values where A+1.645 (A) < 1 are excluded at 95% C.L. • dashed line: 1.645 (A) as function of m • measurement sensitivity: 1.645 (A) = 1 narrow ms range wide ms range

  31. prompt track Ds- vertex “Bs” vertex P.V. Calibrating the Proper Time Resolution • utilize large prompt charm cross section • construct “B0-like” topologies of prompt D- + prompt track • calibrate ct resolution by fitting for “lifetime” of “B0-like” objects period 3 trigger tracks +

  32. p p  D decay B decay Lxy RUN 304720 EVENT 109026 Proper decay time reconstruction PV Detector length scale and proper treatment of detector/selection biases controlled by performing lifetime measurements

  33. Lepton Ds- vertex Bs vertex P.V. l+Ds ct* Projections Bs lifetime in 355 pb-1: 1.48 ± 0.03 (stat) ps World Average value: 1.469 ± 0.059 ps

  34. Hadronic Scan: Combined Preliminary Bs! Ds / Ds

  35. Semileptonic Scan: Combined

  36. Combined Amplitude Scan Preliminary 25.3 ps-1 A/A (17.25 ps-1) = 3.5 How significant is this result?

  37. k k k k = Sst D isolation K-factor ct [cm] pT [GeV/c] Courtesy of J.Kroll Likelihood Data fitted with an unbinned likelihood function to the expected unmixed and mixed distributions Procedure checked on B0 by fitting for md for each event: k=sig,bg k sig pdg (*) H-G.Moser, A.Roussarie, NIM A384 (1997) Amplitude method(*): scan ms space: fix msfit for A: A consistent with 1  mixing detected at the given ms

  38. Measurement of ms ms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1 -0.21 -0.21 limit ms in [16.94, 17.97] ps-1 at 95% CL “Measurement of the Bs-Bs Oscillation Frequency” hep-ex/0606027 – published by Physical Review Letters

  39. inputs: • m(Bd)/m(Bs) = 0.9830 (PDG 2006) •  = 1.21 +0.047 (M. Okamoto, hep-lat/0510113) •  md = 0.507 ± 0.005 (PDG 2006) -0.035 |Vtd| / |Vts| = 0.208 +0.008 (stat + syst) -0.007 |Vtd| / |Vts| • compare to Belle bd (hep-ex/0506079): |Vtd| / |Vts| = 0.199 +0.026 (stat) +0.018 (syst) -0.015 -0.025

  40. CKM Fit including CDF ms measurement

  41. D0 Outlook • Add Same Side Tagging • Add hadronic modes triggering on tag muon • Add more data (4-8 fb-1 in next 3 years) with improved detector – additional layer of silicon between beampipe and Silicon Tracker (Layer0) – better impact parameter resolution Layer0 has been successfully installed in April 2006 - S/N = 18:1 & no pickup noise - First 50 pb-1 of data on tape, first tracks have been reconstructed, and commissioning advancing quickly

  42. CDF Run II Preliminary L=1 fb-1 CDF Outlook BsDs+-+ (Ds +--) • Collecting new integrated luminosity • Squeezing maximum information from the data • we already have: • Systematic use of Neural Networks in signal extraction: • use decays modes previously discarded cause high BG • more signal in already used modes • Use partially reconstructed BsDs*/K and Ds: • large BR • good momentum resolution • Improve Flavor taggers: • OST: +15% D2 • NN to combine OS taggers • OSKT • SSKT: ~+10% D2 • better use of combined PID and kinematics NBs = 220 BsDs+ (Ds-)

  43. Conclusions • Frequency of Bs mixing successfully measured at the Tevatron! • D0 reported the first two-sided limit on ms • ms [17,21] ps-1 @ 90% C.L. • CDF confirmed result with measurement ms =17.33 +0.42 (stat) ± 0.07 (syst) ps-1 |Vtd / Vts| = 0.208 +0.008 (stat + syst) -0.21 -0.007

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