33 rd International Conference in High Energy Physics (Jul 26 th – Aug 2 nd , Moscow, Russia)

1. Bs Mixing at D0 Tania Moulik (Kansas University) presented by Andrei Nomerotski (Fermilab/Oxford) 33rdInternational Conference in High Energy Physics (Jul 26th – Aug 2nd, Moscow, Russia)

2. Mass eigenstates are a mixture of flavor eigenstates: BH and BL have a different mass and may have different decay width. Dm = MH– ML = 2|M12| , DG = GH - GL = 2|G12| B mixing Dominant Diagram for the transition : Time evolution follows the Schrodinger equation

3. In an Ideal Scenario.. “Opposite sign” Oscillations with amplitude = 1.0 and Frequency = Dms. “Same sign”

4. 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

5. X μ+/e+ B μ(e) p- D-S φ n K- K+ Analysis outline • Identify e/m. • PT (e/m) > 2.0 • | h| (e/m) < 1.0/2.0 • Signal Selection • Look for tracks displaced from primary vertex in same jet asm/electron • Two tracks should form a vertex and be consistent with f mass (fp K Kp) or K* mass (K*K  KKp) • KKp invariant mass should be consistent with Ds mass

6. X μ (e)+ B μ(e) π- D-S φ ν 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

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

8. Effect of Neutrino • Need to correct Decay Length for relativistic contraction  need to know Bs momentum • Can estimate Bs momentum from MC (through so called k-factor) at expense of additional uncertainty • Dk/k uncertainty causes additional smearing of oscillations • Only few first periods are useful for semileptonic channels • Sensitivity at DL=0 is crucial All above represents the main difference wrt hadronic channels 200 micron # of periods

9. 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

10. Dilution in Δmd 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:εD2=2.48±0.21±0.08 %

11. 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

12. (Cabibbo suppressed) K*K Fit Components Difficult mode due to K* natural width and mass resolution – larger errors wrt fp mode

13. 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

14. Amplitude Method Amplitude fit = Fourier analysis + Maximum likelihood fit often used in oscillationmeasurements Need to know dilution (from Δmd analysis) If A=1, the Δm’s is a measurement of Bs oscillation frequency, otherwise A=0

15. Cross-check on BdXμD±() 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

16. μ J/ψ vertex PV μ L±σL Measure Resolution Using Data • Ultimately Dms sensitivity is limited by decay length resolution – very important issue • Use J/ψ→μμ sample • Fit pull distribution for J/ψ 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

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

18. Log Likelihood Scan In agreement with the amplitude 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

19. 15% 80% 5% Dms(ps-1) 0 17 21 5% 90% 5% Dms(ps-1) 0 17 21 Interpretation Results of ensemble tests: DZero result : Combined with World (before CDF measurement):

20. Impact on the Unitarity Triangle BeforeBS mixing

21. Impact on the Unitarity Triangle With D0

22. Impact on the Unitarity Triangle With CDF

23. DØ Run II Preliminary Period of oscillations @ 19ps-1 “Golden” Events for Visualization • Weigh events using # of periods

24. Can We See Bs Oscillations By Eye ? • Weighted asymmetry • This plot does not represent full statistical power of our data # of periods

25. 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

26. Combination • 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

27. 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

28. Summary • Established upper and lower limits on Dms using Bs  Ds (fp) mn X mode • Analysis published in PRL 97 (2006) 021802 • Combined with two other channels • Bs  Ds (K*K) mn X • Bs  Ds (fp) en X considerable improvement in sensitivity 14.1  16.5 ps-1, no improvement for Dms interval • Looking forward to a larger dataset with improved vertex detection • If Dms is indeed below 19 ps-1 expect a robust measurement with the extended dataset

29. BACKUP SLIDES

30. b b b b u d s c b b b c s u B Mesons Matter b Anti-Matter d

31. CKM matrix and B mixing Why are we interested to study B meson oscillations Wolfenstein parametrisation - expansion in l. complex

32. B Mixing In general, probability for unmixed and mixed decays Pu,m(B)  Pu,m(B).In limit, G12 << M12 (DG << DM) (Standard model estimate and confirmed by data), the two are equal. ~ 10-4 for Bs system ~ 10-3 for Bd system

33. Constraing the CKM Matrix from Dms CDF+D0 (2006) Dms inputs • And similar expression for • Dms (x2) • x = 1.24 0.040.06 • (from Lattice QCD calculations) • Ratio suffers from lower theoretical • Uncertainties – strong constraint Vtd

34. Excellent Tevatron Performance • Data sample corresponding to over 1 fb-1 of the integrated luminosity used for the Bs mixing analysis • Full dataset is ready (85-90% DAQ efficiency)

35. Muon Triggers • Limitation of data recording. Triggers are needed to select useful physics decay modes. 396 ns bunch crossing rate ~ 2.5 MHz  ~50 Hz for data to be recorded. • Single inclusive muon Trigger: • |η|<2.0, pT > 3,4,5 GeV • Muon + track match at Level 1 • Prescaled or turned off depending on inst. lumi. • We have B physics triggers at all lumi’s • Extra tracks at medium lumi’s • Impact parameter requirements • Associated invariant mass • Track selections at Level 3 • Dimuon Trigger : other muon for flavor tagging • e.g. at 50·10-30 cm-2s-1, L3 trigger rate : • 20 Hz of unbiased single μ • 1.5 Hz of IP+μ • 2 Hz of di-μ • No rate problem at L1/L2

36. μ Sample Opposite-side flavor tagging μD±: 7,422 μDs: 26,710 μD±: 1,519 Tagging efficiency 21.9±0.7% μDs: 5,601±102

37. check Using BdXμD±() • The Amplitude Scan shows Bd oscillations at 0.5 ps-1 • no lifetime bias • (A=1) : correct dilution calibration

38. Detector Effects flavor tagging power, background Decay length resolution momentum resolution  (p)/p = ? % sl = ? SM prediction - Dms ~ 20 ps-1 Trying to measure : Tosc~0.3 X 10-12 s !

39. Sample Composition • Estimate using MC simulation, PDG Br’s, Evtgen exclusive Br’s Signal: 85.6%

40. Flavor tag Dilution calibration • Bd mixing measurement using Bd D* m n X, D* D0 p, D0 K p, and evaluate dilutionin various diution bins. Follows similar analysis outline as Bs mixing. • Form measured asymmetry in 7 bins in visible proper decay length (xM) – Count OS and SS events (compare charge of reconstructed muon with tagger decision) • Fit the c2: • Also include B+ D0 mn X decay asymmetry.

41. Dilution calibration : Results • For final fit, bin the tag variable |d| in 5 bins and do a simultaneuos fit c2(i) where i=1,5. Parameters of the fit : Dm, fcc, 5 Dd, 5 Du = 12 B0 B+ Increasing dilution Increasing dilution Dm = 0.506 0.020 (stat.) ps-1 eD2 = (2.48  0.21) (%) (stat.) e = (19.9 0.2) (%) (stat.)

42. Individual Taggers performance Note : To evaluate the individual tagger performance |dpr| > 0.3 This cut was not imposed for final combined tagger. Final eD2 is higher.

43. dpr Likelihood minimization to get Dms Minimize • Form Probability Density Functions (PDF) for each source Dilution Calibration (From Dmd measurement) Signal selection function (y)

44. Bs Signal and background • Signal PDF: • Background PDF composed of long-lived and prompt components – Evaluated from a lifetime fit. • Long Lived Background – Described by exponential convoluted with a gaussian resolution function. • Non-sensitive to the tagging • Non-oscillating • Oscillating with Δmd frequency • Prompt Background – Gaussian distribution with resolution as fit parameter.

45. more pure more pure Combined flavor tag algorithm • Combine individual tag informations to tag the event. • Get tag on opposite side and construct PDF’s for variables discriminating b (m- ) and b (m+) (Use B+  D0m n X decays in data) • Discriminating variables (xi): Electron/Muon SV Tagger

46. Ensemble Tests • Using data • Simulate Δms=∞ by randomizing the sign of flavour tagging • Probability to observe Δlog(L)>1.9 (as deep as ours) in the range 16 < Δms < 22 ps-1 is 3.8% • 5% using lower edge of syst. uncertainties band • Using MC • Probability to observe Δlog(L)>1.9 for the true Δms=19 ps-1 in the range 17 < Δms < 21 ps-1 is 15% • Many more parameterized MC cross-checks performed – all consistent with above