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SUNY Interview Seminar 29 Jan 2007

This seminar by Tania Moulik explores Bs mixing at the Tevatron, including topics like B meson oscillations, flavor tagging, soft electron identification, analysis results, and constraints on CKM matrix. The presentation covers the historical search for Bs meson oscillation, CKM matrix constraints, and tagger calibration for Bd mixing.

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SUNY Interview Seminar 29 Jan 2007

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  1. SUNY Interview Seminar 29 Jan 2007 Bs Mixing at the Tevatron Tania Moulik (University of Kansas) DØ collaboration Tania Moulik, Bs Mixing at Tevatron

  2. Outline • Introduction to B Mixing. • Flavor Tagging and Bd Mixing • Soft Electron Identification • Electron tagging and combination with other taggers. • Bd mixing – Tagger calibration • Bs Mixing • Analysis Outline • Results from D0 and CDF and interpretation. • Sensitivity studies at D0 • Conclusion Tania Moulik, Bs Mixing at Tevatron

  3. Bs mixing saga • B mesons oscillate (mix) first seen in Bd system– ARGUS(1987)@DESY, CLEO(1989)@CESR (Y(4S)) • Bs mesons oscillate was established by comparing the time-integrated oscillation probability measurements above with measurement at LEP (which contained both Bd and Bs). • But what is the oscillation frequency? • Search started at LEP and continued between 1984-1999. • Formation of B oscillations working group in 1996 at LEP. • Last LEP average : 1999 (D. Abbaneo and G. Boix, JHEP 9908 (1999) or hep-ex/9909033) Dms > 12.3 ps-1 • Search continued at Tevatron….observed in 2006…20 years later.. Tania Moulik, Bs Mixing at Tevatron

  4. Neutral B’s oscillate. 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 Time evolution follows the Schrodinger equation Tania Moulik, Bs Mixing at Tevatron

  5. 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), Tania Moulik, Bs Mixing at Tevatron

  6. CKM matrix and B mixing Wolfenstein parametrisation - expansion in l. complex Tania Moulik, Bs Mixing at Tevatron

  7. Constraints on Vtd (r,h) a g b (0,0) (1,0) On solving the mixing box diagrams : Theoretical Inputs : x = 1.21 0.040.05 fBd BBd-1/2 = 22333 12 Tania Moulik, Bs Mixing at Tevatron

  8. Flavor TaggingSoft Electron Id, Other Taggers, Bd Mixing. Tania Moulik, Bs Mixing at Tevatron

  9. D0 Liquid argon calorimeter Central (|h| < 1) (32 EM Mod.), Forward upto (|h| = 4) h x f = 0.1 x 0.1 (finer in third layer – 0.05x0.05) Preshower added in runII – three layers of scintillators with WLS readout. Soft Electron Identification Tania Moulik, Bs Mixing at Tevatron

  10. Soft electron Id. • Electrons non-isolated in b-jets and of low momenta. Reach shower maximum much earlier than higher momentum electrons. • Need preshower for electrons bremming early on. • Use a narrower cone to reject fake activity. • Soft electron reconstruction separate from standard calorimeter reconstruction. Start with track, extrapolate to calorimeter and cluster towers around track in a narrower Dh x Df region. e p e p EM HAD EM HAD Tania Moulik, Bs Mixing at Tevatron

  11. The Road Method • Steps in the Road Algorithm • Extrapolate track in a Helix inside the magnetic field ( 58.7 cm - inner radius of coil) and then in a straight line. • In every layer of calorimeter, use list of cells belonging to a predefined road -- h rings and f slices spanned by entry and exit points of track in each floor. h road Shower profile suggests : 2 h rings and at most 1 f slice. Neighbouring f slice added if track close to edge. ~ 90% contained in first three floors ~ 90 % of energy contained in road Overall ~ 80% of energy f road Tania Moulik, Bs Mixing at Tevatron

  12. Performance study • Study performance using • signal : ge+e-, J/Ye+e- • background : Ksp+ p- Detector material Profile Use 3 Variables Min Single layer cluster energy in the 3-D CPS cluster Tania Moulik, Bs Mixing at Tevatron

  13. Flavor Taggers • Opposite side Flavor Tagging  Identify flavor of reconstructed BS candidate using information from B decay in opposite hemisphere. Lepton Tag (e,m) Jet-Charge Tag Additionally – Look for a secondary vertex Tania Moulik, Bs Mixing at Tevatron

  14. Tagger development • Develop likelihood for the tag to be from a b or a b • Use B+  D0 m+ n X decay data sample (No mixing, B flavor known on both sides). • Small contribution from B0 decays which can oscillate (2%). Require decay length of B candidate < 500 mm. Tania Moulik, Bs Mixing at Tevatron

  15. Lepton Tag Likelihood • Use tracks in a cone (DR < 0.7) around e/m • Look for Secondary Vertex • Combined likelihood: Tania Moulik, Bs Mixing at Tevatron

  16. Combined Tagger • In absence of lepton, use secondary vertex tagger in combination with event charge. • Use taggers in order of preference • Muon > Electron > SVT Muon Charge d > 0 Intial flavor b d < 0 Initial flavor b Tania Moulik, Bs Mixing at Tevatron

  17. Bd mixing &Tagger calibration • B+ and Bd decays. Tania Moulik, Bs Mixing at Tevatron

  18. Bd mixing and Tagger Calibration • Binned asymmetry fit : • Bin D0 mass distribution in 7 Visible Proper decay length VPDL bins Tania Moulik, Bs Mixing at Tevatron

  19. For final fit, bin the tag variable |d| in 5 bins and do a simultaneuos fit Asymmetry fits in dilution bins Tagger Calibration at DØ Increasing dilution • Dm = 0.506 0.020 (stat.)  0.016 (syst.)ps-1 • eD2 = (2.48  0.210.07) (%) • = (19.9 0.2) (%) fcc= 2.20.9(%) Event-by-event dilution Measurement of Bd mixing using opposite Side flavor tagging PRD 74, 112002 (2006) Tania Moulik, Bs Mixing at Tevatron

  20. Individual Taggers performance Note : To evaluate the individual tagger performance |dpr| > 0.3 This cut was not imposed for final combined tagger. Hence, final eD2 is higher. Tania Moulik, Bs Mixing at Tevatron

  21. Flavor Tag at Tevatron • For comparison of break up, older CDF tagger results quoted.Very recent numbers eD2 (OST) = 1.8% (With Neural network combination of taggers) For individual tagggers at D0, |d| > 0.3 (Overall is higher). CDF DØ Tania Moulik, Bs Mixing at Tevatron T. Moulik, (hep-ex/0701022) , Proc. Beauty 2006

  22. Bs Mixing Tania Moulik, Bs Mixing at Tevatron

  23. μ+/e+ B μ(e) p- D-S φ n K- K+ Selection – Semileptonic Mode X Tania Moulik, Bs Mixing at Tevatron

  24. Bs  Ds e ne X selection • Using road electrons with pT > 2.0 GeV in central |h| < 1.1. • Cuts on calorimeter quantities only: EMF > 0.7, 0.55 < E/P < 1.1 • Using the inclusive muon triggered sample : Events already tagged. Tania Moulik, Bs Mixing at Tevatron

  25. Selection – Hadronic Modes Tania Moulik, Bs Mixing at Tevatron

  26. Detector Effects Decay length resolution flavor tagging power, background momentum resolution (L) ~ 50 m mis-tag rate 40% (p)/p = 5% SM prediction : Dms ~ 20 ps-1 Tosc~0.3 10-12 s Tania Moulik, Bs Mixing at Tevatron

  27. Expected p.d.f. • Prob. for oscillated and non-oscillated events as a function of the decay time or distance (x) for signal : Taking into Missing energy Dilution from flavor tagging Visible proper decay length (VPDL) Tania Moulik, Bs Mixing at Tevatron

  28. Expected p.d.f • Transition to measured VPDL (xM). For j’th mode : • There are contributions from other channels to same final state as well.Overall PDF is sum over all j’s taking into account relative contributions. . Integrate over K-factor distribution Convolution with VPDL resolution Tania Moulik, Bs Mixing at Tevatron

  29. Likelihood minimization for Dms Minimize Proper Decay Length (pxM) , Decay Length Error (psxM), Dilution (pdpr), Mfp Ds mass distribution (pMfp), Signal Selection Variable (py) • Signal PDF: • Background PDF: Fractions estimated from lifetime fit. Tania Moulik, Bs Mixing at Tevatron

  30. Reco. Efficiency vs Lifetime Efficiency after lifetime cuts Depends on the decay length of the Ds+m candidate. Other efficiencies included in sample composition estimation. • Efficiency is ~ 90% for VPDL > 0.05 cm • And > 25% for VPDL < 0.05 cm • Below 0 cm, fake events increase the efficiency. Tania Moulik, Bs Mixing at Tevatron

  31. Decay length Resolution Are errors estimated correctly? UseJ/ψ→mm/ee sample. Get Pull distribution For correct errors ~ 1. Scale Factor : 1.0 for 67 %, 1.8 for 33% Decay Length Error Tosc @ 19ps-1 ~ 0.01 cm Tania Moulik, Bs Mixing at Tevatron

  32. Amplitude Scan • Modify the equation and introduce an amplitude term • Vary Dms and fit for A: • A consistent with 0  no oscillation. • A consistent with 1 and inconsistent with 0  oscillation. • Range of dms for which amplitude is compatible with 0 and incompatible with 1 can be excluded. All values for which A+1.645 s < 1 are excluded at 95% C.L. • Sensitivity : 1.645s = 1 (Will see oscillations if Dms is below this value) Tania Moulik, Bs Mixing at Tevatron

  33. check Using BdXμD±(), and sideband Tania Moulik, Bs Mixing at Tevatron

  34. Dms Scan (Signal region) D(log L) = log Lmin – log L 17 < Dms < 21 ps-1 @ 90% CL assuming Gaussian errors Most probable value of Dms = 19 ps-1 A/A (19.0 ps-1) = 2.5 (from 0) A/A (19.0 ps-1) = 1.6 (from 0) Tania Moulik, Bs Mixing at Tevatron

  35. Significance of the minimum • Simulate Δms=∞ by randomizing the sign of flavor tagging • Prob. 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. error band • Region below 16 ps-1 is experimentally excluded • No sensitivity above 22 ps-1 Tania Moulik, Bs Mixing at Tevatron

  36. CDF Scan D(log L) = log L(A=0) - log Lmin (A=1) log Lmin (A=1) = -17.26 A/A (17.75 ps-1) = 6.05 A (Dms = 17.75 ps-1) = 1.24  0.20 What is the significance of the minimum ? Tania Moulik, Bs Mixing at Tevatron

  37. Likelihood Significance • Perform 350 million experiments with randomized flavor tag. Only 28 trials with min(Dlog L) < -17.26, • p-value = 8 x 10-8 > 5 s (Very small prob. For background fluctuation) (p-value = 5 x 10-7 ~ 5.0 s ) Tania Moulik, Bs Mixing at Tevatron

  38. Combination with other modes at D0 Bs  Ds (fp) e n X Bs  Ds (K*K) mn X Sensitivity increase 14.1  16.5 ps-1 Amplitude peak however no longer significantly separated from 0 but is consistent with 1 @ 19 ps-1 Preferred value ~ 19 ps-1 But 8% expectation for background fluctuation (5% for published result) Tania Moulik, Bs Mixing at Tevatron T. Moulik, A. Nomerotski, FERMILAB-CONF-06-496-E (ICHEP 2006)

  39. Implications of current status Dms is standard model–like (upto present state-of-the art theory predictions) Dms (Not in CKM Fit) = Testing New Physics in the Bs decays ? New Physics enters the function S0(xt) (Inami-Lim function) but theoretical uncertainties make testing difficut using the Dms measurement alone. Will need added experimental information from DGs and CP asymmetry in flavor specific decays afs (New measurements from DØ probing this) Tania Moulik, Bs Mixing at Tevatron

  40. Constraining the CKM Matrix Only Angles Combined Fit Tania Moulik, Bs Mixing at Tevatron

  41. Dms Future prospects at DØ • Detector upgrades • Addition of Layer 0 • Better decay length resolution ~ 10-15% • Improvement in rec. efficiency ~ 40% • Analysis improvements • Addition of Same side tagging ~ 20% improvment in eD2. • Event-by-event scale factors ~ 8% improvement in sensitivity • Finer binning in k-factors ~ 10% improvement in sensitivity Tania Moulik, Bs Mixing at Tevatron

  42. Sensitivity studies Using Analytical Expression Tania Moulik, Bs Mixing at Tevatron

  43. Conclusion • 1 fb-1 data sample was used for the Bs oscillation studies at Tevatron. • D0:90% C.L. interval for Δms: 17 – 21 ps-1 assuming Gaussian errors. Probabilty for fluctuation for higher Dms ~ 5%. • CDF 5.4 s result consistent with D0 result. Dms measured to be: 17.77 ± 0.10(stat) ± 0.07(sys) |Vtd|/|Vts| =  0.2060 ± 0.0007(exp) +0.0081-0.0060 (theor) • Improvements in the pipeline for D0 analysis. • Layer 0 is installed and performing well. • Dms provides strong constraint on the CKM triangle. Vtd Currently limited by theoretical errors. An independent measurement from DØ would be interesting in its own right. • CP violation in B-sector will be measured precisely in the next few years. CKM matrix uptil now consistent with standard model predictions. Improvements in theoretical calculations will help to test the model further. Tania Moulik, Bs Mixing at Tevatron

  44. BACK-UP SLIDES Tania Moulik, Bs Mixing at Tevatron

  45. 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), ~ 10-4 for Bs system ~ 10-3 for Bd system Tania Moulik, Bs Mixing at Tevatron

  46. DØ Detector Muon chambers (identify muon) m/e K Ds K Bs Silicon Microstrip Tracker (SMT) decay length resolution Fiber Tracker + SMT Track momentum resolution Calorimeter + preshower (indentify electron) Tania Moulik, Bs Mixing at Tevatron

  47. Bd mixing and Tagger Calibration • Binned asymmetry fit : Bin D0 mass distribution in 7 Visible Proper decay length VPDL bins • PDF for oscillated and non-oscillated events: K-factor : VPDL resolution Rec. efficiency of jth channel K-factor Tania Moulik, Bs Mixing at Tevatron

  48. Layer 0 Performance At p~ 1 GeV, 50 mm  25 mm ~ 10% improvement Tania Moulik, Bs Mixing at Tevatron

  49. Results of the Lifetime Fit • Different background models are used for cross-check and systematic errors • Trigger biases have been studied • Central values for cτBs= 404 − 416 μm • Statistical error ~10 μm • HFAG value cτBs = 438 ± 12 μm Most important region Tania Moulik, Bs Mixing at Tevatron

  50. Sample Composition • Estimate using MC simulation, PDG Br’s, Evtgen exclusive Br’s Signal: 85.6% Tania Moulik, Bs Mixing at Tevatron

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