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Robyn Madrak Harvard University

Measurement of the Lifetime in the Decay Mode. LEPP Journal Club January 22, 2004. Robyn Madrak Harvard University. Decay Modes. First seen unambiguously at CDF in Only measured mass. Lifetimes measured at CDF/LEP in LEP in baryon decays tagged by .

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Robyn Madrak Harvard University

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  1. Measurement of the Lifetime in the Decay Mode LEPP Journal Club January 22, 2004 Robyn Madrak Harvard University

  2. Decay Modes • First seen unambiguously at CDF in • Only measured mass • Lifetimes measured at • CDF/LEP in • LEP in baryon decays tagged by

  3. Why measure the Lifetime ? • Test theoretical models used in predictions of heavy quark quantities • Not only interesting in themselves - need to extract weak interaction quantities from measureables • Currently, experiment and theory disagree • Besides this measurement, lifetime measured only in semileptonic decay modes • Initially expected to have huge amount of data by now  very competitive measurement

  4. Discrepancy: • Theory predictions work well for • But not (though others claim 0.85 could be accommodated)

  5. Measurements up to Now all semileptonic measurements average b-baryon lifetime

  6. Sources of Lifetime Differences • In the simple spectator model, same lifetime for all hadrons with same heavy quark • The heavier the quark, the more valid the approximation • With more sophisticated theory, lifetimes are different, due to • Pauli Interference • Weak Annihilation (for mesons) • Weak Exchange, or scattering (for baryons)

  7. Sources of Lifetime Differences:1.) Pauli Interference Same final state =>interference Different final states

  8. Sources of Lifetime Differences: 2.) Weak Annihilation • Only in mesons • In B but not B - 0

  9. Sources of Lifetime Differences: 3.) Weak Exchange: Main source of lifetime difference for • Helicity suppressed in mesons, but not baryons (no antiquarks)

  10. The Fermilab p pbar Accelerators • Upgraded for Run II • cm energy of 1.96 TeV (was 1.8) • 36p on 36pbar bunches (was 6X6) • 396 ns bunch crossing time: required many detector upgrades (was 3.5 us)

  11. The CDFII Detector Muon Chambers Time of Flight Central Calorimeter CMU CMP CMX Plug Upgrade Calorimeter Beamline Silicon Vertex Detector (SVXII) Solenoid Drift Chamber (COT)

  12. The CDFII Detector

  13. The COT 8 “superlayers” alternating planes of sense wires (readout) and field sheets(ground) alternating axial and stereo (2 °) superlayers End view: Fraction of the endplate sense plane field sheet 1 cell Closeup of cell layout in 1 superlayer

  14. The COT sense wires Segments (in 8 layers) field sheet track Tracking in a Nutshell: • Form line segments in 4 axial layers • Do axial fit, connecting segments • Form segments in stereo layers • Add stereo segments to fit • Final fit field sheet

  15. This measurement: Full Reconstruction baryon equiv. of golden mode • Reconstruct Cons: • Fewer events - larger statistical error Pros: • Potentially smaller systematic error • Less background due to full reco • Have a signal invariant mass peak  signal and background regions well defined • Good vertex for decay length (use multiple tracks) • Do not rely on MC as in semileptonic case (can’t measure momentum of n  boost(bg) is unknown)

  16. Data Sample Muon stubs (J/y->mm) MC event • 65 pb^-1 with SVX fully functional • J/y sample: from dedicated J/y trigger • Level 1: Two online, opp. Q tracks, pT>1.5 GeV, good track-stub match (CMU or CMX) • Level 2: Auto • Level 3: Full tracking, cut on m(mm), track-stub matching • ReconstructingL->pp: • compute invariant mass of all opp. Q tracks • use p/p mass hypothesis for higher/lower momentum particle (Need maximal efficiency: No dE/dx or TOF) Proton track Pion track

  17. Scheme for Measuring the Lifetime • Use control sample: • kinematically similar • well-known lifetime: ct = 2.7 cm ct = 7.9 cm same cuts except for a few cases (m(L) not m(Ks)) • 4 track kinematic fit: vertex constrain J/y and L or Ks (V0’s) • Mass constrain M(mm) to world average m(J/y) (better mass resolution) • Constrain Ks to “point” back to J/y • Measure control sample lifetime • agrees with world average? If so, measure Lb lifetime

  18. Analysis Cuts • No dE/dx cut • No TOF  use in future with more statistics Lb: Need max efficiency now • pt cut only for p’s from Ks, not efficient for L • get rid of B • remove pairs of real J/y and L or Ks that are not B, Lb 0 0

  19. J/y Candidates • ~600k candidates with 3 or more r-f SVX hits • S/B = 5.1 • width = 17.6 MeV

  20. Ks and L Candidates After Cuts Very clean  Most B/Lb background comes from pairs of real J/y and real Ks, L 0 0

  21. 0 Event Yields: B control sample

  22. 0 Same cuts as for B Event Yields: Lb J/y Ks background: expect 1 event at <= 5.5 GeV

  23. A Lb candidate event in data muon tracks proton pion

  24. Proper Decay Time • Need proper decay time ct for each event • primary vertex • secondary vertex • Need pT(Lb)  500 mm  10 cm

  25. Unbinned Maximimum Likelihood Fit • Fit mass histogram to determine signal/background regions, and background fraction fb • Functional for for ct in signal region: G:detector resolution smearing Exp: real long lived distribution • And in background region: • Fit signal and background simultaneously +{

  26. Control Sample Mystery • The control sample lifetime came out low! ct=363  27 mm PDG = 462 mm significance = 3.7s Why??? • Measured the correct lifetime with 5k B events with full simulation • Checked for fitter bias/bugs using Toy MC (next) 0

  27. Fitter Checks with Toy MC • Want to check fit for any possible bias, bugs • Generate 5k “fake” experiments (Toy MC) these have: • same number of events as in data, same S/B • distributions functional forms same as in data • errors on ct drawn from a histogram (of data errors) B toy MC generated ct = 414 mm ave fit ct = 4130.45 B toy MC - ct PULL fit mean = 4  2 mm width = 1.05  0.02

  28. Answer: COT Tracking Algorithms • Two algorithms: • SL (from Run I) links full line segments (in each of 8 layers) • HL (new) for high pT efficiency • looks for hits along a line to the beamline after finding an outer layer segment • easy to believe it would be biased - IS biased for Ks Segments (in 8 layers) 0 ALSO: by dropping “supplementary” algorithm, we lose NO events Use ONLY HL: ct = 33836mm Use ONLY SL: ct = 41431mm • For this analysis we use SL only

  29. 0 Fit for B Control Sample within 1.5 s of PDG world ave of 462 mm • { G (prompt bg) l - l + (tails) (tails + real long-lived bg)

  30. Fit for Lb Lifetime

  31. Cross Checks and Systematics • Cross Checks: • Fitting Method • Splitting of data into separate samples • Treatment of long-lived particles (V0’s) • Systematics • SVX Alignment • Fitting Model • Gaussian resolution function • “Careful Systematics” (from cross checks)

  32. Alternative Fitting Method:2d Simultaneous Mass and Lifetime Fit • Fit mass and ct distributions simultaneously: • gaussian: • linear: • Fit parameters:

  33. Cross Check #1: Results with 2d Simult. Fit B Lb 0 • With this fit: • With separate fit: • With this fit: • With separate fit:

  34. Cross Check #2: Luminosity Effect in Control Sample • Observed B mass width larger in later data than in earlier • Could not find specific change in detector configuration as cause • Divide data into bins of instantaneous luminosity Low lumi mid lumi hi lumi

  35. 0 Cross Check #2: Luminosity Effect in B Control Sample Standard (separate) fit: • low lum: 43839mm Standard (separate) fit: • hi lum: 38355mm  OK (only 0.8 sdifference) 

  36. Cross Check #3:Treatment of V0 ‘s • Ks and L are long-lived (ct=2.7 cm and 7.9 cm) • Many decay outside of SVX • We use only SVX hits consistent with being on tracks (based on COT-only info) • A reasonable analysis may have rejected all SVX hits for V0’s

  37. 0 V0 Tracking: Cross Check Results for B • With COT-only tracks for Ks, lifetime is smaller: Compare to 414 31 mm • But in that case, the sample changes, and the difference is still < 1 s • OK

  38. d vs. f : 3 barrels: Systematic #1: SVX Alignment Getting the systematic: • Use higher statistics B  J/y K • Try a series of “inferior” alignment tables • Also try alignment where SVX wafers are “bowed” out systematically by 100mm • Quantify variation in lifetime: 5 mm Barrel 1 + + 80 mm Barrel 2 Barrel 3 Projection: RMS = 7 mm

  39. Systematic #2: Fitting Model • Current model is well motivated, but can imagine others: • Convolute background exponentials with gaussian resolution (instead of adding) • B lifetime is 11 mm lower • Add an additional positive going tail in background function • no change • Use mass errors and fit for scale factor on mass errors instead of B width • B lifetime is 22 mm higher • Separately fit sidebands and signal region • B lifetime is 2 mm higher • Take largest variation as sytematic: 22 mm

  40. Systematic #3: Resolution Function • ) Fit for sct: scale factor for resolution - 1.26  0.05 • ) Other studies with inclusive J/y’s (not specifically B’s): • ) Fit those results to a function • ) Scale our errors by function and fix sct to 1.0 in lifetime fit • ) Remeasure B lifetime: 1mm smaller

  41. 0 Luminosity Effect in B Control Sample Revisited: Cross Check of a Cross Check • Separate fit: • low lum: 43839mm • high lum: 38355mm • All OK (only 0.8 sdifference) • BUT with 2-d simultaneousfit: • low lumi: 45540 mm, hi lumi: 32450 mm: 2s difference • STILL USE SEPARATE FIT FOR CENTRAL VALUE, THOUGH • Assign systematic for Lb as = 25mm where ct(hi L) is from the separate fit

  42. 0 V0 Tracking: Results for B Looked into treatment of Ks before… now we look a little deeper… Divide into bins of Lxy(Ks): Cross check from before: this was OK The worry here: Very small lifetime for COT-only in smallest bin of Lxy(Ks)

  43. V0 Tracking: Another Effect Given this and the previous effect, Assign systematic as: Allows SVX hits when appropriate

  44. Summary Systematics: • From Bs lifetime control sample (world average = 1.229 0.080 ps)

  45. Implications with • ct(Lb) = 374  89 mm(this measurement) • ct(B) = 462  5 mm

  46. More Data: Predictions • < 10% error • with 3X more data • Already on tape!

  47. Conclusions • We’ve measured the Lb lifetime in LbJ/yL • From this we extract a lifetime ratio which is consistent both with theory and the current world average (Though the two of those disagree) • This result will be much more interesting with 3 times the data (The data are available now! Work in progress...)

  48. Operator Product Expansion Coefficients ci calculable within perturbation theory Matrix elements contain long-distance physics Terms 1 and 2 calculable both for mesons and baryons Term 3 is for PI, WA, and WS, harder for baryons lifetime differences between mesons begin in term 3, between baryons and mesons in term 2

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