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Measurements of Λb Lifetime at DØ

This paper outlines the motivation, measurement strategy, and results of the Λb lifetime measurements at the DØ experiment at the Tevatron. It includes the measurement of the Λb lifetime in semileptonic and fully reconstructed decay modes. The results are compared to theoretical predictions from the Heavy Quark Expansion (HQE) theory.

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Measurements of Λb Lifetime at DØ

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  1. Λb lifetime measurements at DØ Dmitri Tsybychev, SUNY at Stony Brook, DPF2006, Honolulu On behalf of DØ collaboration Outline: • Motivation • DØ detector and Tevatron • Measurement strategy • Λb lifetime in semileptonic mode • Λblifetime in fully reconstructed mode • Summary

  2. B Hadron Lifetimes • Naive quark spectator model predicts equal lifetimes for all B hadrons (but Bc) • (NLO) QCD  Heavy Quark Expansion predicts deviations in rough agreement with data • Testing HQE, where light quarks are included via 1/mb perturbative expansions • Expect some differences between lifetimes of different species: • τ(B+) ≥ τ(B0) ≈ τ(Bs) > τ(Λb) >> τ(Bc) • Uncertainties are reduced in lifetime ratios • Easier to compare experiment/theory • Experimental and theoretical uncertainties are comparable • Lifetime differences probe the HQE to 3rd order in LQCD / mb • Goal: measure the ratios accurately

  3. Λb Lifetime • Lightest b baryon (udb) • Study exclusive and inclusive channels • Λb→J/ψΛ • Λb→μνΛcX • World average lifetime dominated by measurements in semileptonic decay channels • Previous results mostly from LEP and Run I Tevatron • Currently produced in high statistics only at Tevatron Testing Heavy Quark Expansion theory in three quarks system. D. Tsybychev, DPF 2006

  4. B hadrons at Tevatron • Tevatron produces B hadrons inaccessible at B-factories • Cross sections are large but so are the backgrounds • (bb)100b at 2 TeV ((bb) 1nb at B factories) • Large light quark background • Mostly soft pT processes : Triggers are very important • In Run II, Tevatron has two experiments doing B-physics : CDF and DØ Data samples for these analyzes D. Tsybychev, DPF 2006

  5. DØ Detector • Excellent coverage of Tracking and Muon Systems • 2 T Solenoid, polarity inversed weekly • High efficiency muon trigger with muon Pt measurement at Level1 by toroids • Low background Layer 0 SMT H-disks SMT F-disks SMT barrels D. Tsybychev, DPF 2006

  6. Layer 0 installed inside SMT in April 2006 The detector consists of: • 48 modules mounted on carbon fiber support structure at 1.7 cm radius • 6 f-segments, 8 z-segments • Four sensor types provide 98.4% of acceptance • Sensors 12 and 7 cm lengths D. Tsybychev, DPF 2006

  7. Λb→J/ψΛ Fully reconstructed Models of λ + mass distribution (signal and background) Simultaneous Mass & lifetime likelihood fit Λb→μνΛcX Large statistics Part of momentum lost due to neutrino Split data into VPDL bins Use binned χ2 fit to extract lifetime Λb p Λ π Λb Lifetime Measurement Strategy • Signal selection • λ + mass distribution (signal and background) • Extract lifetime D. Tsybychev, DPF 2006

  8. Event selection Select m, Pt > 2 GeV Reconstruct K0S Add track with Pt > 1 GeV to form Λ+c For a set of discriminating variables x1,…,xn construct P.D.F. fsi(xi)[fbgri(xi)] for signal (background) Combine different variables using likelihood ratios: Use following variables Pt (K0S) Pt of proton Pt (Λ+c ) m(m Λ+c) Isolation ofΛ0b candidate Λb in Semileptonic Decays Λ0bm-νΛ+cX K0sp p+p- D. Tsybychev, DPF 2006

  9. Λb in Semileptonic Decays • Obtain P.D.F. from data for signal and backgrounds • B0dmD-X(D  K0Sp) • B0smD-sX(D  K0SK) • Signal region and side bands • Fit with Gaussian for signal • 4th order polynomial for background • 4437±329 candidate events • m(K0Sp) = 2285.8±1.7 MeV/c2 • Width 20.6 ±1.7 MeV/c2 Estimated B0d K0SpX reflections D. Tsybychev, DPF 2006

  10. Measurements • Split data samples into different bins in visible proper decay length: • Lxy distance in transverse plane between primary vertex and Λb-decay vertex • Correct for K-factor • Obtain from MC • Fit peaks in (K0S p)mass spectra to obtain number of Λb candidates • Fix position and width to the values from full sample • Perform minimization of to obtain Λb lifetime D. Tsybychev, DPF 2006

  11. Λb in Semileptonic Decays • Distribution of expected number of events in each bin • P.D.F. for measured VPDL, with background fraction rcc • P.D.F. for signal is given by • With resolution function obtained from data and convoluted with Gaussian detector resolution D. Tsybychev, DPF 2006

  12. Consistency checks and Systematic Uncertainties • Fit procedure tested with full and toy MC • Consistency checked in statistically independent samples • Split sample by date, muon charge, muon direction D. Tsybychev, DPF 2006

  13. +0.12 t(Lb) = 1.28 (stat)±0.09 (syst) ps -0.11 Λb lifetimein Semileptonic Decays D. Tsybychev, DPF 2006

  14. Lb Lifetime in J/y L mode • New result with 4 times statistics from previous published DØ result • ReconstructLb J/y L • Select J/y in mm mode • Add L inpp • Proton pT > 2.4 GeV/c • Use process with similar topology Bd0 J/y Ks to measure the ratio D. Tsybychev, DPF 2006

  15. Lb Lifetime in J/y L mode • Fit simultaneously for both mass and lifetime • Lifetime PDF • Exponential decay function convoluted with resolution • Single Gaussian for prompt background • Exponential decay for long lived background • Negative and positive Exponential decays for combinatorial long lived background • Mass (PDF) • Single Gaussian for signal • Flat distribution for prompt background • 2nd polynomial for non-prompt background D. Tsybychev, DPF 2006

  16. Lb Lifetime in J/y L mode t(Lb) = 1.298 ±0.137(stat)±0.050(syst) ps t(Bd0) = 1.492 ± 0.075(stat) ± 0.047(syst) ps t(Lb)/t(B0) = 0.870 ±0.102(stat)±0.041(syst) D. Tsybychev, DPF 2006

  17. Conclusions • Two recentresults from DØ with about 1 fb-1 of data • Consistent with world average • The world average ont(Λb)/t(B0) measurements used to show ~2.5 σ lower value than the NLO prediction • Better agreement now • Recent theoretical developments F. Gabbiani et al Phys Rev D70, 094031 (2004) E. Franco et al Nucl Phys B633, 212 (2002) • With more data will be able to address discrepancy soon

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