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Semileptonic B Decays at BABAR

Semileptonic B Decays at BABAR. Vladimir Golubev Budker Institute of Nuclear Physics, Novosibirsk, Russia (for the BaBar Collaboration). Beauty 2005 Assisi (Perugia), June 20 – 24, 2005. Semileptonic B Decays – Why are they Interesting. +. Semileptonic decays

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Semileptonic B Decays at BABAR

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  1. Semileptonic B Decays at BABAR Vladimir Golubev Budker Institute of Nuclear Physics, Novosibirsk, Russia (for the BaBar Collaboration) Beauty 2005 Assisi (Perugia), June 20–24, 2005

  2. Semileptonic B Decays – Why are they Interesting + • Semileptonic decays • theoretically simple at parton level • rate depends on CKM elements |Vcb| and|Vub|, the quark masses mb and mc • the leptonic current factors out cleanly, thus one can probe strong interactions in B mesons • sensitive to QCD corrections, OPE • A precise determination of |Vcb| and |Vub| with reliable errors important for • understanding B decay rates • testing of the unitarity of the CKM matrix and predictions of CP violation in B mesons Beauty2005, Assisi

  3. Semileptonic B decay studies at BABAR 2-nd B tagging: untagged, lepton tag, hadronic tag Beauty2005, Assisi

  4. Inclusive BXcln decays: HQE Expansions • Heavy Quark Expansions, tool to correct for QCD effects • Expansion in terms of 1/mb and as(mb) • Separate short-andlong-distanceeffects at m~ 1 GeV • Perturbative corrections calculable from mbmcas(mb) • Non-perturbativeparameters cannot be calculated • We choose calculation by Gambino & Uraltsev hep-ph/0401063 & 0403166 • Kinetic mass scheme to • Eℓ moments • mX moments • 8 parameters to be fitted • Measure 8 moments, each as a function of minimum lepton energy El kinetic chromomagnetic spin-orbit Darwin Beauty2005, Assisi

  5. Measurement of Electron Energy Moments BABAR Albert et al. Phys.Rev. D 69, 111104 (2004) 47 + 9 /fb • Inclusive e± spectrum in 3x106 electron-tagged (1.4<p*<2.3GeV/c) BB events • Corrected for detector effects • Corrected for non-prompt electrons – lepton charge correlation and MC • Corrected for B0 – B0 mixing • Moments for Ecut = 0.6 … 1.5 GeV • Corrected for the final state radiation • Translated to B rest frame • Subtracted B  Xuℓn decays 156,000 e± Partial BF Lepton Energy Moments Beauty2005, Assisi

  6. high mass charm states Mx2[GeV2/c4] Measurement of Hadron Mass Moments Aubert et al. Phys.Rev. D 69, 111103 (2004) BABAR 7114 Events Events/0.5 GeV2 • BB events tagged by a fully reconstructed hadronic B decays • e± or m± with El > Ecut(0.9…1.6 GeV/c) • Lepton charge – B flavor correlation • Improve mx measurement by kinematic fit to whole event, resolution ~350 MeV • To eliminate dependence of moments on uncertain BF and unknown masses of high mass charm mesons we calibrate mx measurement • Calibrate mx based on MC simulation • Linear relation between mxmeas and mXtrue • Validate calibration with excl. B D(*) ln • Moments corrected for detector effects and bkg D, D* mX2 (GeV2) mxmeas (Gev) Hadron mass moments Beauty2005, Assisi mXtrue (GeV)

  7. HQE Fits to Hadron Mass and Lepton Energy Moments Aubert et al. Phys.Rev.Lett. 93, 011803 (2004) ● = used, ○ = unusedin the nominal fit BABAR MX moments M1x M2x M4x M3x M0l M3l M1l M2l Red line: HQE fitYellow band: theory errors c2/ndf = 20/15 Eℓmoments Beauty2005, Assisi

  8. HQE Fit Results(kinetic mass scheme, scale m=1GeV) BABAR • Separate fits to hadron and lepton moments give consistent results • mG2 and rLS3 are consistent with B-B* mass splitting and HQ sum rules • Considerable improvement in precision for |Vcb| (±2%)and Bcln (1.6%) and quark masses, as well as HQE parameters Beauty2005, Assisi

  9. |Vub| from total B Xuln decay rate • Total b  u ℓ  decay rate from OPE: • |Vub| from inclusive BF(B  Xu l n) (theory error 5%): • Experimentally challenging due to charmed semileptonic background: Beauty2005, Assisi

  10. Charmed semileptonic background can be controlled in limited regions of phase space Non-perturbative Shape Functions are used to extract |Vub| from partial branching fractions. The SF parameters are extracted from b  sg photon spectrum New approach to |Vub| extraction: |Vub| from partial B Xuln decay rates • Differential decay rate parametrization from BLNP Bosch, Lange, Neubert, and Paz, Nucl. Phys. B 699, 335 (2004); Lange, Neubert, and Paz, hep-ph/0504071 • New source of information on SF parameters – moments of the b  c ℓ n lepton spectrum Neubert, Phys.Lett. B 612, 13 (2005) Beauty2005, Assisi

  11. Different types of partial decay rate measurements: - electron endpoint - Ee and q2 regions with neutrino reconstruction - Mx and q2 regions |Vub| from partial B Xuln decay rates hep-ex/0408075 hep-ex/0506036 hep-ex/0408068 q2(Gev/c)2vs. Ee(GeV) q2(Gev/c)2vs. Mx(GeV) q2=(pl+pn)2 Kinematic regions of BXcln decays Beauty2005, Assisi

  12. |Vub| from B Xuln endpoint spectrum On-peak data Off-peak data (On-peak)-(Off-peak) data Histogram – B XclnMC (On-peak)-(Off-peak) data Histogram – B XulnMC Differential B Xue nbranching fraction as a function of electron momentum, corrected for detector effects and final-state radiation (On-peak)−(Off-peak) data Histogram – B  XulnMC Signal extraction: subtraction of BB and continuum background Beauty2005, Assisi

  13. Endpoint spectrum analysis: |Vub| extraction • Determination of DBr for pe>pmin • Extrapolation to total charmless semileptonic decay Br using shape function parameters from B  Xsg spectrum and calculations of DeFazio-Neubert(*) and Kagan-Neubert(**) • Extraction of |Vub| using the OPE relation. • BABAR preliminary results, based on BELLE shape function parameters and DFN approach (hep-ex/0408075): DBr(pe>2.0GeV/c)=(0.5310.032stat0.049syst)×10-3 Br=(2.16 0.24exp 0.27SF)×10-3 |Vub|=(4.40 0.24exp 0.28SF 0.21OPE+mb)×10−3 • Analysis based on BLNP calculation is being completed (*) F.De Fazio and M.Neubert, JHEP 9906, 017 (1999) (**) A.L.Kagan and M.Neubert, Eur.Phys.J. C 7, 5 (1999) Beauty2005, Assisi

  14. Event selection: High energy electron Ee>2GeV Missing momentum used for neutrino parameters estimation Cuts on missing momentum magnitude and direction, and event shape Suppression of b®cenbyshmax=m2D: (slightly modified for B -meson motion in the U(4S) rest frame) q2 − Ee analysis with neutrino reconstruction Acceptance ~ 14%, signal/bkg.~0.6 Beauty2005, Assisi

  15. BABAR preliminary q2 − Ee analysis results Test of neutrino energy resolution using pure control sample of B®D(*)en events Signal extraction for Ee>2.0 GeV and sh<3.5 GeV2, BB background normalization for sh>4.25 GeV2, DBr(Ee>2.0GeV, sh<3.5GeV2)=(3.54±0.33±0.34) ´10-4 (*) B.O.Lange, M.Neubert and G.Paz, hep-ph/0504071 Beauty2005, Assisi

  16. Signal downfeed bcln + other Mx−q2 analysis • Select events with a fully reconstructed B  D(*)X decay (Breco) • Study charmless semileptonic decay of B recoiling against Breco - require lepton with p*lep > 1 GeV • 2D fit to measure partial BF in [Mx<1.7 GeV, q2>8 GeV2] hep-ex/0408068 Mx < 1.7 GeV 1.7 < Mx < 2.2 GeV 2.2 < Mx < 2.8 GeV 82 fb-1 Beauty2005, Assisi

  17. Mx−q2 analysis: the results BABAR preliminary (CKM 2005) • Mx spectrum unfolding and fitting: • Mx−q22-dimentional fit: (*) F.De Fazio and M.Neubert, JHEP 9906, 017 (1999) (**) Bauer, Ligeti and Luke, hep-ph/0111387 Beauty2005, Assisi

  18. BABAR preliminary Inclusive B Xuln decay summary Beauty2005, Assisi

  19. ISGW2 Ball’04 FNAL’04 HPQCD’04 |Vub| from exclusive B Xuln decays Strong nonperturbative effects included in form factors Calculations • Light-Cone Sum Rules: Ball,Zwicky (hep-ph/0406232) • Unquenched Lattice QCD: HPQCD (hep-lat/0408019) and FNAL (hep-lat/0409116) • ISGW2: quark model q2 = (plep + pn )2 Beauty2005, Assisi

  20. Different experimental approaches to measure BF: Exclusive decays, experimental techniques PURITY EFFICIENCY Untagged analysis Neutrino reconstruction B  ℓ , B  ℓ  Semileptonic Tags Recoil of B  D(*) ℓ n B  ℓ  Recoil of B  D(*)X 9 channels (p,r,,,’,a0) Hadronic Tags Beauty2005, Assisi

  21. Use BD(*)ln decays of opposite B to “tag” event Use events with BD(*)ln decays on both sides to check tagging efficiency Measure DB(B0p-l+n) in bins of q2 BF for B+p0l+n l D(*) v tag B signal B v p l Exclusive B pln decays, tagged analysis Discriminating Variables • B0 p-l+n: angle between the B momentum and the plane defined by the D(*)l and pl momenta: cos2fB • B+ p0l+n: angle between B and p0l system: cos B-pl Beauty2005, Assisi

  22. B plntagged analysis: the results BABAR preliminary (CKM 2005) B0p-l+n on-peak datasignal MCBrlv MCb ¨ ulv MCB0B0 MCB+B− MCoff-peak data 232 million BB pairs B+p0l+n Branching Fraction Results in 3 bins of q2 88 million BB pairs Beauty2005, Assisi

  23. Exclusive B p(r)ln decays, untagged analysis • Reconstruct neutrino from full event + ensure good reconstruction quality • Signal yield extracted from common fit to DE and mES in 5 q2bins for p+ln, and 3 bins for r-lnandr0lneach q2 resolution neutrino momentum scaled so DE=0  improved q2 resolution Beauty2005, Assisi

  24. Data Signal MC Comb. Sig. Crossfeed bcln qq Untagged B p(r)ln, signal extraction BABAR preliminary Bp-l+n Brln Branching Fraction Results 83 million BB pairs Beauty2005, Assisi

  25. BABAR preliminary (CKM 2005) Form factor fits and |Vub| extraction pln rln Untagged • The pln untagged sample begins to distinguish among FF calculations: ISGW II is disfavored: Prob(c2) = 3% • Other FF calculations tested are compatible with data Extraction of |Vub| Beauty2005, Assisi

  26. Summary • HQE fits to moments hadronic mass and lepton energy moments: Based on 80 fb-1, ~30% of the current BABAR data, we have made significant advances in the analysis of semileptonic B decays. Br(B Xcln)= (10.61 ±0.16exp ±0.06 HQE)% High precision measurement of mb and mc, and exp. determination of HQE parameters. • Inclusive B Xuln decays, three different approaches: The most precise determination of |Vub| Measurement of partial semileptonic branching fractions for different kinematic regions • Exclusive B p(r)ln decay modes in tagged and untagged samples: Determination of |Vub| Begin to test decay form factor calculations The analyses will be repeated for larger data samples and more precise shape function parameters derived from from B Xsg photon spectrum Beauty2005, Assisi

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