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Semileptonic B Decays at BaBar

Semileptonic B Decays at BaBar. Wolfgang Menges Queen Mary, University of London, UK Teilchenphysik Seminar, Bonn, 4 th May 06. Outline. Introduction & motivation CKM Matrix & Unitarity Triangle PEP-II and BABAR Why semileptonic decays? Measurements Inclusive b  cl n

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Semileptonic B Decays at BaBar

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  1. Semileptonic B Decays at BaBar Wolfgang Menges Queen Mary, University of London, UK Teilchenphysik Seminar, Bonn, 4th May 06

  2. Outline • Introduction & motivation • CKM Matrix & Unitarity Triangle • PEP-II and BABAR • Why semileptonic decays? • Measurements • Inclusive b  cln • Exlusive B  D*ln • Inclusive b  uln • Exclusive B  pln • Howit all fitstogether • Outlook / Conclusions |Vcb| |Vub|

  3. Vud VusVub Vcd VcsVcb VtdVts Vtb | Vub| e-i 1-c2 cA c3(-i) = -c1- c2/2A c2 Ac3(1- -i) -A c2 1 CP violation + O(c4) [ c= sinc ] | Vtd| e-i |Vub| B Decays – a Window on the Quark Sector • The weak and mass eigenstates of the quarks are not the same • The changes in base are described by unitarity transformations Cabibbo Kobayashi Maskawa (CKM) matrix • The only 3rd generation quark we can study in detail

  4. Unitarity Triangle Unitarity of VCKM • this is neatly represented by the familiar Unitarity Triangle • angles a, b, g can be measured with CPV of B decays • side from semileptonic B decays (|Vcb| and |Vub|)

  5. Unitarity Triangle Fits J/yK0: sin2 interference in BDK: g Bd mixing: md kaon decays: εK bul: |Vub| D*l: |Vcb| Bs mixing: ms / md charmless two-body: a http://ckmfitter.in2p3.fr

  6. Consistency tests • Compare measurements in the (r, h)-plane • If the SM is the whole story,they must all overlap • The tells us that this is true (status winter 2005) • But still large enough for New Physics to hide • Precision of sin2b from B factories dominates determination of h • Must improve the others to get more stringent tests • Left side of the UT is • Uncertainty dominated byca. 7% error on |Vub| Measurement of |Vub| is complementary to sin2b

  7. Experiments

  8. PEPII Asymmetric B Factory Collides 9 GeV e−against3.1 GeV e+ • ECM = 10.58 GeV = mass of U(4S) • Boostbg= 0.56 allows measurement of B decay times

  9. ElectroMagnetic Calorimeter 6580 CsI(Tl) crystals 1.5 T solenoid Čerenkov Detector (DIRC) 144 quartz bars 11000 PMTs e+ (3.1 GeV) e- (9 GeV) Drift CHamber 40 stereo layers Silicon Vertex Tracker 5 layers, double sided strips Instrumented Flux Return iron/RPCs/LSTs (muon/neutral hadrons) The BaBar Detector

  10. Data Taking Peak luminosity 1×1034/cm2/s BB production ~10 Hz 1 fb-1 -> 1.1 x 106BB Run 5 Datasets: Run 1/2: ~80 fb-1 Run 3/4: ~140 fb-1 Run 1-4: ~220 fb-1 Run 4 Run 5: collected: ~100 fb-1 expected: ~230 fb-1 Run 3 Run 2 Run 1 Belle: collected: ~560 fb-1

  11. KEKB and Belle Detector

  12. Physics

  13. c/u quark turns into one or more hadrons El = lepton energy q2 = lepton-neutrino mass squared mX = hadron system mass Semileptonic B Decays B → Xlvdecays are described by 3 variables Semileptonic B decays allow measurement of |Vcb| and |Vub| from tree level processes. Presence of a single hadronic current allows control of theoretical uncertainties.b  clv is background to bulv:

  14. |Vcb |

  15. free-quark rate |Vcb| & the „Atomic Physics“ of B Decays Inclusive rate described by Heavy Quark Expansion(HQE) in terms ofas(mb) and 1/mb • Contains b- and c-quark masses, mb and mc mp2 related to kinetic energy of b-quark mG2 related to chromomagnetic operator (B/B* mass splitting) • Darwin term (ρD3) and spin-orbit interaction (ρLS3) enter at 1/mb3 m= scale which separates effects from long- and short-distance dynamics r = mc/mb; z0(r), d(r): phase space factors; AEW = EW corrections; Apert = pert. corrections (asj, askb0)  Strategy: Global fit of all parameters  measure rate, moments (NB: Several HQE schemes exist; operators and coeff’s are scheme dependent)

  16. Inclusive Decays: |Vcb| from b  clv • Fit moments of inclusive distributions: lepton energy hadronic mass • Determine OPE parameters and |Vcb| • Measurements from B Factories, CDF, Delphi • We’ll look at an example: Hadronic Mass Moments

  17. v 2250 2000 1750 1500 1250 1000 750 500 250 0 lepton X Events / 1.8 MeV/c2 BABAR L = 81 fb-1 5.22 5.23 5.24 5.25 5.26 5.27 5.28 5.29 mES[GeV/c2] Hadronic Mass Moments BABAR PR D69:111103 • Select events with a fully-reconstructed B • Use ca. 1000 decay chains B D(*) + (np) + (mK) • Flavor and momentum of “recoil” B are known • Find a lepton with E > Ecut in the recoil B • Lepton charge consistent with the B flavor • mmiss consistent with a neutrino • All remaining particles belong to Xc • Improve mX with a kinematic fit (require mB2=mB1 and mmiss=0) • Yields resolution s (mX) = 350 MeV Fully reconstructedB hadrons

  18. D,D* high mass charm states Mx2[GeV2/c4] Validation: BABAR Hadronic Mass Moments BABAR PRL 93:011803 • Unmeasured particles  measured mX < true mX • Calibrate using simulation • Depends (weakly) on decay multiplicity and mmiss • Validate in MC after applying correction • Validate on data using partially reconstructed D *±  D 0p ±, tagged by the soft p ± and lepton • Calculate 1st-4th mass moments with Ecut = 0.9 … 1.6 GeV  input to HQE fit

  19. Fit Parameters • Calculation by Gambino & Uraltsev • Kinetic mass scheme to • Eℓ moments • mX moments • 8 parameters to determine: • 8 moments available with several Ecut • Sufficient degrees of freedom to determineall parameters without external inputs • Fit quality tells us how well HQE works Benson, Bigi, Mannel, Uraltsev, hep-ph/0410080 Gambino, Uraltsev, hep-ph/0401063 Benson, Bigi, Uraltsev, hep-ph/0410080 kinetic chromomagnetic spin-orbit Darwin

  20. l g W b b v s c Results exp HQE GSL Global fit in thekinetic scheme |Vcb| < 2% mb < 1% mc = 5% Buchmüller, Flächer: hep-ph/0507253 Use inputs from BaBar, Belle, CLEO, CDF & DELPHI: b->clv and b->sg • Based on: • Babar: • PRD69, 111103 (2004) • PRD69, 111104 (2004) • PRD72, 052004 (2005) • hep-ex/0507001 • Belle: • PRL93, 061803 (2004) • hep-ex/0508005 • CLEO: • PRD70, 032002 (2004) • PRL87, 251807 (2001) • CDF: PRD71, 051103 (2005) • DELPHI:EPJ C45, 35 (2006) b→sg b→clν combined

  21. G(w) known phase space factorF(w) Form Factor (FF)w = D* boost in B rest frame +0.030 -0.035 • F(1)=1 in heavy quark limit; lattice QCD says: F(1) = 0.919 Hashimoto et al, PRD 66 (2002) 014503 Exclusive |Vcb| and Form Factors Reconstruct B -> D*+ev as D*+ D0p+ with D0Kp BaBar hep-ex/0602023 • Shape of F(w) unknown • Parametrized with r2 (slope at w = 1) and form factor ratios R1, R2 ~ independent on w hA1 expansion a-la Caprini-Lellouch-Neubert Nucl. Phys. B 530, 153 (1998) -> measure form factors from multi-dimensional fit to diff rate -> measure |Vcb| with

  22. stat MC sys R1 = 1.396 ± 0.060 ± 0.035 ± 0.027 R2 = 0.885 ± 0.040 ± 0.022 ± 0.013 r2 = 1.145 ± 0.059 ± 0.030 ± 0.035 B  D*ln Form Factors and |Vcb| Using latest form factors with previous BaBar analysis: PRD71, 051502(2005) 1D projections of fit result hep-ex/0602023 Factor 5 improvement of FF uncertainty from previous CLEO measurement (1996). 80 fb-1 80 fb-1 |Vcb| = (37.6±0.3±1.3 ) x 10-3 +1.5 -1.3 Reducing FF error: 2.8% -> 0.5% Total sys error: 4.5% -> 3.5%

  23. Excl: |Vcb| = (40.9±1.0expF(1)) x 10-3 (D*lv) Incl: |Vcb| = (42.0±0.2exp±0.4HQE±0.6G) x 10-3 +1.1 -1.3 Summary of |Vcb| Results B->D*lv B->Dlv The new BaBar form factors and |Vcb| result are not included. Work is going on.

  24. |Vub |

  25. 2.31 2.64 Form Factor How to Measure b → uln • Need to suppress the largeb → cln background (×50) • Inclusive: • mu<<mc differences in kinematics • Maximum lepton energy 2.64 vs. 2.31 GeV • < 10 % of signal accessible • How accurately do we know this fraction? • Exclusive: • Explicit reconstruction of final-state hadrons • Bplv, Brlv, Bwlv, Bhlv, … • Need form factors(FF) to describe hadronization effects • Example: the rate for Bplv is • How accurately do we know the FF ?(for vector mesons 3 FFs)

  26. Not to scale! Inclusiveb → uln: Strategies Use kinematic cuts to separate bulv from bclv decays: Experimental resolution not included! • smaller acceptance -> theory error increase • OPE breaks down • shape function to resum non-pert. corrections • measure partial branching fraction DB • get predicted partial rate Dz from theory

  27. Lepton Endpoint BABAR hep-ex/0602023 • Select electrons with 2.0 < El< 2.6 GeV • Push below the charm threshold Larger signal acceptance Smaller theoretical error • Accurate subtraction of background is crucial! • S/B =1/15 for the endpoint El> 2.0 GeV • Measure the partial BF BABAR Data MC bkgd.b clv Data – bkgd. MC signalb ulv total BF is ~2103

  28. q2 (GeV2) b ulv b clv El (GeV) BABAR Extract signal normalize bkg El and q2 BABARPRL 95:111801 • Try to improve signal-to-background • Use pv = pmiss in addition to pe calculate q2 • Define shmax(El, q2) = the maximum mXsquared • Cutting at shmax < mD2 removes b  clv while keeping most of the signal • S/B = 1/2 achieved for El > 2.0 GeV and shmax < 3.5 GeV2 • Measured partial BF Smaller systematic errors

  29. mX and q2 BABAR hep-ex/0507017 • Must reconstruct all decay products to measure mX or q2 • Use (fully-reconstructed) hadronic B tag • Suppress b → c lv by vetoing against D (*) decays • Reject events with K • Reject events with B 0→ D *+(→ D0 p+)l−v • Measure the partial BF in regions of (mX, q2) • mX < 1.7 GeV and q2 > 8 GeV2: DB (10-4) = 8.7± 0.9stat ± 0.9sys (211fb-1)

  30. Theory for b → uln • Tree level rate must be corrected for QCD • Just like b →cln…, and with similar accuracy • …until limited experimental acceptance is considered • Poor convergenceof HQE in region where b →cln decays are kinematically forbidden • Non-perturbative Shape Function (SF) must be used to calculate partial rates m= scale which separates effects from long- and short-distance dynamics AEW = EW corrections; Apert = pert. corrections (asj, askb0)

  31. Shape Function – What is it ? Light-cone momentum distribution of b quark: f(k+) • Fermi motion of b quark inside B meson • Universal property of the B meson (to leading order); subleading SFs arise at each order in 1/mb • Consequences: changes effective mb smears kinematic spectra • SF cannot be calculated  must be measured Rough features (mean, r.m.s.) are known Details, especially the tail, are unknown

  32. Extracting the Shape Function • We can fit inclusive distributions from b→ sg or b → clv decays with theory prediction • Must assume a functional form of f (k+) • Example: • New calculation connects SF moments with b-quark mass mband kinetic energy mp2(Neubert, PLB 612:13) • Determined precisely from b→ sgand b  clv decays • from b→ sg, and from b  c lv • Fit data from BABAR, Belle, CLEO, DELPHI, CDF: • Use SF together with calculation of triple-diff. decay rate Bosch, Lange, Neubert, Paz (BLNP) to get|Vub| Buchmüller & Flächerhep-ph/0507253

  33. 1) BLNP: BaBar BXcl moments mb(SF) = 4.61 ± 0.08 GeV mπ2(SF) = 0.15 ± 0.07 GeV2 2) BLNP: Belle BXsg spectrum mb(SF) = 4.52 ± 0.07 GeV mπ2(SF) = 0.27 ± 0.23 GeV2 3) BLL: BaBar BXcl moments mb(1S) = 4.74 ± 0.06 GeV 1) |Vub|=(5.00±0.37exp±0.46SF±0.28theo)´10-3 +0.46 –0.38SF 4) DGE: HFAG average mb(MS) = 4.20 ± 0.04 GeV 2) |Vub|=(4.65±0.34exp ±0.23theo)´10-3 3) |Vub|=(4.82±0.36exp±0.46SF+theo)´10-3 4) |Vub|=(4.86±0.36exp±0.22theo)´10-3 |Vub| from Branching Fraction For converting a branching fraction into |Vub|the phase space acceptance is needed: Results of different calculations/ HQE parameters using the same partial branching fraction: Example for mx/q2 BLL – Bauer, Ligeti, LukeDGE – Anderson, Gardi

  34. Status of Inclusive |Vub| |Vub| world average winter 2006 |Vub| determined to 7.6% |Vub| [x10-3]: BLNP: Shape Function PRD72:073006(2005) 4.45  0.20exp 0.18SF 0.19theo Andersen-Gardi: DGE JHEP0601:097(2006) 4.41  0.20exp 0.20theo NEW! Numbers rescaled by HFAG. SF parameters from hep-ex/0507243,predicted partial rates from BLNP

  35. Summary

  36. b → sg Inclusive b → clv Eg El mX SSFs ShapeFunction HQE Fit mb Inclusiveb → ulv El Exclusive b → ulv |Vub| q2 B→plv w lv, hlv? mX duality FF WA LCSR LQCD unquenching How it all fitstogether Exclusive b → clv |Vcb| B→D*lv

  37. Conclusions • |Vcb| determined with high precision (2%)  Now |Vub| is important! • Precise determination of |Vub| complements sin2b to test the validity of the Standard Model • 7.6% accuracy achieved so far 5% possible? • Close collaboration between theory and experiment is important Inclusive |Vub| : • Much exp. and theo. progress in the last 2 years Exclusive |Vub| : • Significant exp. progress in the last year, but further improvement in B→p FF calculations needed (also new calculations for B → rln, B → wln, B → hln) • Accuracy of7-8% for exclusive |Vub| in the next few years (?) • Important to cross-check inclusive vs. exclusive results

  38. The Unitarity Triangle: 2004 2005 • Dramatic improvement in |Vub| • sin2b went down slightly  Overlap with |Vub/Vcb| smaller

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