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The b semileptonic branching ratio in Z bb decays

The b semileptonic branching ratio in Z bb decays. Alessia Tricomi University of Catania and INFN Bari & Catania on behalf of LEP Collaborations. Inclusive semileptonic branching fractions. Motivations. Measuring BR(b  X l n )

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The b semileptonic branching ratio in Z bb decays

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  1. The b semileptonic branching ratio in Zbb decays Alessia Tricomi University of Catania and INFN Bari & Catania on behalf of LEP Collaborations

  2. Inclusive semileptonic branching fractions ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  3. Motivations Measuring BR(b  X l n) • Golden route to determine |Vcb| (M. Battaglia talk, CP2 session) • Test of the modelling of heavy hadron dynamics • Input to many HF analysis • Comparison to Y(4S) results Measuring BR(b  c  X l+n) • main bkg for BR(b  X l n) • also input to many HF analyses BR(bcXl+n) BR(b  X l n) |Vcb| Rb AbFB AcFB … ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  4. Analysis techniques Need to distinguish b X l- n from other lepton sources b c X l+n (wrong sign cascade) b c X l-n (same sign cascade) b t-X  l- c X l+n Bkg (leptons from J/y, gluon splitting, misidentified leptons) Several measurement techniques used Common to all LEP experiments: Divide hadronic events in two hemispheres Use lifetime info to tag b hemispheres high b purity samples (> 95%) Look for leptons in the opposite hemispheres ALEPH, DELPHI, OPAL DELPHI ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  5. The key issue First LEP analyses: only lepton spectrum info used to distinguish different components Large dependence on modelling of b fragmentation and decay Present LEP analyses: Use additional information to reduce model dependence (correlation of lepton charge with opposite hemisphere charge estimators) Model depedence reduced, but still sizeable New ALEPH result: Two separate analyses (1. lepton pT info, 2. charge correlation estimators) No NN combination used Good check of systematic effects New treatment of modelling Consistent results wrt usual treatment with completely different approach Fragmentation from ALEPH data No dependence on fragmentation model ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  6. ALEPH new b  X l n: pT analysis pT spectrum counting Weighted sum Relative contributions of cascade and direct BR TAG Hemisphere (B sample) Tight cut on lifetime-mass tag Lepton Analysis Hemisphere Identify leptons Count them and use the pT distribution to separate different components Study of lepton rate wrt pT allows BR(bl) and BR(bcl) to be fitted simultaneously ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  7. ALEPH new b  X l n: charge correlation analysis • TAG Hemisphere Lepton Analysis Hemisphere • P sample: high pT lepton (1.25 GeV/c) Lepton charge • J sample:loose lifetime-mass tag+jet charge cutQHEMI=f(jet ch., i.p. signif.) • Measure fraction of: • Opposite sign Same sign Fit numbers of opposite (same) charge lepton candidates nOC (nSC) to the expected fractions P P P J J counting fP,J& (1- fP,J ): sensitive to bl wrt bcl relative ratio.Can be expressed in terms of Pb= probability to correctly tag Qbdetermined from data using a double tag technique J pT information not used Extract pT spectrum Distinguishbl wrt bclusing charge correlation wrt other b hemisphere ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  8. ALEPH new b  X l nmeasurement 2 methods (3 tags used) “pT analysis”: the pT of the leptons respect to the jet axis used to separate components “charge correlation analysis”: the correlation between lepton charge and charge estimators (hemisphere charge or high pT lepton charge) in the opposite hemisphere gives info about their relative proportions Slightly worse statistical power Reduced dependence on the b  X l n decay modelling Mainly affected by the uncertainty in the rate of b  c X l-n Lepton spectrum extraction possible Larger statistical power But large dependence on b  X l n decay modelling and hence on the shape of the pT spectrum ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  9. ALEPH new b X l n: new approach for the b  l decay modelling New approach: Reweight MC for the measured rates Systematics: vary relative D/D*/D** fraction by their errors Since • The shape of the B0(+)  l spectrum • depends on the following rates: • BR(B Dln) = (1.950.27)% • BR(B D*ln) = (5.050.25)% • BR(B D*X ln) = (2.7 0.7)% • BR(B D1ln) = (0.630.11)% • BR(B D*2ln)= (0.230.09)% • Broad D** • Non resonant D** Bs and b baryon production rates reweighted to latest experimental results and uncertainties in the modelling accounted by enlarging the uncertainties for B BR by 25% • “Standard” treatment: • Fit to CLEO data with ACCMM, • ISGW and ISGW**. • Use • ACCMM Model for central value • ISGW and ISGW** for systematics ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  10. ALEPH new b X l n: decay modelling and fragmentation Energy spectrum of weakly decaying B taken from ALEPH data <Xb> = 0.716  0.006  0.006 [PLB 512 (2001) 30] No dependence on fragmentation model ! (K. Harder talk, HQ-1) “new spectra” in agreement with ACCMM one! • Usual LEP procedure for cl and bcl decay modelling • cl • Use the spectra given by a combined fit of ACCMM to DELCO and MARK III data (C.V. and  s) • bc l • Use the cl models to describe the second step in the process and vary at the same time the BD spectrum obtained by fitting Peterson on CLEO data Correlate the variations with <Xb> analysis ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  11. ALEPH b X l n & b  cX l n results Charge correlation analysis Transverse Momentum Analysis (%) (%) BR(bl) 11.07  0.07stat  0.13sys  0.44mod10.57  0.11stat  0.29sys  0.20mod BR(bcl) 7.52  0.10stat  0.44sys  0.35mod 8.30  0.16stat  0.21sys+0.12mod -0.16 • Two different strategies used to measure BR(bl) &BR(bcl), combined only a posteriori • pT analysis significantly affected by (bl) modelling • Charge correlation analysis less model dependent but suffers for the uncertainty on the rate of b Wcl decay • Agreement between the two results is a crucial consistency check In both cases new treatment for the bl modelling using measured rates of the different c hadrons in the final states Consistent results with standard LEP approach Results averaged using B.L.U.E. technique BR(bl) 10.70  0.10stat  0.23sys  0.26mod W(pT)=0.25 W(Qb)=0.75 BR(bcl) 8.18  0.15stat  0.22sys+0.10mod W(pT)=0.15 W(Qb)=0.85 -0.14 ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  12. LEP BR(b  X l n)average LEP EWWG fit ICHEP ‘02 10.65  0.23 +1s ISGW 11% D** central ACCMM tuned to CLEO data -1s ISGW** 33%D** uncertainty from the modellingof b l dominates (0.15 of 0.23 total error) • Global fit to Heavy Flavoursresults Rb, BR(bl), BR(bcl), BR(cl),c • common input parameter values and systematic definitions used by all experiments • B.L.U.E. technique: take intoaccount correlated systematics • sample composition • b and c lifetimes • B+, B0, Bs, Lb production fractions • gbb, gcc • b and c fragmentation • Lb polarisation • semileptonic decay models ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  13. BR(b X l n) B  X l-n (CLEO+ARGUS)* 10.38  0.32 B  X l-n (BABAR)prel 10.87  0.35 B  X l-n (BELLE)prel 10.90  0.52 B  X l-n (Y(4S)) 10.63  0.25 b  X l-n (LEP) 10.65  0.23 B  D l-n* 2.13  0.22 B  D* l-n* 5.05  0.25 B  D(*)p l-n* 2.26  0.44 B  D10 l-n X* 0.74  0.16 B  X l- v* < 0.65 90% CL B  Xu l-n 0.17  0.05 Total B exclusive 9.61  0.55 The sum of exclusive semileptonic BR agrees with the inclusive within 1.5s *PDG values ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  14. LEP b c  X l n average Global fit to Heavy Flavour results from LEPEWWG Large uncertainties from bl modelling Statistical error sizeable LEP EWWG fit ICHEP ‘02 8.04  0.19 ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

  15. Conclusions • LEP has measured BR(b  l) and BR(b  c  l) with several techniques BR(bl) 0.1065  0.0009stat  0.0015sys  0.0015mod (0.0026ALEPH mod) BR(bcl) 0.804  0.0012stat  0.0013sys  0.0009mod • Average results are consistent with Y(4S) measurements BR(BX l n)|LEP=BR(BX l n) x tB/tb = 0.1082  0.0023 BR(BX l n)|Y(4S)= 0.1063  0.0025 • A careful investigation of systematic errors due to modelling and fragmentation model has been done • New measurements from ALEPH which use a different approach for the (b  l) modelling and have no fragmentation model dependence give results consistent with the usual LEP treatment. This can be considered as a cross-check of the robustness of the analyses. ICHEP '02 - AMSTERDAM, 24-31 July 2002 Alessia Tricomi, ALEPH Bari

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