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Electroweak b physics at LEP V. Ciulli INFN Firenze.

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Presentation on theme: "Electroweak b physics at LEP V. Ciulli INFN Firenze."— Presentation transcript:

1 Electroweak b physics at LEP V. Ciulli INFN Firenze

2 Introduction Two electroweak observable in Z  bb decays: Partial decay width: R b =  bb /  had  measures Forward-bakward b asymmetry: A FB (b)\  measures Often results given as

3 The Electroweak Fit Global fit probability ~ 4.5% (28% without NuTev) Measurements using Z  QQ decays R b anomaly solved long ago… Forward-backward b asymmetry pull > 2 

4 R b vs R c (LEP+SLD) Summer 1996Summer 2003 M t = 174.3  5.1 GeV

5 A FB (b) Very stable over the years Errors significantly reduced –New analysis techniques –Full data reprocessing

6 Asymmetries at the Z pole From fermion direction and helicity: With polarized beams e-e- e+e+ f f  e-e- e+e+ f f  f = e ,b,c f =  only any f f = e ,b,c

7 A FB (b) measured by means of two different techniques b-tagging from semi-leptonic b decays  b quark/anti-quark separation from lepton charge Inclusive b-tagging (impact parameter, secondary decay vertex, etc…)  b quark/anti-quark separation from jet-charge, e.g. The two measurements are systematically uncorrelated Statistical correlation in the same experiment: 25% - 40%

8 A FB (b) with leptons Lepton momentum and transverse momentum separate different sources b-quark direction measured from event thrust, signed by the lepton charge Asymmetry measured from fit to the angular distribution Separation is improved in recent measurements by the use of more variables and neural networks OPAL final (2003)

9 A FB (b) with leptons - 2 A FB diluted by mixing and b  c  l Both mixing and semi-leptonic BR’s measured on same data –Modeling effects, important for those measurements, cancel out in A FB A FB (c) also measured (correlation < 20%) The measurement is largerly dominated by statistical error ALEPH final (2002)

10 A FB (b) with jet-charge A FB measured from forward- backward charge asymmetry  measured on data ! Vertex and identified particles also used to tag the charge  2   2

11 A FB (b) with jet-charge - 2 Main systematics –b purity (efficiencies from double tag methods) –hemisphere correlations (from MC, but much less important than for R b ) –charge tagging calibration charge separation vs cos  Data vs MC

12 QCD corrections Measured asymmetry must be corrected for gluon radiation effects Thrust axis  quark direction correction ~3.5% >  A FB /A FB  1.7%

13 QCD corrections Bias due to event selection –Leptons: 50-70% of total correction –Jet-charge: ~25% only (corrections almost completely absorbed into charge correlations between the hemispheres) Lepton momentum

14 The six electroweak observables measured at LEP and SLC are fitted together with Taking into account statistical and systematical correlations BR’s and mixing mostly correlated to asymmetries Charm hadron production correlated to R c R b, R c, A FB (b), A FB (c), A b and A c LEP + SLD Combination BR(b  l), BR(b  c  l), BR(c  l), , f(D + ), f(D s ), f(c barioni ), P(c  D* + )xBR(D* +  D 0  + )

15 Results  parameter fit:  2 /d.o.f.=51/91 (91/91 if only stat errors are used) Inclusive: A 0 FB (b)=0.1000  0.0019 Leptons: A 0 FB (b)=0.1000  0.0025 A 0 FB (b)=0.0997  0.0016

16 A FB (b) vs centre-of-mass energy Peak data only (+0.6  ): A 0 FB (b)=0.1006  0.0017

17 A b : are LEP and SLD compatible? LEP only (indirect): A b =0.898  0.021 SLD direct: A b =0.925  0.020 Agree within 0.8  LEP+SLD: A b =0.903  0.013 (0.935 SM)

18 From all asymmetries (including quark ones): Final precision < 10 -3 but average prob = 7% Most sensitive measurements A LR A FB (b) disagree by 2.9  ! (  polarisation in the middle of the two) sin 2  e l f e f p t

19 b-quark couplings anomaly? Large deviations in the right coupling g Rb only! Difficult to explain…

20 Conclusions LEP b-asymmetry measurements almost final –2 nd iteration using reprocessed data and improved techniques –only DELPHI inclusive still preliminary LEP results are consistent with SLD A b measurement O(3  ) discrepancy between “hadronic” and “leptonic” mixing angle measurements confirmed Interpretation of this electroweak result difficult

21 Backup slides

22 QCD corrections C QCD (b) = 0.0354±0.0063 C QCD (c) = 0.0413±0.0063

23 LEP + SLD Combination Same inputs used by all experiment for: –Fragmentation –Gluon splitting into b and c quark pairs –Charged particle multiplicity in b and c decays –Lifetimes –Charm decay BR’s –b semi-leptonic decay modeling –Momentum correlation between hemispheres –Light quarks backgrounds

24 Time evolution of A LR and A FB

25 Higgs mass

26 Consequences of new top-mass combination

27  sin 2  e l f e f p t =+0.00003  pull  +0.2  - 0.1

28 Fit constraints in m t vs m H plane Error bands from most sensitive observables

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