Electroweak b physics at LEP

V. CiulliINFN Firenze

Introduction

Two electroweak observable in Zbb decays:

• Partial decay width: Rb = bb /had

measures

• Forward-bakward b asymmetry: AFB(b)\ measures

Often results given as

AbVb gg

22 )()( AbVb gg

22 )()(

2

AfVf

AfVff gg

ggA

The Electroweak FitGlobal fit probability ~

4.5%(28% without NuTev)

Measurements using Z QQ decays

• Rb anomaly solved long ago…

• Forward-backward b asymmetry pull > 2

Rb vs Rc (LEP+SLD)

Summer 1996 Summer 2003

Mt = 174.3 5.1 GeV

AFB(b) • Very stable over the years• Errors significantly reduced

– New analysis techniques– Full data reprocessing

Asymmetries at the Z pole• From fermion direction and helicity:

• With polarized beams

e- e+

f

f

e

f

ff

fe

A

A

AA

4

3)B(A)F(AA

A

4

3A

polpolpolFB

tot

RLpol

tot

BFFB

e-

e+

f

f

f

e

A

A

4

3)B(A)F(AA

A

LRLRLRFB

tot

RLLR

f = e,b,c

f = only

any f

f = e,b,c

AFB(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%

AFB (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

thrustlepQx cos

xAx

dx

dN

N FB3

81

8

31 2

OPAL final (2003)

AFB (b) with leptons - 2

• AFB diluted by mixing and bcl

• Both mixing and semi-leptonic BR’s measured on same data– Modeling effects, important for those measurements,

cancel out in AFB

• AFB(c) also measured (correlation < 20%)

• The measurement is largerly dominated by statistical error

Source A(b)(%) A(c)(%) BR’s 0.034 0.189 Detector 0.015 0.088 Lepton modeling 0.090 0.210 Bkg asymmetries 0.002 0.072 B and D physics 0.032 0.166 Mixing 0.132 0.113 Total 0.169 0.369

ALEPH final (2002)

0.00170.00410.0952 b)(AFB

AFB(b) with jet-charge• AFB measured from

forward-backward charge asymmetry

measured on data !

• Vertex and identified particles also used to tag the charge

2 2

AFB(b) with jet-charge - 2• Main systematics

– b purity (efficiencies from double tag methods)– hemisphere correlations (from MC, but much

less important than for Rb)

– charge tagging calibration

charge separation vs cosData vs MC

QCD corrections• Measured asymmetry must be corrected for

gluon radiation effects

Thrust axis quark direction

correction ~3.5% > AFB/AFB 1.7%

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

The six electroweak observables

measured at LEP and SLC are fitted together with

Taking into account statistical and systematical correlationsBR’s and mixing mostly correlated to asymmetriesCharm hadron production correlated to Rc

Rb, Rc, AFB(b), AFB(c), Ab and Ac

LEP + SLD Combination

BR(b l), BR(b c l), BR(c l), , f(D+), f(Ds), f(cbarioni), P(cD*+)xBR(D*+ D0+)

Results

Source AFB(b)(%) Statistics 0.15 Internal systematics 0.06 Common systematics 0.04 Total systematics 0.07 Total 0.16

parameter fit:2/d.o.f.=51/91(91/91 if only stat errors are used)

Inclusive: A0FB(b)=0.10000.0019

Leptons: A0FB(b)=0.10000.0025

A0FB(b)=0.09970.0016

AFB(b) vs centre-of-mass energy

Peak data only (+0.6):A0

FB(b)=0.10060.0017

Ab: are LEP and SLD compatible?

700186300

Hm

1.53.174 tm

• LEP only (indirect): Ab=0.8980.021

• SLD direct: Ab=0.9250.020

• Agree within 0.8 • LEP+SLD: Ab=0.9030.013 (0.935 SM)

• From all asymmetries (including quark ones):

• Final precision < 10-3

but average prob = 7%

• Most sensitive measurements ALR

AFB(b)disagree by 2.9 !( polarisation in the middle of the two)

sin2elfefp t

b-quark couplings anomaly?

Large deviations in the right coupling gRb only!

Difficult to explain…

Conclusions

• LEP b-asymmetry measurements almost final – 2nd iteration using reprocessed data and

improved techniques– only DELPHI inclusive still preliminary

• LEP results are consistent with SLD Ab

measurement• O(3) discrepancy between “hadronic”

and “leptonic” mixing angle measurements confirmed

• Interpretation of this electroweak result difficult

Backup slides