Forward/Backward asymmetry of������

events at the �P.ANTILOGUS

Universite ClaudeBernard deLyon,IPN Lyon,IN2P3-CNRS,F-69622VilleurbanneCedex, FranceE-mail: Pierre.Antilogus@cern.ch

Forward/Backwardasymmetryof ���� events(� � ���� ) providesthemostprecisemeasurement

of ��������������� ���!"! atLEP I. In this notewewill review :# Thedifferenttechniquesusedto measure� � ���� .# Therecentimprovementsimplementedby theLEPexperimentsin their �$� ���� analysis.# Thecorrectionsappliedto � � ��%� to extract ��������� ����� ��&!'! .

The valueobtainedfor �����(�)�����*�+��&!'! from � � ���� is in favour of a high valuefor the Higgsmasscomparedto what is obtainedwith otherobservableslike �-,%. or /10 . This differenceiscommented.

1 Introduction

Particlesnot producedat LEP, like thetop or theHiggs,canhave measurableeffectsonelectroweakobservablesthroughtheradiativecorrectionsthey induce.In thesamewaythanLEPwasableto predictthetopmasswith aprecisionof 2 10 3-465 beforeitsdiscoveryat theTEVATRON, thepresentelectroweakdatagive throngconstrainsontheHiggsmass(798 = 88 :<;>=? =>; 3-4"5 , 798A@ 196 3-4"5 at 95%cl) B . All measurementssensitive to the fermionscouplingsto the � play an importantrole in thesecon-strains.Theasymmetricforward-backwardproductionof fermionsin the � decaysareamongthem.In this paperthemeasurementof theforward-backward

�asymme-

try performedat LEP I will bepresented.It providestoday, with 7DC and E$FHG , themainconstrainon 7 8 .

1.1 Fermionsasymmetriesand I JLKNMPO Q�RTS+URTV�VIn the StandardModel (SM) the differential cross-sectionin function of the polarangle O betweenthe W ? andthefermion X directionsfor theprocessW : W ?ZY X X at[ \^] 7D_ is `ba`<c6d

IeO 2gfihc6d

I M Ojh kl E VHm nobp c"dIeO

qAll theresults/numbersquotedin thisnoteareupdatedto their summer2001values

1

with

E VHm nobp ] lr<s R s Vwhichcanbeexpressedin termsof therealpartof theeffectivevectorandaxial-vectorneutralcurrentcouplingsof fermion X , tbu%v andt B v :

s V ] w t u%v t B vtbu6v M hxt B v M] w

y{z vy"| vy{z vy"| v M h}fIn the caseof the left-right crosssectionasymmetry( E FHG ) or ~ polarisation

measurements,theresultscanbedirectly expressedin termof s R alone.

At first I>J�K�MPO Q�RTS+URTV�V was introducedfor � pole analysisas the ratio of effectivevectorandaxial-vectorcouplingsof � , including loops,for theon massshell � Y� : � ? vertex : t u6�

t B �] f � r I>J�K�MPO Q�RTS+URTV�V

Thenfor eachfermion, I JLKNMPO VRTV�V is:

t u%vt B v

] f � rj��� V � I JLK�M$O VR�V%VWithin theSM I JLK�M$O VR�V%V canbe correctedto I>J�K�MPO Q�RTS+URTV�V . All the measurements,likeasymmetries,expressedin termof ratioof theseeffectivevectorandaxial-vectorcou-plings,canbetranslatedin a valueof I JLKNMPO Q�RTS+URTV�V . It canbenoticedthan I J�K(M$O Q�R�S URTV�V , intheSM, is themostsensitivequantityto 798 . For examplewith 798�2�f{�H��3-4"5

� 7D87 8 2 rHr �

� I JLK(M$O Q�RTS+UR�V%VI JLK�M$O Q�RTS+UR�V%V

to becomparedto � 7 87 8 2�f l �H�

� 7DC79C

All fermionsasymmetriesdon’t have thesamesensitivity to I J�K�MPO Q�R�S URTV�V ,7D8 . s1� forthe quarksis large ( s�� 2 0.93 and s1� 2 0.67 ) comparedto s Q for the leptons(s Q 2 0.15). For this reasonthe quarks,andmorepreciselythehigh

�quarkasym-

metry, provide a bettersensitivity to I>J�K�MPO Q�RTS+URTV�V :� E$� �obp 2�f�� k � I J�K(MPO Q�RTS+URTV�V ,

� E � �o�p 2r � r � I J�K(MPO Q�RTS+URTV�V and� E$� �obp 2������ � I>J�K(MPO Q�RTS+URTV�V . Neverthelessthesensitivity to I>J�K�MPO Q�RTS+URTV�V for

2

E$� �obp comesfrom s R . In the SM s � is somehow saturated/almostindependentoftheradiativecorrection: it is 2 7 timelesssensitiveto achangein I JLK�M$O Q�RTS+UR�V%V thans R .

FromtheSM modelpointof view E$� �obp , E$� �obp , the ~ polarisationand E F)G measurethesamethings: thevalueof I>J�K�MPO Q�RTS+URTV�V throughs Q or s R accordingto leptonuni-versality.

1.2 TheI J�K�MPO Q�R�S URTV�V measurements

In regardof theprevioussection,thedifferentvaluesof I>J�K�MPO Q�RTS+URTV�V extractedform thedataarenot reallysatisfactoryin theSM framework. As shown in figure1, thereis apooragreement( 2 w � � %) betweentheleptonicmeasurementsof s Q (like in E$� �obp or

E$FHG ) and the indirect hadronicmeasurementsof s Q (like in E � �o�p ). Fixing s Q toits leptonicmeasurementsunderlinefurtherthis point : the

�couplingsextractedthat

wayarequiteaway from theSM prediction(seefigure2).

2 E$� �obp ( E$� �obp ) measurements

2.1 Experimentaltechniques

To performthe E � �o�p measurementthefollowing informationsarerequired:

� A flavour tagto selectthe�P� � �

event

� Thepolarangle/”axis”of the��� � �

production

� A chargetagto signthis��� � �

“axis” alongthe�

direction

Amongthequarksonly the�

and � quarkscanbeeasilytagged.Many methodsto tag� decaysin � �� or� � �

quarkshavebeendeveloped.They rely ondifferentpropertiesoftheheavy quarksproductionanddecay:

� The�

hadronshave a long fly distance( ���&~ � 2 w � mm). Thecharm,presentin the

�decayproducts,further increasesthedistancebetweenthe interaction

region and the secondaryvertices. Suchdecaychain gives verticesclearlyshiftedfrom the primary vertex andproducestrackswith high impactparam-eters(seefigure4). Typical working point in E$� �obp analysisusinglifetime tagcorrespondsto anefficiency in

� 2 0.7- 0.9for apurity 2 0.7- 0.9.Thiswork-ing point is slightly differentto theoneusedin � � analysisfor which a higherpurity is required.

3

sin2θlept

eff

<LEP + SLD>Summer 2001

0.23152 �

± 0.00017

A�

FB b-quark 0.23226 �

±�

0.00031

A�

FB  c-quark 0.23272

�±�

0.00079

<¡ Q¢

FB  > 0.2324

�±� 0.0012

A l Pol.(τ£ )¤ 0.23137 �

± 0.00033

AFB  leptons 0.23099

�± 0.00053

SLD A¥

LR¦ ,A§ l 0.23098

�±�

0.00026

χ¨ 2/dof=12.8/5; Prob(©

χ¨ 2)=.025ª

m« t = 174.3 ±�

5.1 GeV

∆α¬ had­ = 0.02761 ± 0.00036

10 2®

0.23¯

0.231¯

0.232¯

0.233¯

sin° 2θ± lept

eff

mH [

Ge

V]

150200 mt GeV

Figure 1: Valuesof ��������������� ��&!'! extracted fromdifferent measurements.If in average this sub-sampleof electroweak observablespredicts aHiggsmassslightly above100 ²´³Tµ , themasspreferred by each measurementcover a largerange. The main discrepancyis observedbe-tweenthe � ,%. measurementat SLC and the� � ���� measurementat LEP I. Theoverall com-patibility betweenall these measurementsisonly2.5%.

-0.35

-0.33

-0.31

-0.29

-0.54 -0.52 -0.50 -0.48

gAb¶

g Vb

Preliminary

68.3·

95.5 99.5 % CL

SM

Figure2: Valuesof ¸ q{¹ and ¸6º ¹ extractedfrom» � , ¼-½ (¾D¸ �º ¹+¿ ¸ �q ¹ ) and� � ���� (¾D¸ º ¹ÁÀ ¸ q ¹ ).TheSMis excludedat almost99.5% cl.

4

0Â

2500

5000Ã7500Ä

10000Å12500Å15000Å

10Å

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bÉ

→Ê cË →Ê lÌb

É→Ê l

fake lÍcËudsÎData

L3

Muon momentum (GeV/c)

Nu

mb

er o

f m

uo

ns

0Ï

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0Ï

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6Ñ

bÖ

→× cØ →× lbÖ

→× lÙfake l

ÚcØudsÛData

L3

Muon transverse momentum (GeV/c)

Nu

mb

er o

f m

uo

ns

Figure3: Distribution of the Ü momentum,Ý , (left) and transversemomentumrelativeto the closestjet,Ý�Þ , (right) for differentsources.Dueto thehard fragmentationandhighmassof the � , anexcessof events

at high Ý or Ý Þ from �àß�Ü is observed.In the Ü� � ���� L3 analysisá the following cutsare applied:ÝjâäãH²´³Tµ À{å and Ý�Þæâèç{²´³�µ À{å .

� The é and ê hadronshave high semi-leptonicbranchingratios( 2 10% forpromptdecayin � or W ), they takemostof thebeamenergy ( @�ëèì}íjîï2}�N�*�and @�ëèì�íjð2��N� � ), and,dueto theirhighmass,they produceleptonswith ahightransversemomentum(ñ(ò ). For thesereasonssemi-leptonictaggingusingthe ñ andñ(ò of theleptonscangivepuresampleof

�or � quarks(seefigure3).

� The charmcontentof�

and � eventscan be directly identified by exclusivereconstructionof êèó"ôHê decays.The energy andfly distanceof theseêèó'ô�êallow adistinctionbetween

�and � events.

The usualchoicefor the polarangleis the thrustaxis. This choiceinducesspecificQCDcorrectionsbut doesn’t resultin any significantsystematics.Thechargeof theinitial quarkcanbeextracted:

� in a exclusiveway usingthecorrelationbetweenthequarkandits decayprod-ucts(

� Y W ? hxë , � Y êèó : , etc).

� in aninclusivewayby ajet chargemethodwherethesignof thequarkchargeisestimatedby a momentumweightedmeanof thechargedtrackspresentin theForward/Backwardhemispheres:

�öõP÷ îeø ] õ$÷ îeø�ù+úTûjüú ô õP÷ î<ø´ûjüú whereýis typically chosenbetween0.5 and1 (seefigure5). This methodusesthe

factthantracksof highmomentumin a�

jet aremorelikely comingfrom the�

decayitself.

5

10-6

10-5

10-4

10-3

10-2

-8 -6 -4 -2 0 2þ

4 6 8tagging variable B

rate

V

V

OPAL1994 dataMonte Carlo bÿMonte Carlo cMonte Carlo udsÿ

Figure4: ThisOPAL � -tag variablewhich includesdecay length significanceof a secondaryvertex,showsat high value a strong � enrichment. Theeventswith a � -tag valuebelowzero are the resultsof resolutioneffects.This last typeof eventscanbeusedto tunethesimulationto thedataresolution.

0.5

1

-2 0 2

dN/d

Q FB

<QFB>QFB

δf

σf

σf

σFB�

0

0.5

1

-2 0 2Qx

dN/d

Q TOT

sum

quark backward

quark forward QTOT

σf

σf

Figure5: Threeexperimentalobservablesare usedin jet charge analysis:

����������� � ��� whichholdsinformationsontheasymmetryandthechargeseparation ( ! ) ,

� ��&� ����� ¿ ��� which holdsinformationsonchargebiasandresolution( ! ) and���������

(not plottedhere) which holdsinforma-tionsonthechargeseparationandcorrelation.Suchdistributions allow to perform a fine tuning of thesimulationbefore extractingthe � asymmetryitself.

� in a semi-inclusiveway by selectinga subsampleof trackswith a givenprop-erty: for examplewith all thetracksassociatedto a secondaryvertex a “vertexcharge” canbe built holding informationon the charge of the particleat theorigin of thisvertex.

2.2 Improved E � �o�p measurements

Many combinationsof thedifferentflavour andchargetaggingtechniquesdescribedin the previoussectionhave beenused. In this sectionthe last developmentsin theusageof thesetoolswill bepresented.

New lepton analysis

In leptonanalysisa few observableshavebeenusedon topof thepureñ ,ñ ò tags.Im-proved

�samplepurity hasbeenobtainedusingmissingenergy/neutrinotag(ALEPH= ,OPAL � ) or

�lifetime tag(ALEPH = ,DELPHI M ,OPAL � ). Betterchargetagginghas

6

Figure6: Thedistribution of the � -tag variableusedin theALEPHinclusiveanalysisshowsbeforetuningof the simulationa clear excessof eventsin the �region (shadowedarea).

0.7

0.72

0.74

0.76

0.78

0.8

0.82

0.84

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

w b�

wb b-simulation

wb DATA

single tagDELPHI�

0.86

0.88

0.9

0.92

0.94

0.96

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

cos(Θthrust� )

w b�D

wbD b-simulation

wbD DATA

double tag

Figure 7: The probability to identify correctly thechargeof the � quarkin theDELPHI inclusiveanal-ysisfor eventswhere only onehemisphere hasbeentagged (upperplot) and whenthe two hemisphereshavebeentaggedwith oppositecharge (lowerplot).The differencebetweendata and simulationshowsthedifficulty for thesimulationto describecorrectlyall the informationused. Thedirect calibration inthedataof thecharge tagging powerovercomesthisproblem.

beenreachedusingdifferencesbetween� Y �L��� Y���? and

� Y �L��� Y�� : in ob-servableslike the jet chargeof theoppositehemisphereto the lepton(DELPHI M ) orthe energy taken by the tracksnearerthe lepton(OPAL � ). If all theseobservablesimprovedthestatisticalprecisionof theresults,they requirein generalspecificmea-surements/calibrationsto keepthesystematicsundercontrol.

Suchimproved techniquesdecreasedthe statisticalerror up to 20% (50%) forE$� �obp ( E$� �obp ) with in generalconstantif not bettersystematics.

New inclusive analysis

ALEPH ; andDELPHI � provided in 2001resultswith highly inclusive techniques.Theinterestof theseapproachesis to measuredirectly in thedataboth,the

�sample

purity and the charge separation,usingmore of the available information than theusual“

�-tagging+ jet charge” techniques.

For thesamesample,thetotalerroron E$� �obp is reducedby 1.2in DELPHI and1.7in ALEPH comparedto the classicaljet chargeanalysis.Theseresultsareobtainedby :

� addingcharge/�

flavour informationslike secondaryvertex chargeor lepton/Kidentificationcombinedwith their reconstructedñ ,ñ�ò andcharge.

7

0

5000

10000

15000

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25000

30000

35000

40000�

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

DELPHI

Jet Charge distribution for the Barrel ( b-tagging )�

Num

ber

of le

pton

can

dida

tes

�

0

5000

10000

15000

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30000

35000

40000

-1 -0.75 -0.5 -0.25 0 0.25 0.5�

0.75�

1

DELPHI

Jet Charge distribution for the Barrel ( b-tagging )

Num

ber

of le

pton

can

dida

tes

�

Figure8: Distribution for � -taggedeventin theDELPHI leptonanalysisof “the jet-charge oppositehemi-sphere”

�“the chargeof thelepton”. Promptleptonsfrom � produceeventswith a negativevaluefor such

product: thetwo hemisphereshavea � with oppositecharge. Nevertheless� � � mixingor � ß å ß����events(quotedas“ �$ß�� ,wrong” in theplot) mayendup to eventswith a positivevaluefor such product.Figureon theleft : before calibration of thejet charge shape. Figureon theright : after calibration.

� “distinguishing” betweenthe different é hadronsto take advantageof differ-encesin their decay/chargeproperties: é : hasa goodvertex chargeinforma-tion insteadé n

hassomechargeinformationin thefragmentationtracksto tagthesignof the

�/� �

quark.

The E � �o�p obtainedwith suchanalysishave statisticalcorrelationswith other E$� �obpmeasurementsperformedonthesamedatasample.Suchcorrelationshavebeentakeninto accountin the LEP average,for examplea statisticalcorrelationof 2 20% isobservedbetweensuchanalysisandleptonanalysis.

Calibration in the data

Of coursethesimulationcannotdescribeproperlyall thevariablesusedin theseanal-ysis : samplecompositionor charge separationhave to be measureddirectly in thedata. The interestin the analysisdescribedabove rely not only on their improvedstatisticalsensitivity but evenmoreon their relative low sensitivity to theavailabilityof a precisesimulationdescription.

Forexampleall LEPexperimentsobservea 2 w � � %data/MCdiscrepancy in thedistributionof the

�-taggingvariable(seefigure6). To extractthesamplecomposition

in differentbinsof�-tag,ananalysisof thenumberof single( 2� V ) /double( 2! M V )

�-

8

0"0.02"0.04"0.06"0.08" 0.1"

0"

0.1"

0.2"

0.3"

0.4"

0.5"

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0.7"

0.8"

<δ>

<δ>

<δ> thrust>0.90

<δ> no jetcharge

<δ> thrust>0.90 + no jetcharge

<δ> correlation (single tag)

-0.02

0"0.02"0.04"0.06"0.08" 0.1"

0"

0.1"

0.2"

0.3"

0.4"

0.5"

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0.7"

0.8"

flav#

hem$

<β>

<β>

<β> thrust>0.90

<β> no jetcharge

<β> thrust>0.90 + no jetcharge

<β> correlation (double tag)

DELPHI%

Figure9: Thehemisphere charge correlationaspre-dictedbythesimulationwhenonehemisphere (upperplot) or twohemisphereswith oppositecharge(lowerplot) are tagged. The correlationsare plotted as afunction of the neural net variable usedto select �with a definedcharge. Besidethe full flavour net-work (point) resultsusingmodifiedflavour networkareshown.

taggedhemispherescanbeperformedto extracttheefficiency ( V ) to tageachflavourX .In theDELPHI leptonanalysis,thejet chargeof theoppositehemisphereto the

lepton is used. The shapeof the jet charge (a� , � � ) like always is measuredin the

data(seefigure 5). The calibrationgives,almostby construction,a gooddata/MCagreement(seefigure8) for theoverallsamplebut alsofor subsampleswith differentlepton-

�chargecorrelationselectedby agiven ñ ,ñ(ò cuts.This lastpoint is interesting

asit indicatesthantheorigin of thedifferentleptonsis well understood.On importantpoint in the new ALEPH/DELPHI inclusive analysisis the mea-

surementof thechargeseparationin thedatathemself. In DELPHI only hemisphereswith a good

�chargeseparationareused.Thechargetaggingpower is measuredin

thedataby a single/doubletagtechniques.Up to 93%right chargetagginghasbeenobtained(seefigure7). In ALEPH all

�-taggedeventshave beenusedanda charge

separationup to 74%hasbeenobserved.In this lastanalysisaclassical(seefigure5)calibrationof theshapeof thejet chargehasbeenperformed.

In analysisusingsuchkind of self-calibration(for thejet charge,�-tagging, ... )

thedominantsystematicsarein generalstill comingfrom whathasto betakenfromthe simulation. For examplethe hemispherecorrelationsor the behaviour of non-

�eventsinducethemaincontribution to thesystematicsin theinclusiveanalysis.

Neverthelesslots of efforts have beendoneto definecorrectlythe sizeof suchsystematics.For example(seefigure 9) in the new DELPHI inclusive analysisthesourceof the charge correlationbetweenthe hemisphereshasbeeninvestigatedinthe simulationandidentifiedto the jet charge information. Suchcorrelationin thejet chargebeingwell understoodandstudiedin dataasshown in thepurejet charge

9

Table1: Correctionsappliedto theQCD cor-rectedasymmetry(�'&)()*��� ) :�,+ �%� � �-&.(�*��� ¿ /10 >� ���.2 /

Source� E$� �obp � E$� �obp[ \^] 7 _ -0.0013 -0.0034

QEDcorr. +0.0041 +0.0104�43b� � -0.0003 -0.0008Total +0.0025 +0.0062

Table2: Thefollowing reductioncoefficients, 5 ½ , of thetheoretical QCD correction, 687 q:9½ , havebeenestimated(exampleof summer97 analysis): 6'; � q:<½ � 5 ½ 6 7 q:9½and � ; � q:<�=?> � 9��� � 0 çe�@6 ; � q:<½ 2 �-&.(�*���

Exp. �$ôHW ê ó JetCh.ALEPH .74A .07DELPHI .52A .06 .46A .14 .24A .46OPAL .69A .13 .29A .13 .36A .32

DELPHI analysisB , a systematicassociatedto this hemispherecorrelationcan becorrectlydefined.

3 The corrections : E$� �obp Y E � m nobpTo extractfrom themeasuredasymmetriesthepoleasymmetry/I>J�K MPO Q�RTS+URTV�V , correctionshave to beapplied(seetable1) :

� energy shift from 91.26 3-465 (energy usedto averagethe E � �o�p ) to 7 _� initial stateradiation

� � and� � interference

� QCDcorrections(doneseparatelyfor eachmeasurement)

Theeffect of theQCD correctionsreceived lots of attentionover the lastyears.This effort convergedonly in 1999.Themainconclusionof thesestudiesis thanthevisibility of the hardgluon emission,which dominatethe theoreticalQCD correc-tions, is a function of the experimentalmethodusedto extract the asymmetry. Forexample

� experimentalcuts( ex : lowercutontheleptonmomentumremoveeventswithhardgluon)

� eventweight( ex : eventswith hardgluonmayendup in “area” of high back-groundandgeta lowerweightin the E � �o�p extraction)

generatechangesin the QCD correctionsto apply. As shown table 2 the scalingdown of theQCD correctionsdueto theseeffectsis not negligible. For this reason,even if the theoreticalQCD correctionsfor the

�( � ) is C�D�BFE� ] �N� � l ���GA �N� �H���IH

( C�D%B?E� ] ��� � r f l A ��� ���IJ l ), the commonerror in the LEP averagefrom the QCDcorrectionsis only 2 0.0002for E � m no�p ( 2 0.00005for E � m nobp ).

10

AFB

0,bb_

LEPK

Summer 2001<L AM

FB

0,bb_

>N =0.0990 O

±P

0.0017

OPAL jet-chQ

☞1991-950.1028 O

±P

0.0049 ±P

0.0046

L3 jet-ch ☞1994-95

0.0949 O

± 0.0101 ± 0.0056

DELPHI NNR

✍1992-950.0995 O

±P

0.0036 ±P

0.0022

DELPHI jet-ch ☞1992-95

0.1011 O

± 0.0044 ± 0.0015

ALEPH jet-chM

☞1991-950.1012 O

±P

0.0025 ±P

0.0012

OPAL leptonsQ

✍1990-950.0938 O

± 0.0040 ± 0.0022

L3 leptonsK

☞1990-950.0999 O

±P

0.0060 ±P

0.0035

DELPHI leptons ✍1991-95

0.1012 O

± 0.0052 ± 0.0020

ALEPH leptonsM

✍1991-950.0979 O

±P

0.0038 ±P

0.0022

Include Total Sys 0.0007With Common Sys 0.0003S

mtT = 174.3 ± 5.1 GeV

∆αU hadV = 0.02761 ± 0.00036

10 2

0.09W

0.1W

0.11W

AM

FB

0,bb_

mH

X [GeV]

150 200mY t [GeV]

Figure 10: The 9 most precise measurementsof� � ���� andtheoverall LEP I average. Thetotal (fullline) and systematic(dotedline) errors are plotted.Thefirst printederror correspondsto thestatisticthesecondto thesystematic.Thedifferentmeasurements

of � � ���� are in verygoodagreementandgivean av-erage of Z\[ Z^]^_^`�abZ\[ Z^ZHç^c . Theerror of 0.0017in-cludea total systematicof only 0.0007with a com-monpart (systematicspresentin more thanonemea-surement)of 0.0003: the individual measurementsandthefinal average aredominatedby thestatistical

error. This final � � ���� is not significantlycorrelated

to any other quantity than �,d d��� and eventhere thecorrelationis small: 16%.

3.1 Resultssummary

Thereis elevendifferentmeasurementsof E$� �obp performedat LEP I by thefour LEPexperiments.To take into accountcorrectlyin theaveragethestatisticalandsystem-atical correlationsof theseresults,theLEP/SLDHeavy Flavour Working grouphasdevelopedan averagingproceduree which includesmany measuredquantitiescon-nectedto theelectroweakmeasurementsin the

�and � sectorsat the � . In practice

theaverageis performedfor 18 observablesandfor a total of 99 measurements.ThemainmeasurementsandtheLEP averagefor E � �o�p arequotedin figure10.

4 Conclusion

Since1994-1995somediscrepanciesbetweenI J�K�MPO Q�R�S URTV�V extractedfrom E FHG and E � �o�pmeasurementshave beenobserved. Over the last10 years,LEP I datacollectedbe-tween1990and1995havebeenanalysedandre-analysedto improvethe E � �o�p mea-surementandto furthercheckits associatedsystematics.Nevertheless,today, usingE$� �obp , � � andthemeasurementof s Q in the leptonicsector� , thevalueextractedfortheeffective tbu ¹ and t B ¹ arecompatiblewith theStandardModel predictionby lessthan1% (seefigure2). If this is not enoughto claim a discovery, this shouldtrig our½ dominatedby the

» � /�-,%. measurementat SLC

11

attention.Even if some E � �o�p measurementsarestill preliminaryno significantchangeor

improvementareexpectedwith theLEP andSLCdata.The present7 C and E F)G measurementsarecompatiblewith the SM, even if

bothpredicta “too” low Higgs mass,2 1.5 sigmabelow the LEP II 95%exclusionlimit of 114.1 3-465 . If SUSYcouldexplainsuchlow 798 prediction,nodiscrepancybetweenthe I JLKNMPO Q�RTS+URTV�V ,7 8 extractedfrom E F)G and E � �o�p areexpectedin the usualtheoreticalframeworks.

Beforethestartof LHC, to investigatefurther theseresults,improvedmeasure-mentof 79C (LEP II+ TEVATRON :

� 79C 34 Y 25 f94"5 ) , 7 Uhg>S (TEVATRON :� 7 Uhg>S 5.1 Y 2 3-4"5 ) and ikj ; D�BFElnm _,o ( BES+”QCD” : 0.00036Y 0.0002-0.0001

) areexpected.

References

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DELPHI collaboration,Measurementof theForward-Backward AsymmetriesofW : W ? Y � Y � �and W : W ? Y � Y � � usingpromptleptons, DELPHI

2000-101CONF400.3. ALEPH collaboration,D. Buskulicet al. , Phys.Lett. B384, 414(1996)

ALEPH collaboration,Measurementof the�

and � forward-backward asymme-try usingleptons,ALEPH 99-076CONF99-048.

4. OPAL Collaboration,G.Alexanderet al., Z. Phys. C70, 357 (1996)// OPALCollaboration,UpdatedMeasurementof theHeavyQuarkForward-BackwardAsymmetriesandAverage é Mixing UsingLeptonsin Multi-hadronic Events,OPAL PhysicsNotePN226.

5. ALEPH collaboration,Measurementof E � �o�p usinginclusive�-hadrondecays,

CERNEP/2001-0476. DELPHI collaboration,Determinationof E$� �obp using inclusivecharge recon-

structionandlifetimetaggingat LEPI, DELPHI 2001-020CONF468.7. DELPHI collaboration,P. Abreuetal. , Eur. Phys.J.C9, 367(1999).8. D.Abbaneo, NIM A378, 101(1996)

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