Standard Model Results from LEP

Standard Model Results from LEP

Pat WardUniversity of Cambridge

� Introduction

� Electroweak Physics

� QCD and Two-photon

Results

10-1

1

10

10 2

10 3

10 4

80 100 120 140 160 180 200√s / GeV

Cro

ss-s

ecti

on /

pb e+e−→hadronse+e−→µ+µ−

e+e−→γγ (cosθ<0.93)

e+e−→ZZe+e−→WW (prel.)e+e−→WWγ (Eγ> 2.5 GeV)

e+e−→qqγγ (Eγ> 5 GeV)

13/02/2004 K. Sachs

Pat Ward September 2004 1

Introduction

LEP1 1989–1995 � � � � 4.5M Z events / expt

LEP2 1996–2000 161 � � � 209 GeV 10k WW events/expt

� LEP was shut down at the end of 2000, so why are we still giving LEP talks?

� Many physics results still being produced

� In the last 2 years, the 4 LEP experiments (ALEPH, DELPHI, L3 and OPAL)

have submitted for publication � 100 papers

About 25% of these were on LEP1 physics

� The 4 expts have submitted � 120 papers to ICHEP04

Majority on Standard Model physics

Mostly final results, but some new preliminary results too

� This talk will concentrate on results which have been finalized, or are new,

in the last year (but include some older important results too)


Introduction

LEP Standard Model physics covers a wide range of topics; no time in this talk

for, e.g.,

� Precise measurement of the � lifetime (DELPHI)

� � = 290.9 � 1.4 � 1.0 fs (c.f. PDG2004: 290.6 � 1.1 fs)

� � branching ratios and strange spectral functions (DELPHI, OPAL)

� ��

, �� and hard-photon production in two-photon collisions

(DELPHI, L3, OPAL)

� Excited b-hadrons, �� oscillations, B spectral moments (DELPHI)

Will give a brief overview of current results; for more details consult the

experiments’ web pages: http://aleph.web.cern.ch/aleph/

http://delphiwww.cern.ch/

http://l3.web.cern.ch/l3/

http://opal.web.cern.ch/Opal/PPwelcome.html


LEP1 Electroweak Physics� Z lineshape measurements final

since summer 2000

� � = 91.1875 � 0.0021 GeV

� � = 2.4952 � 0.0023 GeV

�� = 41.540 � 0.037 nb

� � = 20.767 � 0.025

� � � �� = 0.01714 � 0.00095

� New measurement of � ��

(DELPHI)

� � new LEP heavy-flavour com-

bination

-0.05

0

0.05

0.1

0.15

89 90 91 92 93 94√s [GeV]

AF

B(√

s)

LEP

AFB

b

AFB

c

� � � �� =0.0998 � 0.0017


LEP1 Electroweak Physics� OPAL have published new mea-

surements of � � , � �

� Use hadronic events with FSR

( � � � � � )

Enriched in up-type quarks

� � � � � � � � � �

� � � ��

� Results:

� � = 300� �� MeV

� � = 381� �� MeV

� Good agreement with SM

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Γhadrons

SMΓhadrons+γ

Γd-type [GeV]

Γ u-ty

pe [

GeV

]Jade E0, ycut=0.08

OPAL

� More precise than earlier measure-

ments (DELPHI, L3, OPAL)


Two-fermion Production at LEP2

µ+

µ-

e-

e+

(Z/γ)*

Non-radiativeµ+

µ-

e-

e+Z0

γRadiative return

� Measure cross-sections and asym-

metries for inclusive and ‘non-

radiative’ events

� LEP combination of preliminary

measurements

Good agreement with SM

� limits on new physics, e.g. �� ,

leptoquarks, RPV squarks, contact

interactions, extra dimensions

Cro

ss s

ectio

n (p

b)

√s

´/s

> 0.85

e+e−→hadrons(γ)e+e−→µ+µ−(γ)e+e−→τ+τ−(γ)

LEPpreliminary

√s

(GeV)

σ mea

s/σ S

M

1

10

10 2

120 140 160 180 200 220

0.8

0.9

1

1.1

1.2

120 140 160 180 200 220


Two-fermion Production at LEP2� At LEP2 energies, � -exchange be-

comes important � can measure

� -Z interference � almost model-

independent determination of � �

in S-matrix fit

� e.g.DELPHI results

� � = 91.1831 � 0.0034 GeV

c.f. � � = 91.1863 � 0.0028 GeV

from standard LEP1 fit assuming

SM interferenceMZ (GeV/c 2)

jtot

had

DELPHI LEP1 data

DELPHI LEP1+LEP2 data

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

91.165 91.17 91.175 91.18 91.185 91.19 91.195

� OPAL, DELPHI have finalized two-fermion measurements; new

combination when all experiments have final results

� Expect small improvements to ��

, but leptons dominated by statistics


WW Cross-sections

W-

W+

e-

e+

Z

W-

W+

e-

e+

γ

W-

W+

e-

e+

ν

� 3 channels: � � � � � � �

� � � � � ��

� � � ��

� LEP combination updated with fi-

nal values from ALEPH, L3

Final combination awaiting OPAL

final results

0

10

20

30

160 180 200

√s (GeV)

σ WW

(pb

)

YFSWW/RacoonWWno ZWW vertex (Gentle)only νe exchange (Gentle)

LEPPRELIMINARY

02/08/2004

Good agreement with theoretical ex-

pectations


W Branching Ratios� LEP combination updated with final values from ALEPH, L3

02/08/2004

W Hadronic Branching Ratio

ALEPH 67.15 ± 0.40

DELPHI 67.45 ± 0.48

L3 67.50 ± 0.52

OPAL 67.91 ± 0.61

LEP 67.49 ± 0.28χ2/ndf = 15 / 11

66 68 70

Br(W→hadrons) [%]

Summer 2004 - LEP Preliminary

02/08/2004

W Leptonic Branching Ratios

ALEPH 10.81 ± 0.29DELPHI 10.55 ± 0.34L3 10.78 ± 0.32OPAL 10.40 ± 0.35

LEP W→eν 10.66 ± 0.17ALEPH 10.91 ± 0.26DELPHI 10.65 ± 0.27L3 10.03 ± 0.31OPAL 10.61 ± 0.35

LEP W→µν 10.60 ± 0.15ALEPH 11.15 ± 0.38DELPHI 11.46 ± 0.43L3 11.89 ± 0.45OPAL 11.18 ± 0.48

LEP W→τν 11.41 ± 0.22

LEP W→lν 10.84 ± 0.09

χ2/ndf = 6.8 / 9

χ2/ndf = 15 / 11

10 11 12

Br(W→lν) [%]


� B(W � � � ) higher than average of B(W � � � ) and B(W � � � ) by 3 �


Single Vector Boson Production� Zee

e-

e+

e-

e+

γZ

q

q--

0

0.5

1

180 190 200 210

√s (GeV)

σ Zee

→qq

ee (

pb)

WPHACT and GRACE

LEP PRELIMINARY

02/08/2004

� We�

e-

e+

e-

ν--

γW

q

q--

0

0.5

1

1.5

180 190 200 210

√s (GeV)

σ Weν

(pb

)

WPHACT and GRACELEP PRELIMINARY

03/08/2004

LEP combination updated with final measurements from ALEPH, L3


Charged Triple Gauge Boson Couplings

W-

W+

Z

W-

W+

γ

� Measured using WW events:

� (WW), �� , W decay angles

� Also We� and� � � channels

� Assuming C, P conservation and

gauge constraints: 14 � 3 cou-

plings: � � � � ��

� LEP combination: combine � ��

curves including correlated system-

atics

OPAL

0

250

500

750

1000

-1 0 1cosθw

Eve

nts/

bin

0

200

400

600

-1 0 1cosθ*

l

Eve

nts/

bin

0

200

400

600

-180 0 180φ*

lE

vent

s/bi

n

0

200

400

-1 0 1cosθ*

jet

Eve

nts/

bin

0

200

400

0 90 180φ* jet

Eve

nts/

bin W+W- → qq

–lν

–

l

data

SM (λ=0)

λ=+0.5

λ=-0.5

background


Charged Triple Gauge Boson Couplings� L3, OPAL values final, ALEPH,

DELPHI values preliminary

� LEP combined results allowing

one free parameter:

� � � � � � � ��

� � � � � � ��

� � � � � � � � ��

� Good agreement with Standard

Model

� Couplings measured with preci-

sion of 2–4%

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.9 1 1.1

g1Z

λ γ

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

0.9 0.95 1 1.05 1.1

g1Z

κ γ

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

-0.1 0 0.1

λγ

κ γ

95% c.l.

68% c.l.

2d fit result

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.9 1 1.1

g1Z

λ γ

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

0.9 0.95 1 1.05 1.1

g1Z

κ γ

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

-0.1 0 0.1

λγ

κ γ

95% c.l.

68% c.l.

2d fit result

LEP charged TGC Combination 2003

LEP Preliminary

� LEP combined confidence levels allow-

ing 2 free parameters


Other Gauge Boson Couplings� Neutral Triple Gauge Boson couplings (ZZ � , Z � � ) are zero in SM

� Limits set from ZZ and Z � channels

� SM Quartic Gauge Couplings either zero or too small to be observed at LEP

� Limits set from WW � , Z � � and� � � � channels

0

0.5

1

100 120 140 160 180 200

OPAL DataSM (KK2F)a0/Λ

2=0.005 GeV-2

ac/Λ2=0.005 GeV-2

aZ

aZ

Eγ > 5 GeV|cosθγ| < 0.95cosθγq < 0.90|Mqq-MZ| < 3 ΓZ

√s [GeV]

σ(e+ e- →

Zγγ

→qq

γγ)

[pb]

-0.1

-0.05

0

0.05

0.1

-0.1 -0.05 0 0.05 0.1

a0/Λ2 [GeV-2]

a c/Λ

2 [G

eV-2

]

V

V

Best FitStandard Model

{a0 , ac{aW, aW}{a0 , ac{aZ, aZ}

Combined (aW=aZ) 95 % C.L.

Combined (aW=aZ) 68 % C.L.

OPAL


W Mass Measurement� A principal aim of LEP2

� Comparison of direct mea-

surement with indirect deter-

mination from EW fits is test of

SM

� Measure by direct reconstruc-

tion of � � or �� mass in

� � � � � �� and

� � � � � � � channels

� Reconstruct event-by-event

mass using beam energy

constraint (kinematic fit) to

improve resolution

m /GeV

Eve

nts

m /GeV

Eve

nts

qqqq

qqlν

OPAL 183-209 GeV ∫ L dt = 677 pb-1

Signal

Combinatorial b/g

Other b/g

0

50

100

150

200

250

300

60 65 70 75 80 85 90 95 100 105

0

50

100

150

200

250

300

60 65 70 75 80 85 90 95 100 105

� Fit mass distribution � � � , using MC

to correct for bias


W Mass Measurement� LEP preliminary values:

� � � � � ��

= 80.411 � 0.032(stat) � 0.030(sys) GeV

� � � � � � � �

= 80.420 � 0.035(stat) � 0.101(sys) GeV

80.0 81.0

MW[GeV]

LEP working groupχ2/dof = 29.6 / 37

ALEPH [1996-2000] 80.379±0.058

DELPHI [1996-2000] 80.404±0.074

L3 [1996-2000] 80.376±0.077

OPAL [1996-1999] 80.490±0.065

LEP 80.412±0.042


� Systematics completely dominate

� � � � channel (and important in

� � �� channel)

� Combined value:

� � = 80.412 � 0.042 GeV

� What has been happening in last

year to improve and finalize mea-

surements?


W Mass Measurement� Main systematics in � � � � channel arise from final state interactions

� Separation of W decay vertices � 0.1 fm � hadronic scale � 1 fm

� W decays have space-time overlap and can exchange colour:

Colour Reconnection

� May also be Bose-Einstein Correlations

between like-sign particles from different

W’s

� Both effects may shift measured � � by

large amount ( � 100 MeV)

Not included in standard MC used to cali-

brate � � measurement

� Much effort to measure CR and BEC ef-

fects to estimate realistic errors


Colour Reconnection

q1

q2

q_

1′

q_

2′

Normalq1

q2

q_

1′

q_

2′

Reconnected

� Effects estimated using phe-

nomenological models

� Look at particle flow between jets in

� � � � � � � events

� Compare particle flow in inter-W re-

gion with intra-W region

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1φresc

fA+

B

N

/fC

+D

N

L3 a)

Particle flow

DataNo CRSKI (kI=3)SKI 100%

RN

� L3 results final, ADO preliminary

� Use largest reconnection probability compatible with data to set ��

� � �

Current LEP combination uses � = 2.1 in SK1 model ( � 55% reconnected)

� � � � � � � � � �

= 90 MeV


Colour Reconnection� Soft particles most affected by FSI

� reduce effects with momentum

cuts or jet cone cuts

Reduces FSI error at expense of

statistical error, as jet directions

less precisely determined

� Final � � analyses will optimize jet

reconstruction

DELPHI preliminary SK1 curves

-100

0

100

200

300

400

500

600

10-1

1 10 102

Standard analysis1 GeV pcut2 GeV pcut3 GeV pcutCone R=1.00 radCone R=0.75 radCone R=0.50 radCone R=0.25 rad

SK1 Model parameter κ

∆mW

/ M

eV/c

2

κ = 0.66

Bose-Einstein Correlations

� BEC between like-sign pions well-established in Z decays

Do they occur between particles from different W’s?

� Currently contributes 35 MeV to � � � � � � � � �

(LUBOEI model)


Bose-Einstein Correlations� Study using two-particle correla-

tions, normalized to ‘no BEC’

� Use � � � � � �� events

to estimate intra-W correlations,

mixed events (or MC) for kine-

matic correlations

� � ��

� ��

� LEP combined data:

� ��

� �� = 0.23 � 0.13

-4-202468

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

(a) OPALdatafull BECintra BECno BEC

Q [GeV]

∆ρ(±

±)

[GeV

-1]

-4-202468

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

(b) OPALdata full BECintra BECno BEC

Q [GeV]

∆ρ(+

−)

[GeV

-1]

� DELPHI, L3, OPAL results now final

� L3, OPAL compatible with no inter-W BEC, DELPHI sees significant effect


Prospects for Final � Measurement� LEP measurements of � � should be finalized soon

� Expect improvements over preliminary measurement

� LEP beam energy uncertainty will decrease from 21 MeV to � 10 MeV

Final LEP beam energy paper recently submitted for publication

� Analyses in � � � � channel will be optimized to reduce total error

� Hadronization and detector effects will be better understood

� In absence of systematics, LEP statistical precision � 21 MeV

With reduced weight for � � � � channel, probably � 25 MeV

� Final total error will probably be in range 32–40 MeV, depending on FSI

results


W Width Measurement� Fits to W mass distributions also

determine � �

� Preliminary LEP average:

� � = 2.150 � 0.091 GeV

1.5 2.0 2.5

ΓW[GeV]

LEP working groupχ2/dof = 19.7 / 24

ALEPH [1998-2000] 2.13±0.14

DELPHI [1997-2000] 2.11±0.12

L3 [1996-2000] 2.24±0.19

OPAL [1996-1998] 2.04±0.18

LEP 2.150±0.091



Standard Model Fit� Standard Model fit by LEPEWWG uses 17 inputs from LEP, SLD, Tevatron:

Z lineshape: � � , � � , �� , � � , � � � ��

� polarisation: � �Polarised lepton asymmetry: � (SLD)

Heavy flavour: � , � � , � � � �� , � � � �� , , �

Inclusive hadronic charge asymmetry

� � , � � , � �

� Updates since summer 2003:

� Tevatron � � : 178.0 � 2.7 � 3.3 GeV

� LEP1 heavy flavours

� Theory calculations including full two-loop corrections for � � and

�� (Awramik, Czakon, Freitas, Weiglein)

Shifts predicted value of � � from �� alone by � 19 GeV


Standard Model Fit� Fit �

� /dof = 15.8/13

� 67% correlation between

� � and � � �

� Largest contribution to ��

from � � � �� (2.4 � )

� � � � �� prefers large � � ,

whereas � � , � � and

lepton asymmetries prefer

small � �Measurement Fit |Omeas−Ofit|/σmeas

0 1 2 3

0 1 2 3

∆αhad(mZ)∆α(5) 0.02761 ± 0.00036 0.02769

mZ [GeV]mZ [GeV] 91.1875 ± 0.0021 91.1874

ΓZ [GeV]ΓZ [GeV] 2.4952 ± 0.0023 2.4966

σhad [nb]σ0 41.540 ± 0.037 41.481

RlRl 20.767 ± 0.025 20.739

AfbA0,l 0.01714 ± 0.00095 0.01650

Al(Pτ)Al(Pτ) 0.1465 ± 0.0032 0.1483

RbRb 0.21630 ± 0.00066 0.21562

RcRc 0.1723 ± 0.0031 0.1723

AfbA0,b 0.0998 ± 0.0017 0.1040

AfbA0,c 0.0706 ± 0.0035 0.0744

AbAb 0.923 ± 0.020 0.935

AcAc 0.670 ± 0.026 0.668

Al(SLD)Al(SLD) 0.1513 ± 0.0021 0.1483

sin2θeffsin2θlept(Qfb) 0.2324 ± 0.0012 0.2314

mW [GeV]mW [GeV] 80.425 ± 0.034 80.394

ΓW [GeV]ΓW [GeV] 2.133 ± 0.069 2.093

mt [GeV]mt [GeV] 178.0 ± 4.3 178.2

Summer 2004


Standard Model Fit

80.2

80.3

80.4

80.5

80.6

130 150 170 190 210

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

Preliminary

68% CL

∆α

LEP1, SLD Data

LEP2, pp− Data

Good agreement between mea-

sured � � , � � and values pre-

dicted by fit excluding direct mea-

surements

0

1

2

3

4

5

6

10020 400

mH [GeV]

∆χ2

Excluded Preliminary

∆αhad =∆α(5)

0.02761±0.00036

0.02749±0.00012

incl. low Q2 data

Theory uncertainty

� � � 260 GeV 95% c.l.


QCD and Two-photon Physics

Will discuss:

� � from event shapes

� � from 4-jet rate

� Unbiased gluon jets

� Coherence effects in Z � 3 jets

� Dead cone effect

� Inclusive jet/hadron production in

two-photon events

No time to cover

� Fragmentation functions and scal-

ing violation

� Fermi-Dirac and Bose-Einstein cor-

relations in Z decays

� b-quark mass effects

� Production of �� , � in Z decays

� Pentaquark searches


� � from Event Shapes

q-

q

e-

e+

g

� Event shapes sensitive to gluon

emission, e.g. thrust:

� � � ��

� Fit event shape distributions with

��

��

+ NLLA ( � � matching)

QCD predictions � �

� Final measurements from all ex-

periments

91 GeV133 GeV (×3)177 GeV (×9)197 GeV (×27)

OPAL

(1/σ

)⋅dσ/

d(1−

T)

PYTHIAHERWIGARIADNE

197 GeV

(MC

− d

ata)

/err

or

91 GeV

(1−T)

1

10

10 2

10 3

0 0.05 0.1 0.15 0.2 0.25 0.3

-5

0

5

0 0.05 0.1 0.15 0.2 0.25 0.3

-505

0 0.05 0.1 0.15 0.2 0.25 0.3


� � from Event Shapes� Combinations by LEPQCD group

� Use variables which are infra-red

safe (soft gluon emission) and

collinear stable (collinear parton

branchings)

� Combination requires care because

of large correlated errors

� Treat hadronization and theory er-

rors as uncorrelated when calculat-

ing weights, but include 100% cor-

relation when calculating hadronisa-

tion and theory uncertainties on com-

bined �

PSfrag replacements

0.110

0.110

0.115

0.115

0.120

0.120

0.125

0.125

0.130

0.130

αs(MZ)

αs(MZ)

1 − T

1 − T

M2

H/s

M2

H/s

C

C

BT

BT

BW

BW

− ln(y3)

− ln(y3)

ALL

ALLPSfrag replacements

0.115

0.115

0.120

0.120

0.125

0.125

0.130

0.130

αs(MZ)

αs(MZ)

ALEPH

ALEPH

DELPHI

DELPHI

L3

L3

OPAL

OPAL

ALL

ALL


� � from Event Shapes� Preliminary combiation of final re-

sults from all experiments

� Relative weights in global fit:

LEP1 46%

LEP2 54%

� � runs as expected from QCD

� Theory error includes variation of

renorm. scale: 0.5 �� 2,

log rescaling fact.: 2/3 � � � � 3/2,

matching scheme and kinematic

cutoffs

PSfrag replacements

αs(Q)

αs(Q)

Q (GeV)

Q (GeV)

100

100

150

150

200

200

0.100

0.100

0.105

0.105

0.110

0.110

0.115

0.115

0.120

0.120

0.125

0.125

αs(Q) fit at single energy

± σstat.

± σtotal

αs(Q) global fit± σstat.

± σstat.

± σtotal

± σtotal

� �

� � �

= 0.1202 � 0.0003(stat) � 0.0007(exp) � 0.0015(hadr) � 0.0044(theo)


� � from 4-jet Rate

q-

q

q

q-

e-

e

q-

q

g

g

e-

e+

� New meas. of � from 4-jet rate

� DELPHI: Cambridge jet algorithm

� Fit to ��

� �

QCD prediction of DE-

BRECEN (Nagy, Trocsanyi) using ex-

perimentally optimized renormaliza-

tion scale

� OPAL: Durham jet finder

� Fit to ��

� �

+ NLLA QCD prediction

with� � =1

10-3

10-2

10-1

10-3

10-2

R4

fit range

DELPHI

xµ=1xµ

opt

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

10-3

10-2

CAMBRIDGECAMBRIDGE ycut

R4da

ta/R

4fit

CAMBRIDGECAMBRIDGECAMBRIDGE


� � from 4-jet Rate

0.090.095

0.10.1050.11

0.1150.12

0.1250.13

100 150 200

DELPHI

αs from Cambridge4-jet rate1/logE fitQCD evolution

√s (GeV)

α s� DELPHI

� �

� � �

= 0.1175 � 0.0030

αS(MZ)=0.1208±0.0038

OPALPreliminary

Durham 4-Jet Rate

√s [ GeV ]

α S

0.08

0.09

0.1

0.11

0.12

0.13

80 100 120 140 160 180 200

� OPAL

� �

� � �

= 0.1208 � 0.0038


Unbiased Gluon Jets with Jet Boost Algorithm� Jet boost algorithm (Eden, Gustafson) relates gluon jets in � � g events to

gg system � unbiased gluon jets

β=cos

=cos

α α

α

β α

α

(a)

αEg

E

E

q

q

−

� Decompose into 2

colour dipoles

E*

(b)

g

q*E

q*E−

� Boost each dipole into

back-to-back frame

Eg*q

EE

*q

*

(c)

−

� Recombine to give

event with gg struc-

ture


Unbiased Gluon Jets with Jet Boost Algorithm� Method used by OPAL to mea-

sure gluon charged multiplicity

for 5 � � � � 20 GeV

� Results consistent with other

measurements, and most pre-

cise for this energy range

� Theoretical fits OK

� Also measure gluon fragmen-

tation function

4

5

6789

10

20

5 6 7 8 9 10 20 30 40Eg (GeV)*

(a)

<ngl

uon>

ch.

3NLO fitFixed αS fitHerwig (gg evts.)

OPAL data (boost)OPAL data (gincl)OPAL data (qqg-qq)CLEO data

-1-0.5

00.5

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

10-5

10-4

10-3

10-2

10-1

1

10

10 210 3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

xE*

Dat

a-P

redi

ct.

σ data

1 N

dngl

uon

ch.

dxE*

Herwig (gg)DGLAP fit

OPAL data

Eg= 14.24 GeV*

(a)


Inclusive Charged Particle Distributions<N

ch>

Ecm [GeV]

JADE

TASSO

ARGUS

AMY

HRS

MARKII

TPC

TOPAZ

ALEPH

3NLO nf = 3χ2/NDOF = 25.1/29

PYTHIAHERWIGARIADNE

0

5

10

15

20

25

30

0 25 50 75 100 125 150 175 200

� Mean charged multiplicity

Well-described by theory or MC

Ecm [GeV]

ξ*

ALEPH

TASSO

QCD MLLA + LPHD nf=3

χ2/NDOF=8.7/12

1.5

2

2.5

3

3.5

4

4.5

20 40 60 80 100 120 140 160 180 200

� Peak of �

� � � � � distribution

Sensitive to coherence effects


Coherence Effects in 3-jet Events� New results from DELPHI

� Measure multiplicity in 30 � cone

perpendicular to event plane in 3-

jet events

� Compare with corresponding quan-

tity in 2-jet events

� Direct evidence for coherence ef-

fects in soft particle production


Dead Cone Effect� QCD predicts gluon radiation

off heavy quark suppressed at

small angles

� DELPHI study using 2-jet

� � � � and � � � � events

� Remove particles associated

with b- or c-quark

� Compare angular distribution of

fragmentation particles in b- and

c-jets

� First DIRECT observation of

‘dead cone’ effect

θ, rad

(dN

b/dθ

)/(d

Nc/

dθ)

q direction estimated bythrustjetvertex

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5


Two-photon Physics� Active area with many new

measurements � test QCD

� e.g. new DELPHI measure-

ment of total hadronic cross-

section at low ��

uses Very Small Angle Tag-

ger at � � 3-15 mrad

� Direct event-by-event recon-

struction of � � for double-

tagged events without un-

folding � small systematic

errors

250

300

350

400

450

500

550

600

650

700

0 20 40 60 80 100 120 W (GeV)

To

tal γ

γ cr

oss

-sec

tio

n (

nb

)

TWOGAM simulationDELPHI single tag

DELPHI double tag

OPAL

L3

DELPHI 189-206 GeV

� DELPHI measurements somewhat higher

than L3 and OPAL


Inclusive jet production� L3 measurements of inclusive

jet production in two-photon

collisions for

� � � � 1

� � � � � 30 GeV

� �� 0.2 GeV�

� � � spectrum well-represented

by power law

� Data higher than NLO QCD

prediction (Frixione, Bertora)

10-3

10-2

10-1

1

10

10 2

20 40 60pt [GeV]

dσ /

dpt [

pb /

GeV

]

Data

NLO QCDPower law fit

L3


Inclusive hadron production

10-3

10-2

10-1

1

10

10 2

10 3

10 4

5 10 15 20pt [GeV]

dσ /

dpt [

pb /

GeV

]

Data π±a)

NLO QCDScale dependenceDirect

L3

Wγγ > 5 GeV

� L3 measurements of inclusive ��

production in two-photon collisions

for

� � � � 1

� � � � � 5 GeV

� Also see excess at high � � com-

pared with NLO QCD calculations

(Binnewies, Kniehl, Kramer)� Similar disagreement seen in ��


Inclusive hadron production� New DELPHI measurement of in-

clusive hadron production

� � � � 1.5

� � � � � 203 GeV

� Good agreement with NLO QCD for

� � � 6 GeV

� Some excess of data over NLO

QCD at high � � , but not at L3 level


Exclusive Particle Production� Several measurements of exclusive

particle production

� e.g. L3 have measured ��

�� and

��

� In range 1.1 � � � � 2.1 GeV,

1.2 � �� 8.5 GeV�

� ��

� �� = 1.81 � 0.47 � 0.22

expect 2 from isospindσ

ee /

dQ2

[pb/

GeV

2 ]Q2 [GeV2]

ρ+ρ−

ρ0ρ0

QCD fit to ρ+ρ−

b) L3

10-3

10-2

10-1

1

10

10 2

1 10

� Fit to QCD expectation � � � ��

� � �� gives � = 2.4 � 0.3 (2.5 � 0.4) for �

��

� ��

�

(expect � = 2)


Summary� Since LEP finished running in 2000, the experiments have continued to

produce lots of new physics results covering a wide range of topics:

Fermion- and boson-pair cross-sections, gauge boson couplings, QCD,

two-photon physics....

� Some important LEP2 measurements still to be finalized, especially � �

� Important to utilize fully the plentiful, high-quality data produced at LEP


Standard Model Results from LEP

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Transcript of Standard Model Results from LEP