standard model physics results from lep2

Standard Model Physics

Results from LEP2

Stephan Wynhoff

CERN

on behalf of the LEP collaborations

Aleph, Delphi, L3, Opal

5th International Symposium on Radiative CorrectionsCarmel, USA

11.-15. September 2000

• Fermion Pair Production

Cross sections and Asymmetries

S-matrix

Contact interactions

• Boson Cross Sections

ZZ Production

Single W Production

• W+W−Production

Cross Section

Branching Fractions

• W Mass Measurement

Mass extraction

Systematic Errors

Comparison of Direct and Indirect Results

• Standard Model Fits

Stephan Wynhoff Standard Model Physics Results from LEP2 2

10-4

10-3

10-2

10-1

1

10

80 100 120 140 160 180 200 220s /GeV

σ/nb

88 90 92 940

10

20

30

1990-19921993-19951996-19981999-2000 (prel.)

mH=114 GeV

e+e– → hadrons

e+e– → HZ → qq_qq

_

e+e–→ W+W−

→ qq_qq

_

e+e–→ ZZ→ qq

_qq

_

L3

• large data statistics at the Z pole:

15 million hadronic and 2 million leptonic events

• above the Z resonance:

luminosity 590 – 630 pb−1 per experiment

• more than 8000 W-pair events per experiment


See LEP2MC workshop proceedingshep-ph/0005309, hep-ph/0007180

2-Fermion Processes

• ZFITTER, KKMCbetter than 0.2% precision in σtot of hadrons, leptons

• KKMC covers LEP, LC, µ-colliders, τ - and b-factories.

4-Fermion Processes

• RacoonWW, YFSWW3σ(W+W−) to 0.4%. (double-pole approx. above thresh-old.)

• WPHACT, grc4f, WTO, ...σ(Weν) to 4-5%. (fermion loop scheme.)

• YFSZZ, ZZTOσ(ZZ) to 2%.

⇓

Excellent match in precision

Experiment ⇐⇒ Theory


e−

e+

f

f

PP

ISR

FSR

• Initial State Radiation

• Pair Production:

• Final State Radiation

√s′:= mass of outgoing lepton pair or γ∗/Z propagator

→ high energy events:√

s′ > 0.85√

s

10 2

10 3

50 100 150 2001

10

10 2

10 3

10 4

50 100 150 200

0

20

40

60

80

100

120

140

160

180

50 100 150 2000

20

40

60

80

100

120

50 100 150 200

√s′ /GeV

Eve

nts

→

√s′ /GeV

Eve

nts

√s′ /GeV

Eve

nts

→

√s′ /GeV

Eve

nts

→

OPAL 205.4 GeV preliminary

(a) hadrons (b) e+e-

(c) µ+µ- (d) τ+τ-


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

0.8

0.9

1

1.1

1.2

120 140 160 180 200 220


Differential cross sections for Muon-, Tau-pair production

Preliminary LEP Averaged dσ/dcosθ (µµ)183 GeV

cosθf

dσ/d

cosθ

(pb

)

189 GeV

cosθf

dσ/d

cosθ

(pb

)

192 GeV

cosθf

dσ/d

cosθ

(pb

)

196 GeV

cosθf

dσ/d

cosθ

(pb

)

200 GeV

cosθf

dσ/d

cosθ

(pb

)

202 GeV

cosθf

dσ/d

cosθ

(pb

)

0

2

4

-1 -0.5 0 0.5 10

1

2

3

4

-1 -0.5 0 0.5 1

0

1

2

3

4

-1 -0.5 0 0.5 10

1

2

3

4

-1 -0.5 0 0.5 1

0

1

2

3

4

-1 -0.5 0 0.5 10

1

2

3

-1 -0.5 0 0.5 1

Use for limits on

• Contact interactions

• Fermion size

• Extra dimensions, TeV strings, gravitons


For

war

d-B

ackw

ard

Asy

mm

etry √s

´/s

> 0.85

e+e−→µ+µ−(γ)e+e−→τ+τ−(γ)

LEPpreliminary

√s

(GeV)

AF

B

mea

s -AF

B

SM

0

0.2

0.4

0.6

0.8

1

-0.2

0

0.2

120 140 160 180 200 220


Fermion Pair - Heavy Flavours

s (again) the ratio of cross sections

0.14

0.16

0.18

0.2

0.22

0.24

0.26

80 100 120 140 160 180 200√s (GeV)

Rb

Rb

LEP preliminary

√s’/√s > 0.1 , 0.85

0.14

0.16

0.18

0.2

0.22

0.24

0.26

0.28

0.3

0.32

80 100 120 140 160 180 200√s (GeV)

Rc

Rc

LEP preliminary

√s’/√s > 0.1 , 0.85

-0.2

0

0.2

0.4

0.6

0.8

1

80 100 120 140 160 180 200√s (GeV)

b-A

sym

met

ry

Ab FB

LEP preliminary

√s’/√s > 0.1 , 0.85

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

80 100 120 140 160 180 200√s (GeV)

c-A

sym

met

ry

Ac FB

LEP preliminary

√s’/√s > 0.1 , 0.85


• Measure Interference between γ and Z (jhad)

-4

-3

-2

-1

0

1

2

3

4

50 75 100 125 150 175 200

s [GeV]

σ int/σ

tot [

%]

no cuts'/s > 0.85

σ0a(s) =

4

3πα2

gafs

+jaf(s−m2

Z) + raf s

(s−m2Z)

2 +m2ZΓ

2Z

, for a = tot, fb,

A0fb(s) =

3

4

σ0fb(s)

σ0tot(s)

, with σ0fb =

4

3(σf − σb) .

• Photon exchange

• Z-Boson exchange

• γ/Z interference

“Standard” fits: fix gaf and jaf ZFITTER, TOPAZ0

S-Matrix fits: fix gaf only ZFITTER, SMATASY


-0.5

0

0.5

1

1.5

91.17 91.18 91.19 91.2mZ [GeV]

j had

tot

68% CL

SM

L3 Z dataL3 data ≤ 189 GeVLEP + Tristan (potential)

MZ[MeV] jtothad corr.

L3 Z data 91 185.2±10.3 0.44±0.59 -0.95L3 ≤ 189GeV 91 187.5±3.9 0.31±0.13 -0.57LEP + Tristan: ±2.3 ±0.04 -0.28


L =1

1 + δef

∑i,j=L,R

ηijg2

Λ2ij

(eiγµei)(fjγµfj),

e+

e−

l+, q−

l−, q

g2 / Λ2

g Coupling, byconvention g2/4π = 1

ηij Helicity amplitudes,choose |ηij| = 0, 1

Λ Energy scale

dσ

d cos θ=

dσSM

d cos θ+ cint(s, cos θ)

1

Λ2+ cci(s, cos θ)

1

Λ4.

ηRR ηLL ηLR ηRLAA ±1 ±1 ∓1 ∓1VV ±1 ±1 ±1 ±1RR ±1 0 0 0LL 0 ±1 0 0

LEP Preliminary 130-202 GeV

Λ- (TeV) Λ+ (TeV)

LL 10.2 12.8

RR 9.7 12.3

VV 17.2 20.4

AA 13.9 17.6

Λ- Λ+

20. 0 20.


Z

Z

e−

e+

f

f

f'

f'

Z

Z

e−

e+

f

f

f'

f'

0

0.5

1

1.5

170 180 190 200

Ecm [GeV]

σZZ

NC

02 [p

b]

LEP Summer 00 - Preliminary20/07/2000

±2.0% uncertainty

ZZTO

YFSZZ


e+ e+

γ

W

Q2→0

e− νe

WMW

2s

f– ‘

f

0

0.5

1

160 170 180 190 200 210

Ecm [GeV]

σ(e+

e– → e

νW)

[pb]

LEP Preliminary21/07/2000

±5.0% uncertainty

grc4fWPHACT IFL/α(t)

W → ff-’


• W-pair production at LEP

e– W–

f

f´νe

W+

f

f´

e+

e–

e+γ,Z

W–

f

f´

W+ f

f´

SM branchings Br(W → qq′) = 67.6%Br(W → lν) = 10.8% per lepton flavour

D E L P H I R u n : E v t :

B e a m :

DA S :

P r o c :

S c a n :

1 0 4 . 4 Ge V 2 9 - Ap r - 2 0 0 0

2 9 - Ap r - 2 0 0 0

1 1 : 4 3 : 3 4

2 9 - Ap r - 2 0 0 0

109372 8483

T a n a g r a

• Fully hadronic: 45.6% 4 jets

High Multiplicity, balanced events

Y

XZ

• Semileptonic: 3×14.6% 2 jets, 1 lepton

Hadronic energy plus high energy lepton ornarrow jet (τ )

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

OOOOO

OOOO

OOOOOO

OOOOOOOOOOOOO

OOOOOOOO

OOOOO



OOOOO

OOOO

OOOOOO



OOOOO

OOOO

OOOOOO

OOOOOOOOOOOOO

OOOOOOOO

OOOOO

OO

OO

OO

OO

OO

OO

OO

OO++++

• Fully leptonic: 10.6% 2 leptons

Low multiplicity, acoplanar, missing energy


0

5

10

15

20

160 170 180 190 200 210

Ecm [GeV]

σWW

[pb]

LEP Preliminary21/07/2000

Gentle 2.1 (±0.7%)

RacoonWW / YFSWW 1.14

0

5

10

15

20

160 170 180 190 200 2100

5

10

15

20

160 170 180 190 200 210

RacoonWWYFSWW 1.14

16

17

18

Excellent agreement with

• RacoonWW• YFSWW

at Ecm > 170 GeV


21/07/2000

Br(W→hadrons) [%]

ALEPH 67.22 ± 0.53

DELPHI 67.81 ± 0.61

L3 68.47 ± 0.59

OPAL 67.86 ± 0.62

LEP 67.78 ± 0.32

66 68 70

Br(W→hadrons) [%]

Summer 00 - Preliminary - [161-207] GeV

DELPHI [161-202] GeV

L3 [161-202] GeV


• Indirect determination of |Vcs|:Br(W → hadrons)

1 −Br(W → hadrons)=

∑ ∣∣∣∣∣V2ij

∣∣∣∣∣

1 +

αs

π

• Direct determination of |Vcs| from tagged charm:

OPAL :Γ(W → cX)

Γ(W → had)= 0.47 ± 0.04 ± 0.06

LEP |Vcs|Indirect 0.989 ± 0.016

Direct 0.95 ± 0.08


21/07/2000

W Leptonic Branching Ratios

ALEPH 11.19 ± 0.34DELPHI 10.33 ± 0.45L3 10.22 ± 0.36OPAL 10.52 ± 0.37

LEP W→eν 10.62 ± 0.20


LEP W→µν 10.60 ± 0.18


LEP W→τν 11.07 ± 0.25

LEP W→lν 10.74 ± 0.10

10 11 12

Br(W→lν) [%]

Summer 00 - Preliminary - [161-207] GeV

DELPHI [161-202] GeV

L3 [161-202] GeV

Indirect extraction from TEVATRON:

Br(W → eν) [%]

CDF 10.50 ± 0.30

D0 10.39 ± 0.35

10.43 ± 0.25


q1

q2

e,µ,τ

ν

q2q1

q4q3

• reconstruct lepton and jets

• impose kinematic constraints:

E and *p conservation → 1C for qq+ν, 4C for qqqqequal masses of recontructed W’s → +1C

• special for qqqq events: jet pairing problem

q2q1

q4q3

q2q1

q4q3

q2q1

q4q3

→ choose best pairing

• also gluon radiation is taken into account→ split into 4 and 5 jet sample (D,O)


L3

minv [GeV]

Num

ber o

f Eve

nts

/ 1 G

eV1999 Data qqeνM.C. reweighted

M.C. background (a)

preliminary

MW = 80.28 ± 0.19 GeV

0

20

40

60

60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

50 55 60 65 70 75 80 85 90 95

MW (GeV/c2)

Eve

nts

per

1 G

eV/c

2

ALEPH Preliminaryµνqq selection

√s = 191.6,195.5,199.5,201.6 GeV

Data (Luminosity = 237 pb-1)

MC (mW = 80.60 GeV/c2)

Non-WW background

DELPHI preliminary

0

10

20

30

40

50

60

70

80

90

50 55 60 65 70 75 80 85 90 95 100

W mass (GeV/c2)

even

ts /

2 G

eV/c

2

0

50

100

150

200

70 80 90m / GeV

Ev

ents

/ G

eV qqqq 4-jet

OPAL 192-202 GeV preliminary

→ compare reweighted Monte Carlo to data (A,L,O)→ convolute differential cross-section with resolution

function (D,O)→ fit Breit-Wigner curve to measured mass spectrum (O)


• all LEP2 data until 1999 included:

80 80.2 80.4 80.6 80.8 81

MW [GeV]

preliminary

80 80.2 80.4 80.6 80.8 81

ALEPH

DELPHI

L3

OPAL

LEP

80.440 ± 0.064 GeV

80.380 ± 0.071 GeV

80.375 ± 0.077 GeV

80.485 ± 0.065 GeV

80.427 ± 0.046 GeV

χ2/dof = 27.1/29

• LEP value is a combination of individual measurementsfrom the experiments for different channels and years

• contribution to total error from

statistics: 30 MeVsystematics: 36 MeV


• LEP energy error ∆Ebeam = 21 MeV

⇒ ∆MW = ∆EbeamEbeam

· MW = 17 MeV

• new LEP spectrometer → precision ∆Ebeam < 15 MeV(not yet achieved)

• Final State Interactions in qqqq events

γ,Z

W–

W+

πo πo

K+ K+

π– π–

ColourReconnection

Bose-Einstein

• cross-talk affects reconstruction of invariant masses


Identical pions close in phase space:Q2 = squared 4-momentum difference

Q [GeV]

R2

WW qqlv data - 189 GeV•

Z→light quark data - 91 GeV¬

L3 preliminary

0.8

1

1.2

1.4

1.6

1.8

2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Intra-W effects:

W± � Zdecays (Z �→ bb)

Cross talk in W+W− → qqqq ?

Q [GeV]

D, (±

,±)/

0.04

GeV

L3 data

inter-W correlation

no inter-W correlation

p < 1.5 GeVL3

0.80.9

11.11.21.31.41.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4

→ BE correlations only inside each W!?


Compare particle flow between jets from

C

D

A

B

• same (A,C)

• different (B,D)

W-Boson

0

2

4

6

0 50 100 150 200 250 300 350particle flowparticle flowparticle flowparticle flowparticle flow φ

1/N

evt d

σ/dφ

ALEPHpreliminary

particle flow

data (@ 189 GeV)WWZZqqγ

0

2

4

6

0 0.5 1 1.5 2 2.5 3 3.5 4norm. particle flownorm. particle flownorm. particle flownorm. particle flownorm. particle flow φr

1/N

evt d

σ/dφ

r

norm. particle flow

data (@ 189 GeV)WWZZqqγ

W-jets W-jets

ALEPHpreliminary

A B C D

0.75

1

1.25

1.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1φr

1/N

evt d

σ/dφ

r(A

+C)/

(B+D

)

ALEPH preliminary

data 189, 196, 200 GeV combined

KoralW, ki = 0.

KoralW, ki = 0.6

KoralW, ki = 2.3

KoralW, ki = 1000. L3, ALEPH:

R =dn/dφ(A+ C)

dn/dφ(B +D)

→ not yet statistically significant, but promising


• ∆MW = mqqqq −mqqlν probes possible effects of FSI

-0.5 0 0.5

∆MW [GeV]

preliminary

-0.5 0 0.5

ALEPH

DELPHI

L3

OPAL

LEP

0.032 ± 0.091 GeV

−0.011 ± 0.112 GeV

0.190 ± 0.130 GeV

−0.103 ± 0.111 GeV

0.005 ± 0.051 GeV

FSI 0.056 GeV

→ no indication for mass shift due to FSI

• FSI systematic error is estimated by comparingMC models

for BE → with and w/o cross-talkfor CR → SK I, SK II, SK II’, ARIADNE I and II,

HERWIG, GH

all experiments are affected in the same way!

• error on qqqq due to BE: 25 MeV CR: 50 MeV

statistical error component: 34 MeV (qqqq only)


typical example

Source Systematic Errors on MW in MeVqq+ν qqqq Combined

Colour Reconnection – 50 13Bose-Einstein Correlations – 25 7LEP Beam Energy 17 17 17ISR / FSR 8 10 8Hadronisation 26 23 24Detector Systematics 11 8 10Other 5 5 4Total Systematic 35 64 36Statistical 38 34 30Total 51 73 47

• due to FSI → contribution to combined MW measurement:

qqqq 27% and qq+ν 73%


• comparison with other W mass measurements:

W-Boson Mass [GeV]

mW [GeV]

χ2/DoF: 0.1 / 1

80 80.2 80.4 80.6

pp−-colliders 80.452 ± 0.062

LEP2 80.427 ± 0.046

Average 80.436 ± 0.037

NuTeV/CCFR 80.25 ± 0.11

LEP1/SLD/νN/mt 80.386 ± 0.025


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.51.5 2.0 2.5ΓW[GeV]

ALEPH 2.17±0.20

DELPHI 2.09±0.15

L3 2.19±0.21

OPAL 2.04±0.18

LEP 2.12±0.11

LEP Preliminary : Summer 2000

• compare direct width determination by CDF

ΓW = 2.06 ± 0.13 GeV


10

10 2

10 3

80.25 80.5mW [GeV]

mH [G

eV]

W-Boson Mass [GeV]ALEPH

DELPHI

L3

OPAL

LEPpp

Average

80.440±0.064

80.380±0.071

80.375±0.077

80.485±0.065

80.427±0.04680.451±0.062

80.436±0.037

mt =174.3±5.1 GeV

∆α(5)

had =0.02755±0.00046


• compare direct MW and M t with fit to electroweak data:

80.2

80.3

80.4

80.5

80.6

130 150 170 190 210

mH [GeV]113 300 1000

mt [GeV]

mW

[G

eV]

Preliminary

68% CL

LEP1, SLD, νN Data

LEP2, pp− Data

• from electroweak fitswith LEP data: M t = 179+13

−10 GeVall data except direct MW: MW = 80.386 ± 0.025 GeV

Standard Model parameter relationsconfirmed at 1-loop level W W

b

t


•α(M2Z) is important ingredient in EW fits → M H

• running of α:

α(s) =α(0)

1 − ∆αlep(s) − ∆α(5)had(s) − ∆αtop

had(s)• ∆αlep(s) known to 3-loop level

• important is contribution from ∆α(5)had

→ less precise

∆α(5)had(s) ∝

∞∫

4M2π

R(s′) ds′

s′(s′ − s)

γ

q

q

γ

• the ratio R = σ(e+e− → hadrons)/σ(e+e− → µ+µ−)

can be measured and calculated in perturbative QCD• recent measurements by BES II are now included:

BES-II (preliminary 1999)

BES-II (1998)

MK1 normalised to pQCD

γγ2

MEA

RpQCD

√ s (GeV)

Sum of exclusive channels→

← Inclusive data {

0

0.5

1

1.5

2

2.5

3

3.5

4

1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25


Mass of the Higgs Boson

0

2

4

6

10 102

103

mH [GeV]

∆χ2

Excluded Preliminary

∆αhad =∆α(5)

0.02804±0.00065

0.02755±0.00046

theory uncertainty

∆α(5)had M H log(M H/ GeV) M H limit

[ GeV] (95% C.L.)

0.02804 ± 0.00065 60+52−29 1.78+0.27

−0.28 < 162 GeV

new BES included0.02755 ± 0.00046 88+60

−37 1.94+0.22−0.24 < 203 GeV

new BES and pQCD0.02738 ± 0.00020 104+59

−39 2.02+0.19−0.20 < 215 GeV

(Jegerlehner et al., Pietrzyk et al., Martin et al., includes new results on MW by Tevatronand on heavy flavours by SLD)

• the new values of ∆α(5)had yield

→ a better error on log(M H/ GeV)

→ a better agreement with Higgs searches at LEP

• the Higgs boson is light . . . but heavier than

M H > 112.3 GeV at 95% CL (limit from direct searches at LEP)


• Measurement of Fermion Pair Production in good agree-ment with Standard Model predictions

• Further improvement on MZ, jhad within S-Matrix ansatz

• No new (contact) interactions below 10-20 TeV

• Cross sections for Single W, W+W−, ZZ agree as well

• LEP measures W mass and width with increased precision

MW = 80.427 ± 0.046 GeVΓW = 2.12 ± 0.11 GeV

. . . in perfect agreement with fit to electroweak data

• outlook on LEP MW:

with 2000 data → statistical error × 0.85→ δMW = 30 − 40 MeV

• there is progress going on in α(M2Z) determination

• although we are always looking for deviations . . .

The Standard Model works

The Standard Model Higgs boson is light(and maybe already observed?)


133 - 202 GeV

0.6

0.65

0.7

0.75

0.8

0.85

0.9

-0.04 -0.02 0 0.02 0.04 0.06 0.08jb

tot

j b fb

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

-0.3 -0.2 -0.1 0 0.1 0.2 0.3jc tot

j c fb

Stephan Wynhoff Standard Model Physics Results from LEP2

133 - 202 GeV

Stephan Wynhoff Standard Model Physics Results from LEP2

standard model physics results from lep2

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