results from the opal experiment - cern indico

OPAL Celebrates the Standard Model

Results from the OPAL Experiment

Richard Hemingway

IPP/Carleton University

On behalf of the OPAL CollaborationLEP Fest, 10th October 2000

• Introductory Remarks

Luminosity .. Y2K Data

• Physics Results

PRELIMINARY LEP2, including√s > 202 GeV

– Standard Model cross-sections and couplings

– Indirect limits on new physics

– Search for SM and MSSM Higgs

– Searches for new particles

• Where do we go from here?

R.J. Hemingway OPAL presentation at LEP FEST 10 October 2000


Apologies .. Consult our Web site

30 minutes is too short to give adequate recognition to the

intensive work over the past few weeks (months) (years) ..

Please consult our public web site

’The OPAL experiment at LEP’

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

where you will find all OPAL physics results and, in particular, 3

Collective Physics Notes prepared for this LEP Fest

• Measurement of Standard Model Processes in e+e-

Collisions at√s > 202 GeV

• Updated Results of Higgs Boson Searches in e+e-

Collisions at the Highest LEP Energies

• New Particle Searches in e+e- Collisions at√s = 200-209

GeV



Special People .. Many Thanks

• OPAL Spokesmen

– Aldo Michelini, early beginnings - 1993

Design, construction, installation, exploitation LEP1

– Rolf Heuer, 1994 - mid 1998

LEP1 to LEP2 transition, precision physics

– Dave Plane, mid 1998 - present

LEP2 highest energies/luminosities

• OPAL Secretary

– Mette Stuwe, early beginnings - present



The OPAL Collaboration .. past/present

• CANADAAlberta, Carleton, CRPP/NRC, Montreal, UBC, Victoria

• FRANCESaclay

• GERMANYAachen, Bonn, Freiburg, Hamburg/DESY, Heidelberg,LMU-Munich, MPI-Munich

• HUNGARYBudapest, Debrecen

• ISRAELTechnion, Tel Aviv, Weizmann

• ITALYBologna

• JAPANICEPP-Tokyo/Kobe

• CERN

• UKBirmingham, Brunel, Cambridge, Manchester, QMW, RAL,UCL/Birkbeck

• USAChicago, Duke, Indiana, Maryland, Oregon, Riverside, Yale



OPAL Luminosity Summary

• Special thanks to the LEP Division and all the technical staff

associated with the LEP program. Each year, 1989-2000,

has been great!

Today: Total luminosity recorded in Y2K almost 200 pb−1

OPAL data taking efficiency in Y2K = 92%



OPAL Data Quality

• Calibration procedures for OPAL have maintained steady

and robust resolutions throughout the LEP2 period

momentum + d0 resolutions

year

σ p/p2 *

10-3

(1/

GeV

)

Z0 calibration runs

Jet + Vertex Chambers + SI Vtx

LEP1

year

σ(d 0)

(µm

)

1995

/96

shut

dow

n:SI

Vtx

re-

inst

alla

tion

LEP1

high energy runs

Jet + Vertex Chambers + SI Vtx

11.11.21.31.41.51.61.71.81.9

2

1995 1996 1997 1998 1999 2000

1012141618202224262830

1995 1996 1997 1998 1999 2000



A Bhabha event at 209 GeV• 10 August 2000, LEP sets energy record at 209 GeV

• Run lasted less than 2 minutes. Pity! 80 nb−1

Z

Y

X



Standard Model Cross-sections• from CERN Yellow Report, LEP2 Physics



A Multihadron event at 208 GeV• One person’s signal is another person’s background!

Y

Z

200 . cm.

Cen t r e o f s c r een i s ( . 0000 , . 0000 , . 0000 )

50 GeV2010 5



Cross-section for hadrons• separate full-energy annihilation events from

Z0 radiative return

1478 non-radiative events at√s = 205 GeV


10 2

10 3

10 4

10 5

80 100 120 140 160 180 200

-5%

0

+5%

-5%0

+5%

120 140 160 180 200

√s / GeV

Cro

ss-s

ecti

on /

pb

OPALpreliminary

e+e-→hadrons

e+e-→hadrons; s′/s>0.7225

(a)

(b)

Curve is ZFITTER prediction



Cross-section for mu-pairs

• separate full-energy annihilation events from

Z0 radiative return



1

10

10 2

10 3

80 100 120 140 160 180 200

-50%

0

+50%

-50%

0

+50%

120 140 160 180 200

√s / GeV

Cro

ss-s

ecti

on /

pb

OPALpreliminary

e+e-→µ+µ-

e+e-→µ+µ-; s′/s>0.7225

(a)

(b)

Curve is ZFITTER prediction



Forward-backward Asymmetries

• Measure Afb for lepton pairs e+e−, µ+µ−, τ+τ−

0.2

0.4

0.6

0.8

1

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

80 100 120 140 160 180 200

Asy

mm

etry

(a) e+e- |cosθe-|<0.7;θacol<10˚

Asy

mm

etry

(b) µ+µ-

s′/s > 0.01s′/s > 0.7225

√s /GeV

Asy

mm

etry

(c) τ+τ-

s′/s > 0.01s′/s > 0.7225

OPAL preliminary

Curve is BHWIDE (e+e−), ZFITTER (µ+µ−, τ+τ−) prediction



Fine Structure Constant• Use non-radiative cross-sections and asymmetries

• ZFITTER, with all other pars fixed, gives αem(√s)

αem(√

s = 190.6 GeV ) = 128.4+2.5−2.3 (SM: 127.9)

105

110

115

120

125

130

135

140

145

150

155

0 25 50 75 100 125 150 175 200Q / GeV

α-1(Q

)

α-1(0)

αSM(Q)α-1

OPAL 2-fermion fits:average:

TOPAZ µµ/eeµµ and qq average:

Fits to leptonic data from:DORIS, PEP, PETRA, TRISTAN

OPALprelim.



QCD at 206 GeV• All QCD observables well represented by PYTHIA, HERWIG

• No evidence for anomalous 4-jet production

0

0.2

0.4

0.6

0.8

1

10-3

10-2

10-1

n=2

n=4

n=3

n=5

PYTHIA

HERWIG

OPAL

(206 GeV preliminary)OPAL

Durham scheme

ycut

n-Je

t Fra

ctio

n



Strong Coupling Constant

• Fit distributions of 1-T, MH, C, BW, BT, and yD23 to NLLA

O(α2s)QCD calculations

• Results consistent with running of αs

αs(√

s = 205.9 GeV ) = 0.107 ± 0.002 ± 0.004

Summary of OPAL measurements of αs(Q)

0.08

0.085

0.09

0.095

0.1

0.105

0.11

0.115

0.12

0.125

0.13

100 120 140 160 180 200

OPAL (preliminary)

αs(MZ)=0.119±0.004

Q [GeV]

α s(Q

)



Cross-section for photon-pairs

• A pure QED process at tree level

At√s=205 GeV events observed/expected = 467/463

At√s=207 GeV events observed/expected = 534/549

0

2

4

6

8

10

12

14

σ [

pb]

OPALpreliminary

cosθ* < 0.90e+e− γγ(γ)

√s [GeV]

σ / σ

QE

D

160 170 180 190 200 210

0.8

1

1.2



Cross-section for WW-pairs

• Isolate all 3 decay channels WW → qqqq, qq�ν�, �ν��ν�

• Cross-sections assume SM W decay fractions

At√s=205 GeV obtain 651, 545, 125 events resp.

At√s=207 GeV obtain 887, 708, 162 events resp.

√s [GeV]

OPAL preliminary

σ WW

[pb]

Data (Preliminary)

Data (Published)SM predictionNo ZWW Vertex

σ(e+e-→W+W-)

0

2

4

6

8

10

12

14

16

18

20

22

150 160 170 180 190 200 210

SM prediction via RACOONWW and YFSWWR.J. Hemingway OPAL presentation at LEP FEST 10 October 2000


A WW → qqqq event at 208 GeV

Y

XZ

200 . cm.

Cen t r e o f s c r een i s ( . 0000 , . 0000 , . 0000 )

50 GeV2010 5



Cross-section for ZZ-pairs

• Isolate decay channels llll, llνν, qqll, qqνν, qqqq

• Cross-sections assume SM Z decay fractions

At√s=205 GeV obtain 77 candidates, expected SM bkgd = 37

At√s=207 GeV obtain 85 candidates, expected SM bkgd = 45

0

0.25

0.5

0.75

1

1.25

1.5

1.75

175 180 185 190 195 200 205√s (GeV)

Cro

ss s

ectio

n (p

b)

OPAL preliminaryσ(e+e- → ZZ)

2000 data

YFSZZ

No evidence for non-zero neutral TGCs (ZZγ, ZZZ)R.J. Hemingway OPAL presentation at LEP FEST 10 October 2000


A ZZ event at 205 GeV

Run : even t 13394 : 5924 Da t e * * * * * * * T i me 105638

Ebeam102 . 699 Ev i s 213 . 8 Em i s s - 8 . 4 V t x ( - 0 . 05 , 0 . 04 , - 0 . 62 )

Bz=4 . 023 Bunc h l e t 1 / 1 Th r us t =0 . 7139 Ap l an=0 . 0045 Ob l a t =0 . 4498 Sphe r =0 . 1625

C t r k ( N= 25 Sump=174 . 8 ) Ec a l ( N= 36 SumE= 63 . 2 ) Hc a l ( N=17 SumE= 31 . 3 )

Muon ( N= 2 ) Se c V t x ( N= 2 ) Fde t ( N= 0 SumE= 0 . 0 )

M_mm~93 . 1GeV M_qq~90 . 6GeV

Y

XZ

200 . cm.

Cen t r e o f s c r een i s ( - 47 . 1442 , - 3 . 7475 , 0 . 0000 )

50 GeV2010 5



Charged Current TGCs

• Combine WW cross-section and angular distributions with

single-W cross-section

OPAL Preliminary

0

0.5

1

1.5

2

2.5

3

-1 -0.5 0 0.5 1∆κγ

∆ lo

g L

0

0.5

1

1.5

2

2.5

3

-0.4 -0.2 0 0.2 0.4∆g1

z

∆ lo

g L

0

0.5

1

1.5

2

2.5

3

-0.5 -0.25 0 0.25 0.5λ

∆ lo

g L

angulardistributions

event rate

single W

combined

No anomalous behaviour: SM values look OK.

Obtain limits on CP-violating TGCs via spin density matrix.

No evidence for anomalous QGCs, eg WWγγ.



Precision MW Measurement

• With 480pb−1 (prior to Y2K)

mW = 80.485 ± 0.052(stat) ± 0.039(sys)GeV

• With final statistics, expect

mW = 80.xxx± 0.040(stat) ± 0.025(sys)GeV

0

50

100

150

200

250

300

60 65 70 75 80 85 90 95 100

0

50

100

150

200

250

300

60 65 70 75 80 85 90 95 100

m /GeV

Eve

nts

m /GeV

Eve

nts

qqqq

qqlν

OPAL 183-209 GeV 617 pb-1

Signal

Combinatorial b/g

Other b/g

Data shows no FSI (BEC,CR),M(qqqq) =M(qqlν).

Emphasise need for good LEP energy determination.

Estimate all-LEP error 30-35 MeV (SM indirect = 26 MeV).R.J. Hemingway OPAL presentation at LEP FEST 10 October 2000


Non-SM Physics ...indirect limits

ff, γγ,ZZ, ... cross-sections and couplings in agreement with

SM. They provide limits on possible new physics (generally

model dependent).

• 4-fermion Contact Interactions

Mass limits 8-15 TeV with g2/4π = 1

• Sneutrino exchange (RPV) in s-channel

λ131, λ121 coupling limits within 100-300 GeV range

• Z ′ exchange in s-channel

Exclusion limits in 400-750 GeV range

• QED Λ cut-off parameters in e+e− → γγ

Both Λ+,Λ− above 330 GeV

• Excited electron in t-channel γγ

Mass limit ∼ 300 GeV assuming e∗eγ = eeγ coupling

• Low scale quantum gravity in µ+µ−, τ+τ−, γγ,ZZMass limits 830-900 GeV

General conclusion: New Physics is far beyond EW scale



Contact Interactions

15 10 5 0 5 10 15(TeV)

OPAL PreliminaryΛ- Λ+

ee

µµ

ττ

ll

qq

qq+ll

uu

dd

ud

LL RR LR RL VV AA LLRR LRRL ODB



Limits on possible Z ′

-10

-5

0

5

10

Z´ mass [GeV]

Mix

ing

angl

e [m

rad

]

300 500 700 1000 2000

E6 χE6 ψE6 ηLR

OPAL preliminary



Gravity in Extra Dimensions

λ/MS4 (TeV−4)

∆(−l

ogL)

λ/MS4 =

−0.2+0.9−1.0 TeV−4

combined

183−209 GeV

γγ

µµ+ττ

ZZ

OPAL preliminary

0

1

2

3

4

5

6

-8 -6 -4 -2 0 2 4 6 8

95% CL lower limit on MS is

0.83 TeV with λ = −1 and 0.90 TeV with λ = +1



Standard Model Higgs Decays

6 %+

Z ee, µµ

τ+

9 %

H bb( ττ )b

b

τ_

Z ττ (qq)

ll

_

H bb

b

b

60 %

Z qqq q

18 %

H bb

b

b

Z νν

ν ν

H bb

b

b

4-jet channel, data=18, bkgd=19.1+-2.4

Missing E ch, data=26, bkgd=28.1+-2.9

Tau channel, data=4, bkgd=2.7+-0.4

e/mu channel, data=9, bkgd=5.9+-1.0

TOTAL, data=57, bkgd=55.7+-5.8



Higgs: Individual Channel Mass Distributions

OPAL Preliminary

Reconstructed Mass (GeV)

Eve

nts/

4 G

eV 4 jetOPAL Data4-fermion2-fermionsignal (mh=115 GeV)


Eve

nts/

4 G

eV Missing E


Eve

nts/

4 G

eV Tau


Eve

nts/

4 G

eV e/µ

0

2

4

6

8

40 60 80 100 1200

2

4

6

8

60 80 100 120

0

0.5

1

1.5

2

40 60 80 100 1200

1

2

3

4

60 80 100 120



Higgs: Mass Distribution,all Y2K data


Eve

nts

/ 3 G

eV OPAL Preliminary Y2K 200-209 GeV

Higgs 115 GeV

Higgs 110 GeV

Background

0

2

4

6

8

10

12

40 50 60 70 80 90 100 110 120 130



Higgs: Mass Distribution,only 207 GeV data


Eve

nts

/ 3 G

eV OPAL Preliminary 206-209 GeV

Higgs 115 GeV

Higgs 110 GeV

Background

0

1

2

3

4

5

6

7

8

40 50 60 70 80 90 100 110 120 130



Higgs: 1-CLb: background-only hypothesis

mH(GeV)

1-C

Lb

OPAL Preliminary

√s=192−209 GeV

10-6

10-5

10-4

10-3

10-2

10-1

1

100 105 110 115 12010-6

10-5

10-4

10-3

10-2

10-1

1



SM Higgs Mass .. 95% CL lower limit

OPAL Preliminary

√s=192−209 GeV

mH (GeV)

Num

ber

of S

igna

l Eve

nts

0

10

20

30

40

50

100 105 110 115 1200

10

20

30

40

50

mH > 107.9 GeV at 95% CL



More Higgs Searches

• SUSY Higgs

– e+e− → h0Z0: as HZ search

– e+e− → A0h0 → bbbb: Data=7, Bkgd=8.4+-1.3

– e+e− → A0h0 → τ+τ−bb: Data=5, Bkgd=3.4+-0.5

For tan(β) > 1.2, determine

mh > 80.7 GeV, mA > 83.2 GeV.

– e+e− → H+H− → τ+ντqq′: Data=160, Bkgd=168

– e+e− → H+H− → qq′qq′: Data=279, Bkgd=302

mH± > 72.3 GeV assuming Br(τν + qq) = 1

• Special Higgs Decays

– Invisible h0, eg.→ χ̃01χ̃

01: Data=35, Bkgd=53

mh0 > 107.2 GeV assuming SM prod rate

– Fermiophobic h0 → γγ: Data=16, Bkgd=19

mh0 > 104.6 GeV assuming SM prod rate



Limits in CMSSM parameter space

mh-max: A specific benchmark MSSM scan which provides the

most conservative range of excluded tan(β) values.

1

10

0 20 40 60 80 100 120 140

1

10

mh (GeV/c2)

tanβ

Excluded

TheoreticallyInaccessible

mh-max

Exclusion: tan(β) region 0.8-2.0



Search Channel List

e+e− → hZ h → bb bb̄qq̄, bb̄νν̄, bb̄�+�−

h → γγ (qq̄, �+�−, νν̄)+γγh → χ̃0χ̃0 qq̄, �+�−+ �E

e+e− → hA h, A → bb̄, ττ bb̄bb̄, bb̄τ+τ−

h → AA bb̄bb̄bb̄e+e− → H+H− H+ → qq̄, τν qq̄qq̄, qq̄τν, τντνe+e− → χ̃+χ̃− χ̃− → W∗χ̃0 jets (+�±), �+�−+ �E

(In)Direct RPV jets, �±, ν(χ̃0 → γG̃) jets, �+�−, γγ+ �E

e+e− → χ̃02χ̃

01 χ̃0

2 → Z0χ̃01 2 jets + �E

χ̃02 → γχ̃0

1 γ+ �Ee+e− → χ̃0

1χ̃01 χ̃0

1 → γG̃ γγ+ �Eχ̃0

1 Lifetime non-pointing γRPV Decays jets, �±, ν

e+e− → �̃+�̃− �̃− → �−χ̃01 �+�−+ �E

(χ̃0 → γG̃) �+�−γγ+ �E(In)Direct RPV 2,4,6×�±+ �E�̃± Lifetime Kinked Tracks

Stable, Chargede+e− → ν̃ν̃ (In)Direct RPV �+�−�+�−

jets + �Ee+e− → t̃1t̃1 t̃1 → cχ̃0

1 2 jets + �Et̃1 → b�+ν̃ 2 jets + �+�−+ �E(In)Direct RPV �+q�−q

e+e− → NN̄ N → �W jets +�±

e+e− → L+L− L+ → νW jets, �±+ �Ee+e− → �∗+�(∗)− �∗+ → �+γ �+�−γ(γ)e+e− → ν∗ν̄∗ ν∗ → νγ γ(γ)+ �Ee+e− → �∗�, ν∗ν �∗ → �Z jets, �±, ν



Single Photon Recoil Mass

Sensitive to Gravity in Extra Dimensions, GMSB scenarios, eg.

χ̃01 → γG̃, and MSSM χ̃0

2 → χ̃01γ, and excited neutrinos, eg.

ν∗ → νγ.

Standard Model process is e+e− → ννγ(γ).

2000 data: √s = 200 - 209 GeV

∫Ldt = 166.3 pb-1

Nobs = 526Nexp = 530 ± 26

Mrecoil (GeV)

Eve

nts

/ 4 G

eV

0

10

20

30

40

50

0 25 50 75 100 125 150 175 200



Non-SM Physics ...direct searches

In general a null search provides limits on cross-sections,couplings, masses, ...

• Leptoquarkse+e− → LqLq, Lq → lq Data=54, Bkgd=55

• Heavy Leptonse+e− → NN,N → lW Data=63, Bkgd=51

• Excited Leptonse+e− → l∗+l∗−, l∗ → lγ Data=6, Bkgd=4e+e− → l∗+l−, l∗ → lγ Data=642, Bkgd=691

• Stable, long-lived, massive particlesSensitive toQ/e = ±1,±2/3 Data=0, Bkgd=1

• Single top via FCNCe+e− → tc(u) Data=21, Bkgd=23

M(l∗) > 103 GeV

M(�̃) > 97 GeV,M(χ̃±) > 101 GeV , for long-lived

σtop < 0.36 pb, assuming Br(t → bW ) = 1



Non-SM Physics ...direct SUSY searches

MSSM searches of two types are conducted

(A) MSUGRA, with/without RPC, where LSP = χ̃01 (stable),

which leads to topologies with jets, leptons, AND Missing Energy

(B) GMSB, where LSP = G̃ [χ̃01 → γG̃, �̃→ lG̃], which give

MORE leptons and photons

• Scalar Leptons

e+e− → �̃+�̃−, �̃→ lχ̃01

• Scalar top/bottom quarks

e+e− → t̃t̃, t̃ → cχ̃01, bχ̃

±

e+e− → b̃b̃, b̃ → bχ̃01

• Charginos

e+e− → χ̃±χ̃∓, χ̃± → χ̃01W

±

• Neutralinos

e+e− → χ̃02χ̃

01, χ̃

02 → χ̃0

1Z0

No compelling evidence for data in excess of SM bkgd

Cross-section limits are determined

Calculate exclusion regions in MSSM parameter space



MSSM exclusion limit for stop

0

25

50

75

100

40 60 80 100 120

m ( t∼1 ) [ GeV ]

m (

χ∼10

) [ G

eV ]

OPAL Preliminary

t∼ → c χ

∼10 θ t = 0.0∼

θ t = 0.98∼

CDF

LEP

1



MSSM exclusion limit for gauginos

20

40

60

80

100

80 90 100 110m ( χ∼ 1 ) [GeV]

m (

χ∼10 )

[GeV

]

OPAL Preliminary

tan β =1.5

+−

FORBIDDEN (THEORY)

FORBIDDEN (THEORY)



MSSM exclusion limit for neutralino

0

20

40

60

1 10

E x c l u d e d

a t 9 5 % C L

3 9 . 0G e V

3 6. 0G e V

t a n β

m (

χ∼

10 ) [

Ge

V

]

O P A L P r e l i m i na r y

Absolute lower limit on lightest neutralino

mχ̃01

> 39.0 GeV for m0 > 500 GeV

mχ̃01

> 36.0 GeV for any m0



Summary

• OPAL data taking in 2000 very successful∫ Ldt almost 200 pb−1

• Many ongoing physics analyses, both LEP1 and LEP2

• All results from LEP data in good agreement with SM

predictions.

• We look forward to several years of continuing physics

analysis and

.....perhaps, in some forgotten corner, unexpected new

physics


results from the opal experiment - cern indico

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