Download - Recent Electroweak Results from the Tevatron

Recent Electroweak Results Recent Electroweak Results from the Tevatronfrom the Tevatron

Weak Interactions and Neutrinos WorkshopDelphi, Greece, 6-11 June, 2005

Dhiman ChakrabortyDhiman Chakraborty

Northern Illinois UniversityNorthern Illinois University

For the DØ and CDF CollaborationsFor the DØ and CDF Collaborations

CDF

WIN'05, June, 2005 Tevatron Electroweak Results Dhiman Chakraborty

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OutlineOutline• The Tevatron and the collider experiments at Fermilab: DØ & CDF

• W and Z production cross sections in collisions, asymmetries,

lepton universality, …

• Measurements of W mass and width

• Diboson production

• Summary

For further details and updates, see

http://fcdfwww.fnal.gov/physics/ewk/ (CDF)

http://www-d0.fnal.gov/Run2Physics/WWW/results/ew.htm (DØ)

pp

http://fcdfwww.fnal.gov/physics/ewk/

http://www-d0.fnal.gov/Run2Physics/WWW/results/ew.htm


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Run 2 at the TevatronRun 2 at the Tevatron

• pp

pp Collisions at TeV,

= 0.8 fb-1 recorded,

≈ 0.4 fb-1 analyzed so far,

≈ 4-8 fb-1 expected by 2009.

96.1s

Ldt

Ldt

Ldt

L > 1032 cm-2s-1 achieved


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The DThe DØ and CDF Ø and CDF ExperimentsExperiments

CDF


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W, Z Production Cross W, Z Production Cross SectionsSections

Motivation:

• Important tests of the SM

• Leptonic (e, ) decays ⇒ clean signal

• Important for detector and luminosity calibrations

Limiting factors:

• Luminosity: L/L ≈ 6%

• PDF’s: ≈ 1.5%

• Lepton identification efficiencies, statistics, …

Many of these cancel out in (W)/(Z) ratios!


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General Analysis MethodGeneral Analysis Method

• Look for high-pT e, ,from decays of W, Z in

• Model signal and physics backgrounds using Monte Carlo,

• Model instrumental backgrounds using data,

• Design selection criteria to maximize expected signal significance,

• Estimate efficiencies using data, acceptance using Monte Carlo and

detector simulation,

•

LdtA

NNB

bkgobs

XZWpp /


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Some Some (W), (W), (Z) results at(Z) results at √√s=1.96 TeVs=1.96 TeV

DØ:

• 2865 ± 8 (stat) ± 76 (syst) ±186 (lum) pb

• 2989± 15 (stat) ± 81 (syst) ±194 (lum) pb

CDF:

• 2786 ±12 (stat) +60-54 (syst) ± 166 (lum) pb

• 253.1±6.8 (stat) +8.9-8.1 (syst) ±15.1(lum) pb

• 11.02 ± 0.18 (stat) ± 0.14 (syst)

Theory: R = 10.69±0.013

)()( eWBWpp

)()( WBWpp

)()( ZBZpp

)()( WBWpp

)()(

)()(

ZBZpp

WBWppR


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W, Z Cross Section W, Z Cross Section SummarySummary

• Theory: NNLO calculations by Hamberg, van Neerven, Matsuura (1991).


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W mass in the Standard W mass in the Standard ModelModel

• MW expressed in terms of MZ, mt, and electroweak coupling:

• Total uncertainty is dominated by the radiative correction term

r (≈0.067) which, in turn, is dominated by mt and MH. All

others have been measured with precisions of ~10-4 or better.

• Thus, measurements of mt and MW together constrain MH.

• Need a precision of 30 MeV on MW to match the current

precision of 4.3 GeV on mt.

)1)(/1(2 22

2

rmmGm

ZWF

EMW


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W mass at the TevatronW mass at the Tevatron

• Charged lepton momenta are the most precisely measured.

• Only the transverse component of neutrino momenta can be

inferred after measuring the recoil system.

• Combine l± and transverse momenta to form W transverse

mass:

• Use leptonic Z decay events to model soft hadronic recoil

system.

)cos1(22 TlTT ppM


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ppTT’s in W/Z Production & ’s in W/Z Production & DecayDecay

• Run 2 goal: calibrate charged lepton pT to 0.01% (now at

0.03%). CDF W→ CDF Z→

DØ Z→DØ Z→ee


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W transverse massW transverse mass

CDF W→CDF W→e

DØ W→

DØ W→e


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CDF W Mass Summary CDF W Mass Summary

• 200 pb-1 of data analyzed by CDF.

• Combined uncertainty on MW (76 MeV) smaller than Run 1(79 MeV).

• DØ finalizing calorimeter calibration.

• Expect to achieve MW<40 MeV per experiment by the end of Run 2.

Source of uncertainty (MeV) W→e (Run 1b) W→ (Run 1b)

Lepton E scale, p resolution 70 (80) 30 (87)

Recoil E scale and resolution 50 (37) 50 (35)

Background 20 (5) 20 (25)

Production & decay model 30 (30) 30 (30)

Statistics 45 (65) 50 (100)

Total 105 (110) 85 (140)


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Lepton Universality in W Lepton Universality in W DecaysDecays

Many systematic uncertainties cancel out.

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2

)(

)(

)()(

)()(

eg

g

eW

W

eWBWpp

WBWppU

012.0998.0 eg

g

syststat 04.002.099.0 eg

g

Similarly,


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W width: Indirect W width: Indirect MeasurementMeasurementUsing direct measurements of leptonic partial widths & Z

lineshape, one can extract the total width of the W boson from

CDF measurements from Run 2:

)()()(

)()()(

)()(

)()(

WllZZpp

ZlWWpp

llZBZpp

lWBWppR

Channel (W) [MeV] ∫Ldt (pb-1)

e+ 2079±41 72

e 2056±44 194

World avg. 2124±41


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W width: Direct W width: Direct MeasurementMeasurementDØ: 177 pb-1 W→e sample

Method• Generate MC templates with different W widths

• Compare to the tail of MTW distribution

Main systematic uncertainties• Hadronic response and resolution: ~64 MeV• Underlying event: ~47 MeV• EM resolution: ~30 MeV


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W Charge AsymmetryW Charge Asymmetry• Probes the proton structure:

• Relies on lepton charge

identification

• Convolution of W production

asymmetry and V-A decay:

dlddld

dlddldA l /)(/)(

/)(/)()(


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• Sensitive to new physics

• QED radiative corrections

• Main background: jets

misidentified as electrons

• Systematic uncertainties:

– Background estimation

– PDFs

– Detector modeling

– Z/ pT in Monte Carlo

eedM

eeZd )/( *


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ZZ//→ → ee ee Forward-Backward Forward-Backward AsymmetryAsymmetry

• An extension of the analysis

• V-A nature of fermion-Z coupling

leads to asymmetry in lepton

production angle w.r.t beam axis:

• AFB vs. mass has different

sensitivity to u, d quarks in proton

• Mix of V and A couplings change

with mass.

dM

d

1

0

0

1

1

0

0

1

)(cos)(cos

)(cos)(cos

)(cos)(cos

)(cos)(cos

dd

dd

d

d

dd

dd

dd

AFB


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Diboson Production: WDiboson Production: W, WW, , WW, WZ, ZWZ, Z

• Measure cross sections, look for physics beyond the SM.

• Background for top pair production, Higgs, NP searches.

• Only leptonic final states used to keep backgrounds manageable.

• Similar to LEP and Run1 measurements.

Test for Anomalous Couplings via Leff:

Where V = Z,

In the SM, g1V=V=1; V=0.

Determine from data: g1V=g1

V-1; V; V=V-1

VWWM

VWWWVWVWWggLW

VVVWWVWWV

†2

†††1 )(/


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Example: WExample: W anomalous anomalous couplingcoupling

Binned likelihood fits to ET() gives 1d, 2d results on ,

1d limits at 95% CL (= 2 TeV):

-0.88<<0.96,

-0.20< <0.20

2d limit at 95% CL (= 2 TeV)


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Diboson SummaryDiboson Summary

No significant departure observed from SM predictions in any channel.


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SummarySummaryA successful electroweak physics program is shaping

up with the ongoing Run 2 of the Tevatron– W mass measurement is on course to an unprecedented

∽30 MeV precision (CDF ⊕ DØ, all channels combined).

– Most systematic uncertainties scale with (∫Ldt)-½. [⇐Run 1]

– Inclusive cross section measurements have reached high precision and are in good agreement with SM expectations.

– Competitive results on W width and lepton universality.

W<50 MeV should be possible with ∫Ldt ≈2fb-1 .

– W charge asymmetry measurements pressing PDFs.

– No significant departure from SM found so far, but we’ll stay alert …


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Backup SlidesBackup Slides


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CDF W mass CDF W mass measurementmeasurement

• Precision of O(10-4) requires detailed modeling of measured line shapes.

• QCD corrections to W/Z production (RESBOS for pT(W/Z)): MW=±13 MeV.

• QED corrections to W/Z decays (FS ’s w/ WGRAD): MW=15-20 MeV.

• Muon momentum calibration using J/ψ & Υ decays: MW=±25 MeV.

• Electron energy calibration using E/p: MW=±55 MeV.

• Hadronic recoil energy estimated by vectorially summing energies in all

calorimeter towers except those associated with the charged lepton.

Hadronic response parametrized using Z→ events. MW=±37 MeV.

• Background from cosmic rays, QCD jets faking isolated l±: MW=±20

MeV.