Measurement of the W Boson MassMeasurement of the W Boson Mass

Yu ZengYu Zeng

Supervisor: Prof. KotwalSupervisor: Prof. Kotwal

Duke UniversityDuke University

4/11/2007 PHY 352 Seminar 2

OutlineOutline

• Introduction to the Standard ModelIntroduction to the Standard Model• Motivation of W mass measurementMotivation of W mass measurement• Method (calibration, simulation …)Method (calibration, simulation …)• Result and discussionResult and discussion• Future prospectsFuture prospects

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The Standard Model (SM)The Standard Model (SM)• It is a special relativity quantum field theory in which the It is a special relativity quantum field theory in which the

dynamics is generated from the assumption of local gauge dynamics is generated from the assumption of local gauge invariances.invariances.

• It is renormalizable (divergences can be absorbed into It is renormalizable (divergences can be absorbed into parameters such as masses and coupling strengths.)parameters such as masses and coupling strengths.)

• Encompasses Electroweak theory and QCDEncompasses Electroweak theory and QCD

• The The onlyonly elementary particle theory that has been verified elementary particle theory that has been verified experimentally.experimentally.

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The Standard Model (SM)The Standard Model (SM)• Number of “elementary particles” in SM:Number of “elementary particles” in SM:

Physical QuantityPhysical Quantity No.No.

Mass of quark 6

Mass of lepton 3

Masses of W±,Z, Higgs 3

Coupling strength 2

Quark EWK mixing parameter 4

Strong CP violation 1

Neutrino mass 3

Neutrino mixing parameter 4

12 leptons + 36 quarks + 12 mediators + 1 Higgs = 61

• Parameters needed to SM completely predictive:Parameters needed to SM completely predictive:

Total = 26

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MotivationMotivation• W mass is a fundamental parameter in SM.W mass is a fundamental parameter in SM.• Precise W mass and top quark mass values constrain the mass of Precise W mass and top quark mass values constrain the mass of

undiscovered Higgs.undiscovered Higgs.

• With ultimate precision can set With ultimate precision can set limits on new particles in loopslimits on new particles in loops

(Higher order radiative corrections from loop diagrams involving (Higher order radiative corrections from loop diagrams involving other particles contribute to the observed W boson mass)other particles contribute to the observed W boson mass)

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Radiative CorrectionsRadiative Corrections

• Top quark mass and the Higgs boson mass dominate radiatTop quark mass and the Higgs boson mass dominate radiative correctionsive corrections

2

80.380 0.526 1 0.054ln ( , , ,...)174 100

t HW EM S Z

m mm f m

13 MeV shift to Mass of W if M_t≈2.1GeV△

Arouse few MeV shift to Mass of W

• Currently W mass uncertainty dominates the above Currently W mass uncertainty dominates the above relationshiprelationship

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Motivation cont’dMotivation cont’dExample: Relations among the masses of W, t and HiggsExample: Relations among the masses of W, t and Higgs

• Loop effects of the Loop effects of the masses of W and t to masses of W and t to that of Higgs are quite that of Higgs are quite different in size. W different in size. W mass uncertainty mass uncertainty dominates.dominates.

http://acfahep.kek.jp/acfareport/node181.html

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History of W Boson StudyHistory of W Boson Study• Experimental effortExperimental effort

19831983 Discovery of the W at Discovery of the W at CERN’s proton-antiproton cCERN’s proton-antiproton collider by UA1 & UA2 collaollider by UA1 & UA2 collaborationsborations

19961996 CERN’s e+e- collider LE CERN’s e+e- collider LEP increased its c.m. energy aboP increased its c.m. energy above 161 GeV which is threshold ve 161 GeV which is threshold for W pair productionfor W pair production

19851985 Tevatron, the second p Tevatron, the second proton-antiproton collider, waroton-antiproton collider, was commissioned at Fermilabs commissioned at Fermilab

2000 2000 four LEP experiments four LEP experiments (ALEPH, DELPHI, L3, OPAL) (ALEPH, DELPHI, L3, OPAL) ceased data takingceased data taking

19871987 Fermilab observed its f Fermilab observed its first W candidateirst W candidate

Now Now CDF and D0 at Fermilab CDF and D0 at Fermilab are still runningare still running

W boson mass has been measured with increasing precision by those experiments

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Collider Detector at Fermilab (CDF)Collider Detector at Fermilab (CDF)

Muon Detector

Central Hadronic Calorimeter

Central Outer Tracker

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The CDF Detector The CDF Detector

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The CDF Detector (Quadrant)The CDF Detector (Quadrant)

Central Hadronic Calorimeter

Central E&M CalorimeterProvides precise measurement of electron energy

Provides precise measurement of track momentum

Provides measurement of hadronic recoil objects

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Particle IdentificationParticle Identification• Particle detectors measure long-lived particles produced frParticle detectors measure long-lived particles produced fr

om high energy collisions: electrons, muons, photons and om high energy collisions: electrons, muons, photons and “stable” hadrons (protons, kaons, pions)“stable” hadrons (protons, kaons, pions)

• Quarks and gluons do not appear as free particles, they hadQuarks and gluons do not appear as free particles, they hadronize into a jet.ronize into a jet.

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W Boson ProductionW Boson Production

Process a) dominates (80%), Process b) implies the existence of net transverse momentum. Process a) dominates (80%), Process b) implies the existence of net transverse momentum.

,

,

e

e

u d g W e

u d g W e

eW e

Lepton Pt carries most information of W mass

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2 (1 cos( )) 2 ( ) (1 cos( ))l l lT T T T T Tm p p p p u

W Mass Measurement (1)W Mass Measurement (1)• Invariant mass of lepton-neutrino cannot be reconstructed since neutrino Invariant mass of lepton-neutrino cannot be reconstructed since neutrino

momentum in beam direction is unknown. However, we can use transverse massmomentum in beam direction is unknown. However, we can use transverse mass

1). Relatively insensitive to the production dynamics of W.

2/ ~ ( / )WT T T Wm m p M

2). Sensitive to detector response to recoil particles.

Features of transverse mass spectrum:

Angle between 2 pt

eW e

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W Mass Measurement (2)W Mass Measurement (2)• Another way is to use transverse momentum spectrum of Another way is to use transverse momentum spectrum of

leptonlepton

/ ~ ( / )WT T T Wm m p M

1). Better resolution than neutrino pt

2). Sensitive to the W boson production dynamics

Features of transverse momentum of lepton:

→ relatively insensitive to the recoil response of detector

Sensitive to both W production dynamics & the recoil response

• A third way is to use transverse momentum spectrum of A third way is to use transverse momentum spectrum of neutrinoneutrino

Features of transverse momentum of neutrino:

eW e

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W Mass Measurement (3)W Mass Measurement (3)

Source: A. Kotwal 2007 Aspen talk

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Tracker calibration

EM Calorimeter calibration

• Detector CalibrationDetector Calibration

W Mass Measurement StrategyW Mass Measurement Strategy

• Fast SimulationFast SimulationNLO event generator

Detector response simulation

Hadronic recoil modelling

W mass templates, bule for 80 GeV, red for 81 GeV

Data

+ Backgrounds

Binned Likelihood Fit W boson mass

4/11/2007 PHY 352 Seminar 18

Event Selection for W & ZEvent Selection for W & Z• Select clean W and Z samples to get maximum ratio of S/N.Select clean W and Z samples to get maximum ratio of S/N.

Trigger info: lepton Pt>18 GeV

Central leptons selection: |eta|<1

Final Analysis: lepton Pt>30 GeV

W boson further requires: u<15 GeV and missing Et>30GeV

Z boson: two charged leptonsCollected data used (02/2002-09/2003) ~ 1/10 of data on tape.

Number of W events comparable to 4 LEP experiments combined.

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Detector CalibrationDetector Calibration

• Tracker calibrationTracker calibration

1). Calibration of COT using comic rays

2). J/psimu+mu- and Upsilonmu+mu- are used to scale COT momentum

3). Using Zmu+mu- invariant mass fit to further check

• EM Calorimeter calibrationEM Calorimeter calibration1). Using Ecal/p ratio to scale COT momentum

2). Using Ze+e- mass fit to further check calorimeter energy scale

4/11/2007 PHY 352 Seminar 20

BackgroundsBackgrounds

• Largest background comes from ZLargest background comes from Zmu+mu-mu+mu-• WWtau nutau numu nu nu eventsmu nu nu events• Cosmic raysCosmic rays• Kaon decays in flightKaon decays in flight• QCD jet events where one jet contains one non-isolated muon QCD jet events where one jet contains one non-isolated muon

For WFor Wmu numu nu

For WFor We nue nu• ZZe+e-e+e-• WWtau nutau nue nu nue nu nu• QCDQCD

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Transverse Mass Fitting resultsTransverse Mass Fitting results

W e W

background background

4/11/2007 PHY 352 Seminar 22

Transverse Mass UncertaintiesTransverse Mass Uncertainties

Combined electron and muon uncertainty is 48 MeV

4/11/2007 PHY 352 Seminar 23

Other W Mass Fits – Lepton Pt (Et)Other W Mass Fits – Lepton Pt (Et)

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Other W Mass Fits – Neutrino PtOther W Mass Fits – Neutrino Pt

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Combined ResultsCombined Results

280413 48( ) , ( ) 44%Wm stat sys MeV P

• Combine all 6 fitting results:Combine all 6 fitting results:

Best single precise measurement!

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Implications for Standard ModelImplications for Standard Model

332476HM GeV

392885HM GeV

• Uncertainty down from 29 MeV to Uncertainty down from 29 MeV to 2525 MeV MeV• Central value up from 80392 MeV to Central value up from 80392 MeV to 8039880398 MeV MeV• Previous SM Higgs mass prediction fromPrevious SM Higgs mass prediction from

• 95% CL upper limit on Higgs mass lowers from previous 199 GeV to 189 GeV95% CL upper limit on Higgs mass lowers from previous 199 GeV to 189 GeV

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The Implications for TevatronThe Implications for Tevatron

In 2004, the estimated upper limit for Higgs mass is 250 GeV, however Tevatron only reach upper limit 170 GeV, people think Tevatron has no chance to

find Higgs.

Now Tevatron is back into the competition.

4/11/2007 PHY 352 Seminar 28

Future Prospects at CDFFuture Prospects at CDF

• Mw uncertainties are dominated by statistics of calibration dMw uncertainties are dominated by statistics of calibration data. Current analysis only used 1/10ata. Current analysis only used 1/10thth of data on tape. of data on tape.

• Detailed study of PDFs (Parton Distribution Fuction) to reduDetailed study of PDFs (Parton Distribution Fuction) to reduce systematic uncertainties. ce systematic uncertainties.

• Magnetic field within COT is not uniform, need to fix that.Magnetic field within COT is not uniform, need to fix that.• Calibrate sag of wires in COT due to gravityCalibrate sag of wires in COT due to gravity• ……

Goal: Delta_mw<25 MeV from 1.5 fb^-1 of CDF data

For Example:For Example:

4/11/2007 PHY 352 Seminar 29

ReferencesReferences• Ashutosh Kotwal, Aspen Conference on Particle Physics (2007)Ashutosh Kotwal, Aspen Conference on Particle Physics (2007)• CDF Note 8665CDF Note 8665• http://acfahep.kek.jp/acfareport/node181.htmlhttp://acfahep.kek.jp/acfareport/node181.html• William Trischuk, Collider 2 Cosmic Rays (2007)William Trischuk, Collider 2 Cosmic Rays (2007)• Oliver Stelzer-Chilton, PhD thesis, University of Toronto (2006)Oliver Stelzer-Chilton, PhD thesis, University of Toronto (2006)• Andrew Gordon, PhD thesis, Harvard University (1998) Andrew Gordon, PhD thesis, Harvard University (1998) • Al Goshaw, Phy346 Lecture notes, Duke University (2007)Al Goshaw, Phy346 Lecture notes, Duke University (2007)

AcknowledgementAcknowledgement

Prof. Ashutosh KotwalProf. Ashutosh Kotwal

4/11/2007 PHY 352 Seminar 30

Backup Slides …Backup Slides …

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Choices of SM Parameters (1)Choices of SM Parameters (1)

Physical QuantityPhysical Quantity No.No.

Fermion masses (6 quark + 3 lepton) 9

Higgs Boson 1

Quark weak mixing parameter 4

Strong CP violation parameter 1

Strong interaction coupling constant 1

Fundamental EWK parameters 3

Neutrino masses 3

Neutrino mixing parameter 4

Total = 26

Can be chosen from:

2sinem z F W Z Wg g G m m v

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Choices of SM Parameters (2)Choices of SM Parameters (2)2sinem z F W Z Wg g G m m v

Follow the pattern that parameters are masses and coupling constants.

Choice 1. Choice 2.

Choice 1.

Choose parameters measured most precisely.

4/11/2007 PHY 352 Seminar 33

MotivationMotivation

• The EWK sector of SM is constrained by three The EWK sector of SM is constrained by three precisely measured parameters:precisely measured parameters:

• At lowest order, these parameters are related by:At lowest order, these parameters are related by:2 2

2 2 2

/ 2

/ 2

cos

W F W

Z F W

W Z W

M G sin

M G sin cos

M M

5

( ) 1/127.918(18)

1.16637(1) 10

91.1876(21)

EM Z

F

Z

M

G

M GeV

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Blind Analysis TechniqueBlind Analysis Technique

• A random [-100,100] MeV offset is added in the likelihood A random [-100,100] MeV offset is added in the likelihood fitter, thus all W mass fits are blindedfitter, thus all W mass fits are blinded

• Blinding offset is removed after the analysis was frozon.Blinding offset is removed after the analysis was frozon.• Benefit: allowing study data in detail while keeping W maBenefit: allowing study data in detail while keeping W ma

ss value unknown within 100 MeV. Helps to avoid biased ss value unknown within 100 MeV. Helps to avoid biased analysis.analysis.

4/11/2007 PHY 352 Seminar 35

Why two coupling constantsWhy two coupling constants

e e

e e

W

e e

4eg 2

1 /

eW

W Z

gg

M M

( / )Z e Z Wg g M M

Thus, only two counpling constants:

1) e2/(4hc)=1/137; 2) S for strong coupling