Measurement of the W Boson Mass

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Measurement of the W Boson Mass

• Yu Zeng
• Supervisor Prof. Kotwal
• Duke University

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Outline

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

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The Standard Model (SM)

• It is a special relativity quantum field theory
  in which the dynamics is generated from the
  assumption of local gauge invariances.

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

• Encompasses Electroweak theory and QCD

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

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The Standard Model (SM)

• Number of elementary particles in SM

12 leptons 36 quarks 12 mediators 1 Higgs
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• Parameters needed to SM completely predictive

Physical Quantity 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
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Motivation

• W mass is a fundamental parameter in SM.

• Precise W mass and top quark mass values
  constrain the mass of undiscovered Higgs.

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

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

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Radiative Corrections

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

13 MeV shift to Mass of W if ?M_t2.1GeV
Arouse few MeV shift to Mass of W

• Currently W mass uncertainty dominates the above
  relationship

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Motivation contd

• Example Relations among the masses of W, t and
  Higgs

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

http//acfahep.kek.jp/acfareport/node181.html
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History of W Boson Study

• Experimental effort

1983 Discovery of the W at CERNs proton-antiproton collider by UA1 UA2 collaborations 1996 CERNs ee- collider LEP increased its c.m. energy above 161 GeV which is threshold for W pair production
1985 Tevatron, the second proton-antiproton collider, was commissioned at Fermilab 2000 four LEP experiments (ALEPH, DELPHI, L3, OPAL) ceased data taking
1987 Fermilab observed its first W candidate Now CDF and D0 at Fermilab are still running
W boson mass has been measured with increasing
precision by those experiments
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Collider Detector at Fermilab (CDF)
Muon Detector
Central Hadronic Calorimeter
Central Outer Tracker
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The CDF Detector
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The CDF Detector (Quadrant)
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Particle Identification

• Particle detectors measure long-lived particles
  produced from high energy collisions electrons,
  muons, photons and stable hadrons (protons,
  kaons, pions)

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

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W Boson Production

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

Lepton Pt carries most information of W mass
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W Mass Measurement (1)

• Invariant mass of lepton-neutrino cannot be
  reconstructed since neutrino momentum in beam
  direction is unknown. However, we can use
  transverse mass

Features of transverse mass spectrum
1). Relatively insensitive to the production
dynamics of W.
2). Sensitive to detector response to recoil
particles.
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W Mass Measurement (2)

• Another way is to use transverse momentum
  spectrum of lepton

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W Mass Measurement (3)
Source A. Kotwal 2007 Aspen talk
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W Mass Measurement Strategy

• Detector Calibration

Data

• Fast Simulation

Binned Likelihood Fit
W boson mass
NLO event generator Detector response
simulation Hadronic recoil modelling

Backgrounds
W mass templates, bule for 80 GeV, red for 81 GeV
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Event Selection for W Z

• Select clean W and Z samples to get maximum ratio
  of S/N.

Trigger info lepton Ptgt18 GeV Central leptons
selection etalt1 Final Analysis lepton Ptgt30
GeV W boson further requires ult15 GeV and
missing Etgt30GeV Z boson two charged leptons
Collected 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 Calibration

• Tracker calibration

1). Calibration of COT using comic rays 2).
J/psi?mumu- and Upsilon?mumu- are used to scale
COT momentum 3). Using Z?mumu- invariant mass
fit to further check

• EM Calorimeter calibration

1). Using Ecal/p ratio to scale COT momentum 2).
Using Z?ee- mass fit to further check
calorimeter energy scale
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Backgrounds
For W?mu nu

• Largest background comes from Z?mumu-
• W?tau nu?mu nu nu events
• Cosmic rays
• Kaon decays in flight
• QCD jet events where one jet contains one
  non-isolated muon

For W?e nu

• Z?ee-
• W?tau nu?e nu nu
• QCD

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Transverse Mass Fitting results
background
background
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Transverse Mass Uncertainties
Combined electron and muon uncertainty is 48 MeV
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Other W Mass Fits Lepton Pt (Et)
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Other W Mass Fits Neutrino Pt
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Combined Results

• Combine all 6 fitting results

Best single precise measurement!
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Implications for Standard Model

• Uncertainty down from 29 MeV to 25 MeV
• Central value up from 80392 MeV to 80398 MeV
• Previous SM Higgs mass prediction from

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

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The 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.
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Future Prospects at CDF
For Example

• Mw uncertainties are dominated by statistics of
  calibration data. Current analysis only used
  1/10th of data on tape.
• Detailed study of PDFs (Parton Distribution
  Fuction) to reduce systematic uncertainties.
• Magnetic field within COT is not uniform, need to
  fix that.
• Calibrate sag of wires in COT due to gravity

Goal Delta_mwlt25 MeV from 1.5 fb-1 of CDF data
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References
Acknowledgement

• Prof. Ashutosh Kotwal

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

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Backup Slides
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Choices of SM Parameters (1)
Physical Quantity 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
Can be chosen from
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Choices of SM Parameters (2)
Choice 1.
Choice 2.
Choice 1.
Follow the pattern that parameters are masses and
coupling constants.
Choose parameters measured most precisely.
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Motivation

• The EWK sector of SM is constrained by three
  precisely measured parameters
• At lowest order, these parameters are related by

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Blind Analysis Technique

• A random -100,100 MeV offset is added in the
  likelihood fitter, thus all W mass fits are
  blinded
• Blinding offset is removed after the analysis was
  frozon.
• Benefit allowing study data in detail while
  keeping W mass value unknown within 100 MeV.
  Helps to avoid biased analysis.

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Why two coupling constants
Thus, only two counpling constants
1) ae2/(4phc)1/137 2) aS
for strong coupling