Transcript Measurement of the W Boson Mass

Measurement of the W Boson Mass
Yu Zeng
Supervisor: Prof. Kotwal
Duke University
Outline
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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 = 61
• Parameters needed to SM completely predictive:
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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
  mt 2 
 mH 
mW  80.380  0.526  

1

0.054ln

100   f (  EM ,  S , mZ ,...)
  174 



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
relationship
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Motivation cont’d
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
CERN’s proton-antiproton
collider by UA1 & UA2
collaborations
1996 CERN’s e+e- 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)
Central Hadronic Calorimeter
Provides precise
measurement of
electron energy
Central E&M Calorimeter
Provides
measurement
of hadronic
recoil objects
Provides precise
measurement of
track momentum
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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.
W   e  e
u  d  g  W   e  e ,    
u  d  g  W   e  e ,    
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
mT  2 pTl  pT (1  cos( ))  2 pTl  ( pTl  uT )  (1  cos( ))
Angle between 2 pt
Features of transverse mass spectrum:
W   e  e
1). Relatively insensitive to the production dynamics of W.
 mT / mT ~ ( pTW / MW )2
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
Features of transverse momentum of lepton:
W   e  e
1). Better resolution than neutrino pt
→ relatively insensitive to the recoil response of detector
2). Sensitive to the W boson production dynamics
 mT / mT ~ ( pTW / MW )
• A third way is to use transverse momentum spectrum of
neutrino
Features of transverse momentum of neutrino:
Sensitive to both W production dynamics & the recoil response
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W Mass Measurement (3)
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Source: A.PHY
Kotwal
2007 Aspen talk
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W Mass Measurement Strategy
• Detector Calibration
Tracker calibration
EM Calorimeter calibration
• Fast Simulation
Data
Binned Likelihood Fit
W boson mass
NLO event generator
Detector response simulation
Hadronic recoil modelling
+
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W mass templates, bule for 80 GeV, red for 81 GeV
Backgrounds
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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 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 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mu+mu- and Upsilonmu+mu- are used to scale COT momentum
3). Using Zmu+mu- invariant mass fit to further check
• EM Calorimeter calibration
1). Using Ecal/p ratio to scale COT momentum
2). Using Ze+e- mass fit to further check calorimeter energy scale
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Backgrounds
For Wmu nu
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Largest background comes from Zmu+muW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e+e• Wtau nue nu nu
• QCD
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Transverse Mass Fitting results
W  e
background
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W  
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:
mW  80413  48(stat  sys)MeV , P(  2 )  44%
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
M H  8539
28 GeV
M H  7633
24 GeV
• 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_mw<25 MeV from 1.5 fb^-1 of CDF data
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Acknowledgement
Prof. Ashutosh Kotwal
References
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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
em g gz GF mW mZ sin2 W v 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)
em g gz GF mW mZ sin2 W v
Choice 1.
Follow the pattern that parameters are
masses and coupling constants.
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Choice 2.
Choose parameters measured most precisely.
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Motivation
• The EWK sector of SM is constrained by three
precisely measured parameters:
 EM ( M Z )  1/127.918(18)
GF  1.16637(1) 105
M Z  91.1876(21) GeV
• At lowest order, these parameters are related by:
M W2   / 2GF sin 2W
M Z2   / 2GF sin 2W cos 2
M W  M Z cos W
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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
e
e
e
g
e
e
Z0
W
ge  4
gW 
ge
1   MW / M Z 
e
2
gZ  ge (M Z / MW )
Thus, only two counpling constants:
1) e2/(4hc)=1/137;
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2) S for strong coupling
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