Transcript A New Precise Measurement of the W Boson Mass by CDF

Measurement of the W Boson Mass at CDF-II
Yu Zeng
Duke University
(for the CDF Collaboration)
APS April Meeting, Atlanta, GA
March 31, 2012
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Motivation for Precision Measurements
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W boson mass is an important parameter of the SM.
Precise knowledge of W boson mass, together with top quark mass,
constrains the mass of the unobserved Higgs boson, and possibly
new particles beyond the SM.
- For the same constraining power on Higgs mass:
¢ m t = 0:9 GeV $
¢ mW ¼ 5 MeV
(before Feb. 2012)
m H < 158 GeV at 95% C:L:
- World-average of W mass (Feb. 2012)
m W = 80399 §23 MeV
¢ mW
- Progress on
has the biggest
impact on Higgs constraint!
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W Boson Production and Decay at CDF
Selected candidates
from 2.2/fb data set
W ! eº
W ! ¹º
~470k
~625k
q¹q
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Production is dominated by
annihilation (~80%)
pT
Lepton carries most of the W boson mass information
- measured to 0.01%

Soft hadronic recoil in calorimeter due to initial state QCD radiation
- measured to 0.5%
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Measurement Strategy
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q
Use transverse quantities
mT =

m T ; pl ; pº
T
T
2(pl pº ¡ p
~ l ¢~
pº)
T T
T
T
Precise measurement of charged lepton
- Track momentum calibration
- EM calorimeter energy calibration
- Cross-check and additional calibration using Z’s

Hadronic recoil
- Calibrate with Z boson events
- Cross-check with W boson events
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Extract W mass values using templates
- Use parameterized custom fast simulation to model detector effects.
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Track Momentum Scale
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Constrain momentum scale and
tracker non-linearity using J/y
- Measure magnetic field non-uniformity
- Eliminate mis-alignment effects
- Tune energy loss model
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Further
¨constrain momentum scale
using
- Cross-check of non-beam-constrained
(NBC) and beam-constrained (BC) fits
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Apply to Z decays as cross-check
- Use world-average Z mass as additional
calibration
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Calorimeter Energy Scale
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Calibration of calorimeter energy scale
is vital for electron measurement
Calorimeter energy calibrated using peak
of electron E/p distribution from Ws
- Apply calibrated track momentum scale
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Energy loss tuned with high E/p tail
Correct for
ET
dependence
Measure Z mass in electrons using
calibrated scale
Combine
Z ! ee E/p-based calibration with
mass for maximum precision
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Hadronic Recoil Model
pZ
T
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Hadronic recoil
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- 2-vector sum of energy in calorimeter excluding
measured lepton
- contribution from underlying events added back in
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Detector response model
tuned using Z events
pl l
- Use mean & rms of T projections on apT
set of orthogonal axes defined by boson
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u
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Recoil Model Validation with W events
electron
muon
Recoil projection on lepton direction
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Recoil projection perpendicular to lepton
Good agreement in recoil distribution between simulation and
data for W boson events
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W Mass Fits and Uncertainties
muon
electron
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Result
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Combined electron and muon fits:
m CD F = 80387 §12st at §15syst = 80387 §19 MeV=c2
W
- Most precise measurement of W boson mass to date
- Exceed the precision of all previous measurements combined
- Reduces uncertainty of world average by ~30%
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Combined with D0 W mass result:
m D 0 = 80376 §23 MeV=c2
W
- New SM Higgs constraint:
m H < 152 GeV=c2 @95% C:L:
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Conclusion
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CDF has achieved the most precise measurement of the W boson mass
m CD F = 80387 §19 MeV=c2
W
- arXiv: 1203.0275. Accepted for publication by Phys. Rev. Lett..
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Expect CDF
¢ m W < 15 MeV=c2
with full CDF dataset already recorded
The W mass will continue to play an important role as a stress test of the
Standard Model.
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Thank you!
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Collider Detector at Fermilab (CDF)
Muon
detector
Central
hadronic
calorimeter
Central
outer
tracker
(COT)
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Signal Simulation
MW = 80 GeV
MW = 81 GeV
+
DukeSim =
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Generator-level signal simulation
- RESBOS for W/Z production and decay
- PHOTOS for multiple radiative photons off charged lepton
- Cross-check PHOTOS with HORACE
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Detector simulation and template fitting
- Custom fast Monte Carlo (DukeSim)
- Simulate detector response to e/m/g/recoil etc.
- Generate finely-spaced templates as a function of the fit variable
- Perform binned maximum-likelihood fits to data
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We extract
m ; pl ; W
pº mass fits from 6 kinematic distributions
-
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T
T
T
for electron and muon channel
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Backgrounds
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