Precision Measurement of the

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Precision Measurement of the W Boson Mass with
CDF
Chris Hays, University of Oxford
Les Rencontres de Physique de la Vallee
d'Aoste March 7, 2007
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W Boson Mass
Given precise measurements of mZ and aEM(mZ), we
can predict mW
paEM
mW2
(on-shell scheme)
v2GF (1 - mW2/mZ2)(1 - Dr)
Dr O(3) radiative corrections dominated by tb
and Higgs loops
DmW µ mt2
DmW µ ln (mH/mZ)
C. Hays, University of Oxford
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W Mass Prediction and Measurement
W mass uncertainty from input parameters
Next talk
Direct W mass measurement
W mass predicted much more precisely (13 MeV)
than measured (29 MeV) Need to reduce dmW
to further constrain mH and other new physics
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C. Hays, University of Oxford
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Tevatron Run II
Each experiment has collected gt2 fb-1 of 1.96 TeV
vs pp collisions Current Run II gt15x Run I
data set
Year 2002 2003
2004 2005 2006
Month 1 4 7 10 1 4 7 10 1 4
7 10 1 4 7 10 1 4 7 10
Run II CDF
First Run II W mass measurement uses 200 pb-1 of
CDF data
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W Z Boson Production and Decay
Dominant production mechanism qq(') annihilation
s(W ln) 2775 pb After event selection (l,
n ET gt 30 GeV) 51,128 W mn candidates 63,964
W en candidates
s(Z ll) 254.9 pb After event selection (l
ET gt 30 GeV) 4,960 Z mm candidates 2,919 Z
ee candidates
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CDF Detector
High-precision tracking drift chamber dpT/pT
0.05 pT 2 for 40 GeV m
High-precision electromagnetic calorimeter dET/ET
13.5/vET Å? 1.7 3 for 40 GeV e
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Measurement Strategy
W en
Calibrate l track momentum with
mass measurements of J/y and U decays to
m Calibrate calorimeter energy using track
momentum of e from W decays Cross-check with Z
mass measurement, then add Z's as a calibration
point
Calibrate recoil measurement with Z decays to
e, m Cross-check with W recoil distributions
Z mm
Combine information into transverse mass mT
vETET(1 - cosDf) Statistically most powerful
quantity for mW fit
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Alignment and Corrections
Align tracker using cosmic-ray data Determine
track-level corrections from electron-positron
differences Use ratio of calorimeter energy to
track momentum Curvature biases affect e, e-
differently, but calorimeter measurement
independent of charge
L 200 pb-1 CDF Run II Preliminary
Statistical uncertainty of track-level
corrections leads to dmW 6 MeV
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Mass Measurements
Template mass fits to J/y, U, Z resonances in
muon decay channels Fast detector simulation
models relevant physical processes internal
bremstrahlung ionization energy loss multiple
scattering Simulation includes event
reconstruction and selection Detector material
model Map energy loss and radiation lengths in
each detector layer One material parameter
determined from data Overall material scale
g
m
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Momentum Scale Calibration
606,701 J/y mm candidates Fit mass as a
function of mean inverse pT Slope affected by
energy loss modelling Scale detector material by
0.94 to remove slope
Use calibrated momentum scale to measure Z mass
Constrain tracks to originate from the beam
line Improves resolution by a factor of 3
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Full Electron Simulation
Response and resolution in EM calorimeter
Energy loss into hadronic calorimeter
EM Calorimeter
Energy loss in solenoid
Track reconstruction in outer tracker
Bremstrahlung and conversions in silicon
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Energy Scale Calibration
Calibrate calorimeter energy with peak of W
electron E/p distribution One free parameter
for X0 scale (set with high E/p region)
Material scale 1.004 0.009
L 200 pb-1 CDF Run II Preliminary
Calorimeter Energy lt Track Momentum Energy loss
in hadronic calorimeter
Calorimeter Energy gt Track Momentum Energy loss
in tracker
Energy scale uncertainty 0.034
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Scale Energy Dependence
Apply energy-dependent scale to each simulated
electron and photon Determine energy dependence
from E/p fits as functions of electron ET Scale
1 (6 7) 10-5 ET/GeV - 39 Most energy
dependence implicitly accounted for by detector
model
(dmW 23 MeV)
L 200 pb-1 CDF Run II Preliminary
L 200 pb-1 CDF Run II Preliminary
Energy Scale
Energy Scale
Z data
W data
ET (e) (GeV)
ET (e) (GeV)
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Z Mass Measurement
Fit Z mass using scale from E/p calibration
L 200 pb-1 CDF Run II Preliminary
Measured value consistent with world average
value (91188 MeV) Incorporate mass fit into
calibration to reduce scale uncertainty
dmW 30 MeV
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Boson pT Model
Model boson pT using RESBOS generator with
tunable non-perturbative parameters g2
parameter determines position of peak in pT
distribution Measure g2 with Z boson data (other
parameters have negligible effect on W mass) g2
0.685 0.048 dmW 3 MeV
muon channel
electron channel
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Recoil Measurement
Calculate recoil by summing over calorimeter
towers, excluding Towers with lepton energy
deposits Towers near the beam line
e
Electron Remove 7 towers (shower) Muon Remove
3 towers (MIP) Model tower removal in simulation
dmW 8 (5) MeV for e (m)
L 200 pb-1 CDF Run II Preliminary
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Recoil Model
L 200 pb-1 CDF Run II Preliminary
Components Recoil scale (R umeas /
utrue) Recoil resolution Spectator and
additional interactions (contribute to
resolution) Calibrate scale with momentum
balance along bisector axis (h) Calibrate
models of recoil resolution and spectator
interactions using momentum resolution along
both axes dmW 11 MeV
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Recoil Model Checks
Apply model to W boson sample, test consistency
with data Recoil distribution Sensitive to
scale, resolution, boson pT u
distribution Sensitive to lepton removal,
efficiency model, scale, resolution, W
decay Directly affects mT fit result
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Production, Decay, Background
Boson pz determined by parton distribution
functions Vary PDFs according to
uncertainties dmW 11 MeV
Brem?trahlung reduces charged lepton pT Predict
using NLO QED calculation, apply NNLO
correction dmW 11 (12) MeV for e (m)
Background affects fit distributions QCD
Measure with data Electroweak Predict with
MC dmW 8 (9) MeV for e (m)
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W Mass Fits
Mass fit results blinded with -100,100 MeV
offset throughout analysis Upon completion,
offset removed to determine final
result Transverse mass fits
muon channel
electron channel
mW 80417 48 MeV (stat sys) for e m
combination (P(c2) 7)
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W Mass Fits
Fit ET, ET distributions and combine with mT to
extract most precise result Electron ET
fit Muon pT fit
mW 80388 59 MeV (stat sys) for lepton pT e
m combination (P(c2) 18)
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W Mass Fits
mW 80434 65 MeV (stat sys) for neutrino pT
e m combination (P(c2) 43)
Electron ET fit Muon ET fit
mW 80413 48 MeV (stat sys) for six-fit
combination (P(c2) 44)
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W Mass Uncertainties
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W Mass Result
New CDF result is world's most precise single
measurement Central value increases 80392 to
80398 MeV World average uncertainty reduced 15
(29 to 25 MeV)
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Previous Higgs Mass Prediction
Predicted Higgs mass from global electroweak
data mH 8539-28 GeV (lt 166 GeV at 95
CL) Direct search from LEP II mH gt 114.4 GeV at
95 CL
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New Higgs Mass Prediction
Predicted Higgs mass from global electroweak
data mH 8036-26 GeV (lt 153 GeV at 95
CL) Direct search from LEP II mH gt 114.4 GeV at
95 CL
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Effect on New Physics Models
Supersymmetry now preferred at 1s level...
New world average
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New W Mass Projections
New projected Tevatron precision as a function of
luminosity
New projection with 1.5 fb-1 of data dmW lt 25
MeV with CDF
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Summary
W mass excellent probe for new particles coupling
to the electroweak sector CDF has made the
single most precise W mass measurement mW
80413 34 MeV (stat) 34 MeV (sys) 80413
48 MeV (stat sys) New SM Higgs mass
prediction mH 8036-26 GeV Mass has moved
further into LEP-excluded region Expect CDF dmW
lt 25 MeV with 1.5 fb-1 already collected Will
squeeze SM in conjunction with Tevatron Higgs
results Electroweak data will probe more new
physics after the Higgs
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Backup
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Filling in the Pieces
Precision electroweak data will continue to guide
us to the next physics Today dmW 25 MeV, mH
lt 153 GeV at 95 CL After Higgs dmW 15
MeV, SUSY predicted at 95 CL? After SUSY
dmW 10 MeV, more new physics?
mW
SM
measurement
mW
SM
measurement
mW
measurement
MSSM
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Electron mT Signed c
High c2 dominated by a few bins with large
fluctuations
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Tevatron Run I Uncertainties
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Tracker Alignment
Central Outer Tracker Open-cell drift chamber
Wires strung under tension between two
endplates Model endplate distortions and
constructional variations using a
cell-to-cell endplate alignment
Determine individual cell tilts shifts using
cosmic-ray data Fit a single 'dicosmic' to
track segments on opposite sides of the
chamber Measure cell displacement
(Kotwal, Gerberich, Hays,
C. Hays, University of Oxford
NIM A 506, 110 (2003))
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Weak Boson Physics
Z boson parameters measured precisely by LEP
17 million measured Z candidates dmZ 2.1 MeV,
dGZ 2.3 MeV
Tevatron goal World's most precise W boson
measurements Expect 15 million measured W
candidates
Vtb
GW
mW
xfa(xp,Q)
WWZ coupling
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Alignment Example
Inner 'Superlayer'
Before alignment
Cell Shift (microns)
Cell number (f)
After alignment
CDF Run II preliminary
Cell number (f)
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Wire Alignment
Wire shape along z-axis determined
by Gravitational sag Electrostatic effects
Apply additional correction based on cosmic ray
study Compare parameters of incoming and
outgoing tracks from a cosmic ray muon
Dc (cm-1)
Final correction removes z-dependent curvature
biases
z (cm)
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Electron Track Model Validation
Fit Z mass reconstructed from electron track
momenta
L 200 pb-1 CDF Run II Preliminary
Measured value consistent with world average
value (91188 MeV)
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Calorimeter Energy Calibration
Calibrate electron energy using electron track
momentum First step validate model of
electrons in tracker Additional physical
effects beyond those associated with muons
Photon radiation and conversion in tracker
e
e-
g
e-
e-
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Combined Momentum Scale
Dp/p (1.50 0.19) 10-3
Systematic uncertainties
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Transverse Mass Distribution
mW 81 GeV
mW 80 GeV
Distribution peaks just below mW and falls
sharply just above mW
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Energy Loss Model
Use GEANT to parametrize energy loss in solenoid
and hadronic calorimeter Energy loss in hadronic
calorimeter
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Previous W Mass Projections
Previously projected Tevatron precision as a
function of luminosity
Projection with 2 fb-1 of data dmW 40 MeV per
experiment
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Higgs Mass Prediction
Predicted Higgs mass from global electroweak
data mH 8539-28 GeV (lt 166 GeV at 95
CL) Direct search from LEP II mH gt 114.4 GeV at
95 CL
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U Mass Measurement
L 200 pb-1 CDF Run II Preliminary
34,618 U mm candidates Short lifetime allows
a track constraint to the beam line Improves
resolution by a factor of 3
L 200 pb-1 CDF Run II Preliminary
Test beam constraint by measuring mass using
unconstrained tracks Correct by half the
difference between fits Take correction as a
systematic uncertainty
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Momentum Scale Calibration
Magnetic field along z-axis causes curvature in
transverse plane mv2/R evB, pT eBR CDF
Insufficient precision on B and R for W mass
measurement In-situ calibration (1) Apply
relative alignment of drift chamber wires (2)
Determine momentum scales such that J/y, U, and
Z mass measurements result in the
world-average values
Combine results to obtain scale for mW measurement
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Effect on New Physics Models
Additional space-time symmetry (Supersymmetry)
would affect the W mass
Previous world average
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Effect on New Physics Models
Supersymmetry now preferred at 1s level...
New world average
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