Chris Hays Duke University

1
W Mass Measurement at the Tevatron
DØ
Chris HaysDuke University
CDF
TEV4LHC Workshop Fermilab 2004
2
Past, Present, and Future
Precision of direct measurements
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2004 LEP 42 MeV
1990 UA2 900 MeV
2000 DØ 91 MeV
1989 UA1 2.9 GeV
1995 CDF 180 MeV
1983 UA1 5 GeV
Tevatron 59 MeV
W mass now known to better than 5 parts in 10,000
(?MW34 MeV)
Looking forward Tevatron Run 2 has quadrupled
Run 1 CDF, DØ data sets CDF has analyzed first
200 pb-1 of data and determined uncertainties
CDF 79 MeV DØ 84 MeV
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2011 Tevatron lt15 MeV
2006 CDF/DØ lt34 MeV
2004 CDF 76 MeV
2005 CDF/DØ lt59 MeV
2008 CDF/DØ lt20 MeV
2010 ATLAS/CMS lt20 MeV
LHC lt15 MeV
1 fb-1
0.4 fb-1
0.2 fb-1
4 fb-1
10 fb-1
C. Hays, Duke University, TEV4LHC
3
W Mass Fit Distributions
Transverse mass mT2 2pTET(1-cos(Df)) m
2 pT u Low sensitivity to pTW Recoil
modelling crucial
ET -(pT u)
u
Lepton pT m 2 pT Insensitive to recoil
pTW modelling crucial
(u)
u
CDF RUN II PRELIMINARY
pT
mT
CDF RUN II PRELIMINARY
C. Hays, Duke University, TEV4LHC
4
CDF Run 2 Uncertainties
Scale and resolution m ??W 30 MeV e ??W
70 MeV
Tower removal m ??W 10 MeV e ??W 20
MeV
Transverse mass fit
Backgrounds m,e ??W 20 MeV
Production and decay model m,e ??W 30 MeV
CDF RUN II PRELIMINARY
Statistics m ??W 50 MeV e ??W 45 MeV
Scale and resolution m,e ??W 50 MeV
C. Hays, Duke University, TEV4LHC
5
Production and Decay Model Uncertainties
CDF RUN II PRELIMINARY
Parton Distribution Functions
QED Radiative Corrections
e Uncertainty determined from CTEQ eigenvectors
MRST e W charge asymmetry will reduce
uncertainty
e Single-photon FSR modelled
l
??W 5 MeV with 4 fb-1
v LHC No charge asymmetry v Constrain PDFs
with lepton h distributions from W and Z
e 2-photon FSR needs to be added
??W 5 MeV with 4 fb-1
??W 10 MeV achievable
(also at LHC)
C. Hays, Duke University, TEV4LHC
6
Production and Decay Model Uncertainties
pTW Model
e Uncertainties determined from RESBOS parameters
g1, g2, g3 e Parameters constrained by Run 1 Z
pT distribution
g2 0.68 0.12 GeV2 g3 -0.60 0.30
pT fit ??W 27 MeV mT fit ??W 13 MeV
CDF RUN II PRELIMINARY
??W 5 MeV with 4 fb-1 (pT fit 10 MeV)
v LHC Large Z statistics
??W 5 MeV achievable (also for pT fit)
GW
e 70 MeV uncertainty from world average of direct
measurements
??W 5 MeV with 4 fb-1
(also at LHC)
C. Hays, Duke University, TEV4LHC
7
CDF Run 2 Muon Momentum Calibration
Set momentum scale using J/? and upsilon decays
to muons
e Uncertainty from difference in scales
CDF RUN II PRELIMINARY
??W 15 MeV
?p/p
e Post-alignment-correction uncertainty
??W 20 MeV
J/?
e Resolution uncertainty (from Z decays)
??W 12 MeV
??W 5 MeV with 4 fb-1
lt1/pT(?)gt (GeV-1)
(also at LHC)
CDF RUN II PRELIMINARY
CDF RUN II PRELIMINARY
Upsilon
Z
C. Hays, Duke University, TEV4LHC
8
CDF Run 2 Electron Calibration
Set energy scale using E/p peak
??W 35 MeV
Tune upstream passive material using tail of E/p
distribution
??W 55 MeV
CDF RUN II PRELIMINARY
CDF RUN II PRELIMINARY
Correct for non-linearity
??W 25 MeV
e Resolution uncertainty (from Z decays)
??W 12 MeV
CDF RUN II PRELIMINARY
??W 15 MeV with 4 fb-1
(also at LHC)
C. Hays, Duke University, TEV4LHC
9
CDF Run 2 Recoil Measurement
Measure hadronic recoil by summing over all
calorimeter towers Remove towers with energy
deposited by lepton
0.1 x 0.25 ???
Estimate removed recoil energy using towers
separated in ?
438 1243 92
m ??W 10 MeV e ??W 20 MeV
9 MeV
??W 5 MeV with 4 fb-1
(also at LHC)
CDF RUN II PRELIMINARY
Removed muon towers
C. Hays, Duke University, TEV4LHC
10
CDF Run 2 Recoil Model
Parametrize hadronic response R
umeas/utrue Resolution model incorporates terms
from underlying event and jet resolution
utrue given by pT(Z) ??W 20 MeV
?
Tune parameters using Z ?? events
?
?
u
Resolution at low pT(Z) dominated by underlying
event
Resolution at high pT(Z) dominated by jet
resolution
CDF RUN II PRELIMINARY
??W 42 MeV
Model underlying event with minimum-bias data
(inelastic collisions)
??W 10 MeV with 4 fb-1
(also at LHC)
C. Hays, Duke University, TEV4LHC
11
CDF Run 2 Backgrounds
Muons
CDF RUN II PRELIMINARY
Muons
Electrons
??W 20 MeV
??W 5 MeV with 4 fb-1
(also at LHC)
C. Hays, Duke University, TEV4LHC
12
Milestones
Improve understanding of passive material
(E/p tail) Improve COT alignment Understand
W/Z differences in recoil model W pT
constrained with Z pT
CDF RUN II PRELIMINARY
Detailed understanding of upsilon/Z
systematics PDF constrained with W charge
asymmetry Include 2-photon FSR
C. Hays, Duke University, TEV4LHC
13
Ratio Method
Can also measure mass by measuring Z transverse
mass for each lepton
W transverse mass measurement gives ratio of W to
Z masses
Potentially removes energy calibration uncertainty
Lower Z statistics (not an issue at LHC)
Are there additional systematics?
pTW uncertainty could be larger than mT
measurement
DØ Run 1 Experience (82 pb-1)
C. Hays, Duke University, TEV4LHC
14
Summary
Now have Run 2 experience (first milestone hit)
Most systematics shrinking with increasing data
Experimental issues (CDF)
Recoil differences between W's and Z's
Electron energy calibration (passive material)
Should be soluble (still early)
Preliminary Run 2 results available
Theoretical issues
Modelling 2-photon FSR (in progress)
Update PDFs (need input from W charge asymmetry
measurement)
Update pTW parameters (need input from Z pT
measurement)
Potential Improvements at the LHC
Can large Z statistics get recoil model below 10
MeV uncertainty?
Can PDF uncertainty get below 10 MeV?
C. Hays, Duke University, TEV4LHC