cigdem issever on the behalf of cdf collaboration
DESCRIPTION
Measurement of the Forward-Backward Charge Asymmetry from W -> e n Production in Collisions at = 1.96 TeV hep-ex/0501023. Cigdem Issever On the behalf of CDF Collaboration. more information: http://fcdfwww.fnal.gov/physics/ewk/2004/w_charge_asym/w_charge_asymmetry.html. - PowerPoint PPT PresentationTRANSCRIPT
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22.05.2005IOP 2005
Cigdem Issever, University of Oxford
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Measurement of the Forward-Backward Charge Asymmetry from W -> e Production in Collisions at
= 1.96 TeV hep-ex/0501023
Cigdem IsseverOn the behalf of CDF Collaboration
s
pp
more information:http://fcdfwww.fnal.gov/physics/ewk/2004/w_charge_asym/
w_charge_asymmetry.html
CDF
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22.05.2005IOP 2005
Cigdem Issever, University of Oxford
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PDFs significant for many measurements at the Tevatron
How can we gain insights into parton momentum distribution functions (PDFs) of the proton at the Tevatron???
Measure W Charge Asymmetry
Ad(W ) /dyW d (W ) /dyWd(W ) /dyW d (W ) /dyW
PDFs not well known at high x (high )Don’t already know the answer, need to measure.Can improve many of our upcoming measurements.
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Reason for W Charge Asymmetry?
u quarks carry more momentum than d quarks
xf(
x,Q
2)
x
Q2 = 100 GeV2
MRST2004NLO
u
d
10-3 10-2 10-1
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22.05.2005IOP 2005
Cigdem Issever, University of Oxford
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Reason for W Charge Asymmetry?
W+
dp
u
u
d
pu
u
e+
W- W+
protonantiproton
W+ and W- boosted in forward and backward direction; Information about u(x)/d(x) convoluted in W Asymmetry
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sensitive to PDF’s !
Measure Charge Asymmetry of Lepton Channel
Asymmetry in W production complicated by unknown pz
use lepton asymmetry:
which convolves W production with V-A decay.
uncertainty on PDF’s significant error in several measurements at CDF !
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CDF Run1 Result
Forward Tracking and Charge ID is the key!!
Measurement at large has high impact on PDF fits
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22.05.2005IOP 2005
Cigdem Issever, University of Oxford
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Calorimeter Seeded OI Si-Tracking (PHX)
Forward Tracking outside COT
Seed tracksSi-track
Si-Hit
1) Reconstruction of Seed Track with
o Calorimeter cluster position
o Event vertex
o Calorimeter cluster ET
2) Reconstruction of Si-Tracks
o Using same algorithm as the COT OI Si-Tracking to attach Si hits.
vertex
Seed tracks
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W Event Selection• Baseline electron selection
• PHX track optimized for good charge identification:– #hits > 4
– 2 < 8
– 2 > 0.5
– Pull Ratio
• ET > 25 GeV
• Missing ET > 25 GeV
• 50 < MT < 100 GeV/c2
• No other EMO with ET>25 GeV to suppress QCD and DY
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Needs to be corrected for
- background contributions and
- charge misidentification.
Raw, uncorrected Charge Asymmetry
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Charge Identification
Incorrectly identified charge dilutes asymmetry.We can correct for it, if we know f=#wrong/total.
MC does not include residual misalignments. Must measure it in data as function of .
Measure f with Ze+e-.Use central (|| < 1.5) leg to tag charge of other leg.
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PHX Charge Misidentification Rate
Smaller than 6% in forward
Error calculated with Bayesian prescription
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Correction of Raw Asymmetry for Charge MisID
with f+ charge mis-id rate of positrons, f- charge mis-id rate of electrons
A charge dependence of charge id would directly bias measurement, hence: A = (Araw+ f+ - f-) / (1 - f+ - f-)
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We correct the asymmetry in each bin for backgrounds from
–W eAsymmetric, measured from MC, 4%
–Z e+ e-
Asymmetric, measured from MC, 1%
–QCDSymmetric, measured from data using
Isolation vs MEt.
Background Corrections
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QCD Background (BKG)
upper limit
We use (0.5 ± 0.25)*BKG. Becomes significant for >1.8
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Corrected Asymmetry with stat + total uncertainties
No evidence of CP asymmetry, level of agreement 2/dof=9.5/11.We are using method by R. Barlow, PHYSTAT2003, SLAC, Stanford, California, September 8-11, 2003, for incorporating the asymmetric uncertainties in the error propagation using likelihood distributions.
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For higher ET electrons, is closer to yW.
More information can be gained by
measuring A in bins of electron ET.
Current statistics allow use of two ET bins.
NEW : ET Dependence of Asymmetry
25 GeV < ET < 35 GeV
35 GeV < ET < 45 GeV
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Results for 25 GeV < ET < 35 GeV
--- MRST02__ CTEQ6.1M
NLO RESBOS
Corrections for charge mis-id and backgrounds are measured and applied for each ET bin.
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Cigdem Issever, University of Oxford
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Results for 35 GeV < ET < 45 GeV
--- MRST02__ CTEQ6.1M
NLO RESBOS
2 probabilities are 11% for CTEQ and 0.6% for MRSTInclusion of our results will further constrain fits and improve predictions.
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Outlook & Conclusion
• Results will be included into new PDF fits.
• CDF is beginning to explore a new method which fully reconstructs the W direction, see talk of Boyoung Han at APS 2005.
• CDF Run2 W Charge Asymmetry measurement provides new PDF constraints, particularly when separated by ET range.
• Measured Asymmetry for the first time in different ET bins.
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Backup Slides
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Systematic• Statistical uncertainty on charge mis-id rate dominates
• Uncertainty from QCD jet background is small and other backgrounds are negligible
• Using COT tracks, when available, results in no significant difference
• Checked that variation of detector alignments within their uncertainty has no significant effect on asymmetry
• No difference between asymmetry in the EAST and WEST
• Studied possible biases introduced by detector effects in the MC and found no significant effects.
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Results: W Charge Asymmetry w. total error
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Corrected W Charge Asymmetry Et>25 GeV
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CDF Run 2 Detector
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22.05.2005IOP 2005
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Plug/PES Alignment Align PLUG calorimeter with COT tracks: allow offsets in x,y,z and a rotation in phi
global misalignments2<<8mrad;x,y ~3mm
internal misalignments~1mrad, r~1mm
PES Octant Boundary
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Background Determination Method Is
ola
tion
MET [GeV]
0.
1
0.
4
0.
7
1.0
20 60 100 140
Signal Region