dσ/dy Distribution of Drell-Yan Dielectron Pairs with 2.1 fb -1

Authors: Jiyeon Han, Arie Bodek, Willis Sakumoto, Y.S.Chung
Last Updated: Mar, 26th, 2010

2fb -1 Public Note

Abstract

We report on the measurement of the rapidity distribution, dσ/dy, over the full kinematic range for e+e- pairs in the Z boson region mass range of 66 to 116 GeV/c2 from 2.1 fb-1 of proton anti-proton collisions at (s)^½ = 1.96 TeV taken by the CDF detector at Fermilab

Data set and Trigger Efficiency

The dataset is based on 2.1 fb-1 of integrated luuminosity taken with two triggers: an inclusive single central electron trigger and a two electron trigger (central or forward). The trigger efficiency is determined as a function of electron ET and is close to 100%

Trigger Efficiency Z(CC) Trigger Efficiency Z(CP) Trigger Efficiency Z(PP)
1.0 0.997 ± 0.001 0.992 ± 0.001

Event Selection

Two high pt electrons selected with ET > 25GeV for Z(CC) and Z(CP), ET > 20 GeV for Z(CP). For CC events one electron is required to pass tight electron ID cuts and the other loose. For the CP topology oppositely charged electron pairs are required and the central electron is required to pass the tight cuts and the plug electron the standard plug ID cuts. For the PP topology both electrons are required to pass the standard plug ID cuts and have the same pseudo-rapidity and at least one electron must have a silicon track to reduce background. The Z rapidity of the PP samples reaches |y|=2.9.
167778&plusmn410 Z boson events are selected.


Z(CC) Z(CP) Z(PP)
Events 50752 ± 225 86203 ± 294 31415 ± 177
Mass 90.60 ± 0.02 90.53 ± 0.02 90.26 ± 0.03
Width 2.92 ± 0.02 3.12 ± 0.02 3.22 ± 0.04

Acceptance and Efficiency

Geometric and kinematic acceptances are modeled using the PYTHIA generator combined wuth a GEANT simulation of the CDF detector and are given below.

Z(CC) Z(CP) Z(PP)
Acceptance 0.107 ± 0.001 0.233 ± 0.001 0.095 ± 0.001
Efficiency 0.905 ± 0.008 0.690 ± 0.003 0.758 ± 0.037

Background

The QCD background is estimated from the electron isolation energy distribution and is given below.

Z(CC) Z(CP) Z(PP) Total
0.24 ± 0.03(stat.) % 1.55 ± 0.12(stat.) ± 0.42(sys.) % 3.40 ± 0.72(stat.) ± 0.21(sys.) % 1.50 ± 0.15(stat.)%

EWK backgrounds are evaluated using simulation and are determine to be 0.41 ± 0.02% in total.

Result

σ = 256.6 ± 0.7(stat.) ± 2.0(sys.) pb.
σ(y>0) = 256.4 ± 1.0(stat.) pb.
σ(y<0) = 256.9 ± 0.9(stat.) pb.

σ(CC) = 255.3 ± 1.1(stat.) pb.
σ(CP) = 256.9 ± 0.9(stat.) pb.
σ(PP) = 257.0 ± 1.5(stat.) pb.
The luminosity uncertainty of 6% is not included in the above systematic errors.
In comparing to theory, the theory predictions are scaled to the measured cross sections.
The best description of the data is presently provided by a NLO calculation using the NLO CTEQ6.1M PDF.

Plots

The relation between the parton momentum fraction and the rapidity of the Z boson
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The relation between the parton momentum fraction and the rapidity of the Z boson (log scale)
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Dielectron invariant mass distributions of the three toplogies
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Z rapidity distribution
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Trigger efficiency as a function of Z boson rapidity.
Blue: Z(CC), Magenta: Z(CP), Red: Z(PP)
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dN/dy ratio distribution of data to Pythia simulation.
This is used as a correction factor (acceptance correction) for the input dsigma/dy to take out the model dependence.
(The correction factor is data-driven.
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dN/dy ratio distribution of data to Pythia simulation after the acceptance correction.
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Zpt distribution of data (black points) and Pythia simulation (blue histogram).
CDF MC (Pythia simulation) includes W/Z boson Pt tuning using Run I measurement.
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cos(theta) distribution of data (black points) and Pythia simulation (blue histogram).
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Background Fraction of the total events(CC+CP+PP)
Histograms are stacked up.
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Data and background distribution
Brown: EWK background, Green: QCD background
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dσ/dy distribution of positive and negative rapidity.
No PDF or luminosity uncertainty included.
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dσ/dy distribution of Z/γ* -> e+e-.
No PDF or luminosity uncertainty included in data.
NLO calculation with NLO CTEQ6.1M PDF theory prediction compared to data.
Theory prediction scaled to total cross section from data.
Combined uncertainties (stat+sys) are considered.
Residual.
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dσ/dy distribution of Z/γ* -> e+e-.
No PDF or luminosity uncertainty included in data.
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Fraction of data and theory prediction II.
NLO calculation with NLO CTEQ6.1 PDF and NLO CTEQ6.6 PDF theory predictions used.
Theory prediction scaled to total cross section from data.
The statistic and systematic uncertainties are considered.
Yellow error band shows the CTEQ6.6M PDFs uncertainty. (CL = 90 %)
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Fraction of data and theory prediction I.
NLO calculation with NLO MRST PDF(2004) and NLO calculation with NLO MSTW PDFs(2008) theory predictions used.
Theory prediction scaled to total cross section from data.
The statistic and systematic uncertainties are considered.
Yellow error band shows the MSTW2008 PDFs uncertainty. (CL = 68 %)
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Fraction of data and theory prediction I.
NNLO calculation with NNLO MRST PDF(2006) and NNLO MSTW PDFs(2008) theory predictions used.
Theory prediction scaled to total cross section from data.
The statistic and systematic uncertainties are considered.
Yellow error band shows the MSTW2008 PDFs uncertainty. (CL = 68 %)
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