Measurement of the forward-backward asymmetry of electron and muon pair-production in $pp$ collisions at $\sqrt{s}=7$ TeV with the ATLAS detector
This paper presents measurements from the ATLAS experiment of the forward-backward asymmetry in the reaction $pp\rightarrow Z/\gamma^{*}\rightarrow l^{+}l^{-}$, with $l$ being electrons or muons, and the extraction of the effective weak mixing angle. The results are based on the full set of data collected in 2011 in $pp$ collisions at the LHC at $\sqrt{s}$ = 7 TeV, corresponding to an integrated luminosity of 4.8 fb$^{-1}$. The measured asymmetry values are found to be in agreement with the corresponding Standard Model predictions. The combination of the muon and electron channels yields a value of the effective weak mixing angle of $0.2308 \pm 0.0005 \mathrm{(stat.)} \pm 0.0006 \mathrm{(syst.)} \pm 0.0009\mathrm{(PDF)}$, where the first uncertainty corresponds to data statistics, the second to systematic effects and the third to knowledge of the parton density functions. This result agrees with the current world average from the Particle Data Group fit.
12 March 2015
Contact: Standard Model conveners
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e-print arXiv:1503.03709 - pdf from arXiv
Figures
Figure 01a
Dilepton invariant mass distributions obtained from the event selections described in the text, for the (a) CC electron, (b) CF electron and (c) muon channels. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 01b
Dilepton invariant mass distributions obtained from the event selections described in the text, for the (a) CC electron, (b) CF electron and (c) muon channels. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 01c
Dilepton invariant mass distributions obtained from the event selections described in the text, for the (a) CC electron, (b) CF electron and (c) muon channels. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 02a
Distributions of the cosine of the polar angle in the Collins–Soper frame (cosθCS*) obtained from the event selections described in the text, for the (a) CC electron and (b) muon channels. The corresponding distribution for the CF electron channel is shown using both (c) a linear and (d) a logarithmic scale. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 02b
Distributions of the cosine of the polar angle in the Collins–Soper frame (cosθCS*) obtained from the event selections described in the text, for the (a) CC electron and (b) muon channels. The corresponding distribution for the CF electron channel is shown using both (c) a linear and (d) a logarithmic scale. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 02c
Distributions of the cosine of the polar angle in the Collins–Soper frame (cosθCS*) obtained from the event selections described in the text, for the (a) CC electron and (b) muon channels. The corresponding distribution for the CF electron channel is shown using both (c) a linear and (d) a logarithmic scale. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 02d
Distributions of the cosine of the polar angle in the Collins–Soper frame (cosθCS*) obtained from the event selections described in the text, for the (a) CC electron and (b) muon channels. The corresponding distribution for the CF electron channel is shown using both (c) a linear and (d) a logarithmic scale. Data are shown by open circles and the total expectation is shown as a line with a band representing the total uncertainty (statistical and systematic added in quadrature). The data-driven estimate for the multi-jet background and the simulation-based estimates for all other backgrounds are shown by the shaded areas.
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Figure 03a
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull value (Δ/σ) for each mass bin, where Δ is the difference between data and simulation and σ is the sum in quadrature of the data and simulation uncertainties.
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Figure 03b
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull value (Δ/σ) for each mass bin, where Δ is the difference between data and simulation and σ is the sum in quadrature of the data and simulation uncertainties.
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Figure 03c
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull value (Δ/σ) for each mass bin, where Δ is the difference between data and simulation and σ is the sum in quadrature of the data and simulation uncertainties.
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Figure 04a
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels in a narrow region around the Z pole, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 04b
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels in a narrow region around the Z pole, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 04c
Detector-level forward-backward asymmetry (AFB meas) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels in a narrow region around the Z pole, after background subtraction. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties for MC are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 05a
Forward-backward asymmetry (AFB obs) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 05b
Forward-backward asymmetry (AFB obs) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 05c
Forward-backward asymmetry (AFB obs) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the systematic uncertainties on the data. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 06a
Forward-backward asymmetry (AFB cor) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration and dilution effects are corrected. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the data systematic uncertainties. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 06b
Forward-backward asymmetry (AFB cor) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration and dilution effects are corrected. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the data systematic uncertainties. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 06c
Forward-backward asymmetry (AFB cor) values as a function of the dilepton invariant mass for the (a) CC electron, (b) CF electron and (c) muon channels. Leptons are unfolded to Born level to account for mass bin migration and dilution effects are corrected. For the data, the black inner error bars represent the statistical component and the lighter outer error bars the total error (statistical and systematic added in quadrature). The boxed shaded regions for the MC expectations represent only the statistical uncertainty; theoretical uncertainties are included in the data systematic uncertainties. The lower panel of each plot shows the pull values (Δ/σ, as defined for figure 3).
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Figure 07
Comparison of the results of this analysis with other published results for sin2θ efflept. This includes the most precise measurements from LEP and SLC, and the leptonic sin2θ efflept measurements from the hadron collider experiments CMS [5], D0 [4], and CDF [3]. Also shown are the values of sin2θ efflept from the LEP+SLC global combination [2] (which includes all sin2θ efflept measurements performed at the two colliders) and from the PDG global fit [46]. The vertical dotted line shows the central value of the ATLAS combined measurement reported here, while the vertical dashed line represents that of the current PDG global fit [46].
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Tables
Table 01
Absolute systematic uncertainties on the AFB rmobs values, after unfolding for mass-bin migration. Approximate values in three invariant mass intervals are given.
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Table 02
The sin2θefflept measurement results in each of the three studied channels: electron central- eff central, electron central-forward and muon. Results of the statistical combination of both electron channels and all three channels are shown as well.
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Table 03
Contributions to the systematic uncertainties on the sin2θefflept values extracted from the eff three analysis channels and on the combined result. Null entries (denoted by “–”) correspond to uncertainties that do not apply to a specific channel. Higher-order corrections include NLO QCD and NLO EWK contributions. Other sources include the effect of pileup, background uncertainties, lepton trigger/reconstruction/identification efficiency uncertainties, muon momentum resolution and effects of detector misalignment.
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Table 04
Comparison of the results of this analysis with other published results for sin2θefflept. The comparison includes the most precise measurements from LEP and SLC, and the results from the leptonic sin2θefflept measurements from the hadron collider experiments CMS, D0, and CDF. Also shown are the values of sin2 θ lept from the LEP+SLC global combination (which includes all sin2θefflept measurements performed at the two colliders) and from the PDG global fit. Each Δ/σ column shows the difference between the result and the quoted reference value, divided by the quadratic sum of the associated uncertainties.
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Auxiliary material
Figure 01
Comparison of the extracted sin2θefflept values when using different PDFs in the templates. Only PDF systematic errors are shown. Errors on MSTW and CT10 are calculated using the CT10 error set, while those on ATLAS-epWZ12 and HERA1.5LO are calculated using the ATLAS-epWZ12 error set.
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