Measurement of the forward Z boson production cross-section in pp collisions at s = √7 TeV

The paper is available here.

arXiv:1505.07024, LHCB-PAPER-2015-001

More detailed information: ANA note 2014-070

Abstract

A measurement of the production cross-section for Z bosons that decay to muons is presented. The data were recorded by the LHCb detector during pp collisions at a centre-of-mass energy of 7 TeV, and correspond to an integrated luminosity of 1.0 fb−1 . The cross-section is measured for muons in the pseudorapidity range 2.0 < η < 4.5 with transverse momenta pT > 20 GeV/c. The dimuon mass is restricted to 60 < Mμ+μ− < 120 GeV/c2 . The measured cross-section is

σZ→μ+μ− = (76.0 0.3 0.5 1.0 1.3) pb

where the uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity. This result is in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. The cross-section is also measured differentially as a function of kinematic variables of the Z boson. Ratios of the production cross-sections of electroweak bosons are presented using updated LHCb measurements of W boson production. A precise test of the Standard Model is provided by the measurement of the ratio

W +→μ+νμ + σW − →μ−ν ̄μ)/σZ→μ+ μ− = 20.63 0.09 0.12 0.05,

where the uncertainty due to luminosity cancels.

Figures

(Note, eps versions are available under attachments).

Caption Figure
Figure 1: Invariant mass of dimuon candidates. Fig1.png
Figure 2: (a) Differential cross-section as a function of yZ compared with the prediction of Fewz configured with various PDF sets. Different predictions are displaced horizontally for visibility. (b) Normalised differential cross-section as a function of yZ compared to the predictions of ResBos and Powheg + Herwig. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. Fig2a.png Fig2b.png
Figure 3: Normalised differential cross-section as a function of pT,Z on (a) logarithmic and (b) linear scales. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. The measurements are compared to the predictions of ResBos and Powheg + Herwig. Fig3a.png Fig3b.png
Figure 4: Normalised differential cross-section as a function of φZ on (a) logarithmic and (b) linear scales. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. The measurements are compared to the predictions of ResBos and Powheg + Herwig. Fig4a.png Fig4b.png
Figure 5: Normalised differential cross-section as a function of pT,Z on (a) logarithmic and (b) linear scales. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. The measurements are compared to MC@NLO + Herwig (HW) and MC@NLO + Herwiri (HERWIRI). Herwig is configured with two choices of the root mean-square-deviation of the intrinsic kT distribution, 0 and 2.2 GeV/c. Fig5a.png Fig5b.png
Figure 6: Normalised differential cross-section as a function of φ</sup<subZ on (a) logarithmic and (b) linear scales. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. The measurements are compared to MC@NLO + Herwig (HW) and MC@NLO + Herwiri (HERWIRI). Herwig is configured with two choices of the root mean-square-deviation of the intrinsic kT distribution, 0 and 2.2 GeV/c. Fig6a.png Fig6b.png
Figure 7: LHCb measurements of electroweak boson production cross-sections compared to NNLO pQCD as implemented by the Fewz generator using various PDF sets. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. Fig7.png
Figure 8: Two dimensional plots of electroweak boson cross-sections compared to NNLO predictions for various parameterisations of the PDFs. The outer, shaded (yellow) ellipse corresponds to the total uncertainty on the measurements. The inner, shaded (purple) ellipse excludes the beam energy and luminosity uncertainties. The uncertainty on the theoretical predictions corresponds to the PDF uncertainty only. All ellipses correspond to uncertainties at 68.3% confidence level. Fig8.png
Figure 9: Ratios of electroweak boson production RW +Z , RW − Z , RWZ , RW , compared to various theoretical predictions. The shaded (yellow) bands indicate the statistical and total uncertainties on the measurements, which are symmetric about the central value. Fig9.png
Supplementary:Figure 1: Comparison of LHCb Z cross-section measurements to theoretical prediction. The uncertainty due to the beam energy is not displayed. Fig1-S.png
Supplementary:Figure 2: Normalised differential cross-section as a function of yZ . The shaded (yellow) bands indicate the measurements. These are compared to Fewz with various PDF sets. Fig2-S.png
Supplementary:Figure 3: Extrapolation of LHCb Z boson cross-section measurements to the ATLAS fiducial volume (muons with pT > 20 GeV/c and invariant mass 66 < Mμμ < 116 GeV/c2 ) [1]. Fig3-S.png
Supplementary:Figure 4: Normalised differential cross-section as a function of Z boson y. The shaded (yellow) bands indicate the measurements, which are compared to MC@NLO + Herwiri (HERWIRI) and MC@NLO + Herwig (HW). Herwig is configured with two choices of the root mean-square-deviation of the intrinsic kT distribution, 0 and 2.2 GeV/c. Fig4-S.png
Supplementary: Figure 5: Differential W+ and W -cross-sections as a function of muon η. Measurements, represented as bands corresponding to statistical (orange (blue) for W +(W -)) and total (yellow (light blue) for W +(W -))) uncertainty, are compared to NNLO predictions with different parameterisations the PDFs (black (blue) markers for W +(W -)) displaced horizontally for presentation. Fig5-S.png
Supplementary: Figure 6: Extrapolation of LHCb W boson cross-section measurements to the ATLAS fiducial volume (muons with pT > 20 GeV/c and invariant mass 66 < Mμμ < 116 GeV/c2 ) [1]. Fig6-S.png
Supplementary: Figure 7: W+ to W -cross-section ratios as a function of muon η. Measurements, represented as bands corresponding to the (orange) statistical and (yellow) total uncertainty, are compared to (black markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs. Fig7-S.png
Supplementary: Figure 8: Lepton charge asymmetries in W decays as a function of muon η. Measurements, represented as bands corresponding to the (orange) statistical and (yellow) total uncertainty, are compared to (open black markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs. Fig8-S.png
Supplementary: Figure 9: Lepton charge asymmetries in W decays as a function of muon η. Measurements, represented as bands corresponding to the (yellow) total uncertainty, are extrapolated to the ATLAS fiducial volume (MT > 40 GeV/c2 and Emiss > 25 GeV) and compared to the (open black markers) ATLAS determinations [1]. Fig9-S.png
Supplementary: Figure 10: Lepton charge asymmetries in W decays as a function of muon η. Measurements, represented as bands corresponding to the (yellow) total uncertainty, are extrapolated to the CMS fiducial volume (pT > 25 GeV/c) and compared to the (open black markers) CMS determinations [2]. Fig10-S.png
-- KatharinaMueller - 2015-06-04
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