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Measurement of the forward $Z$ boson production cross-section in $pp$ collisions at $\sqrt{s}$ = 7 TeV

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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 $2.0 < \eta < 4.5$ with transverse momenta $p_{T} > 20$ GeV/c. The dimuon mass is restricted to $60 < M_{\mu^{+}\mu^{-}} < 120$ GeV/c$^{2}$. The measured cross-section is $$\sigma_{Z\rightarrow\mu^{+}\mu^{-}} = (76.0 \pm 0.3 \pm 0.5 \pm 1.0 \pm 1.3) \text{pb}$$ 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 $$\frac{\sigma_{W^{+}\rightarrow\mu^{+}\nu_{\mu}} + \sigma_{W^{-}\rightarrow\mu^{-}\bar{\nu}_{\mu}}}{\sigma_{Z\rightarrow\mu^{+}\mu^{-}}} = 20.63\pm0.09\pm0.12\pm0.05,$$ where the uncertainty due to luminosity cancels.

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Invariant mass of dimuon candidates.

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(a) Differential cross-section as a function of $y_{Z}$ 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 $y_{Z}$ 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.

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Normalised differential cross-section as a function of $p_{T,Z}$ 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 .

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Normalised differential cross-section as a function of $\phi^{*}_{Z}$ 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 .

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Normalised differential cross-section as a function of $p_{T,Z}$ 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 $k_{T}$ distribution, 0 and 2.2 $ {\mathrm{ Ge V /}c}$ .

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Normalised differential cross-section as a function of $\phi^{*}_{Z}$ 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 $k_{T}$ distribution, 0 and 2.2 $ {\mathrm{ Ge V /}c}$ .

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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.

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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.

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Ratios of electroweak boson production $R_{\it W^{+}Z}$, $R_{\it W^{-}Z}$, $R_{\it WZ}$, $R_{\it W}$, 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.

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Animated gif made out of all figures.

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Tables and captions

Contributions to the relative uncertainty on the total $ Z $ boson cross-section.

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Contributions to the relative uncertainty on the electroweak boson cross-section ratios.

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Inclusive differential cross-sections for $ Z$ boson production as a function of $y_{Z}$. Uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity. No candidates are observed in the 4.250 -- 4.500 bin.

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Inclusive differential cross-sections for $ Z$ boson production as a function of $p_{T,Z}$. Uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity.

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Inclusive differential cross-sections for $ Z$ boson production as a function of $\phi^{*}_{Z}$. Uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity.

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$ W ^+$ to $ W ^-$ boson production cross-section ratios as a function of muon $\eta$. Uncertainties are due to the sample size, systematic effects and the beam energy. These supersede the results in Ref. \cite{wmu}.

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Lepton charge asymmetries as a function of muon $\eta$. Uncertainties are due to the sample size, systematic effects and the beam energy. These supersede the results in Ref. \cite{wmu}.

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Supplementary Material [file]

Supplementary material full pdf

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supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2015-001. The files are: supplementary.pdf : An overview of the extra figures Fig*-S.pdf, Fig*-S.png, Fig*-S.eps, Fig*-S.C : The figures in various formats

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Created on 15 June 2019.Citation count from INSPIRE on 15 June 2019.