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Measurement of forward $W$ and $Z$ boson production in $pp$ collisions at $\sqrt{s} = 8\mathrm{\,Te V}$

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Abstract

Measurements are presented of electroweak boson production using data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s} = 8\mathrm{\,Te V}$. The analysis is based on an integrated luminosity of $2.0\mathrm{\,fb}^{-1}$ recorded with the LHCb detector. The bosons are identified in the $W\rightarrow\mu\nu$ and $Z\rightarrow\mu^{+}\mu^{-}$ decay channels. The cross-sections are measured for muons in the pseudorapidity range $2.0 < \eta < 4.5$, with transverse momenta $p_{\rm T} > 20{\mathrm{\,Ge V\!/}c}$ and, in the case of the $Z$ boson, a dimuon mass within $60 < M_{\mu^{+}\mu^{-}} < 120{\mathrm{\,Ge V\!/}c^{2}}$. The results are \begin{align*} \sigma_{W^{+}\rightarrow\mu^{+}\nu} &= 1093.6 \pm 2.1 \pm 7.2 \pm 10.9 \pm 12.7{\rm \,pb} \, , \sigma_{W^{-}\rightarrow\mu^{-}\bar{\nu}} &= \phantom{0}818.4 \pm 1.9 \pm 5.0 \pm \phantom{0}7.0 \pm \phantom{0}9.5{\rm \,pb} \, , \sigma_{Z\rightarrow\mu^{+}\mu^{-}} &= \phantom{00}95.0 \pm 0.3 \pm 0.7 \pm \phantom{0}1.1 \pm \phantom{0}1.1{\rm \,pb} \, , \end{align*} where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. The evolution of the $W$ and $Z$ boson cross-sections with centre-of-mass energy is studied using previously reported measurements with $1.0\mathrm{\,fb}^{-1}$ of data at $7\mathrm{\,Te V}$. Differential distributions are also presented. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.

Figures and captions

(left) Template fit to the (left panel) positive and (right panel) negative muon $p_{\rm T}$ spectra in the full $\eta^{\mu} $ range for $ W $ candidates. Data are compared to fitted contributions from $ W \!\rightarrow \mu\nu $ signal and QCD, electroweak and heavy flavour backgrounds. (right) Invariant mass distribution of dimuon pairs in the $ Z $ candidate sample.

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(top) Differential $ W ^+$ and $ W ^-$ boson production cross-section in bins of muon pseudorapidity. (bottom) Differential $ Z $ boson production cross-section in bins of boson rapidity. Measurements, represented as bands, are compared to (markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs.

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Summary of the $ W $ and $ Z $ cross-sections. Measurements, represented as bands, are compared to (markers) NNLO predictions with different parameterisations of the PDFs .

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Two-dimensional plots of electroweak boson cross-sections compared to NNLO predictions for various parameterisations of the PDFs. The uncertainties on the theoretical predictions correspond to the PDF uncertainty only. All ellipses correspond to uncertainties at 68.3$\%$ CL.

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Summary of the cross-section ratios. Measurements, represented as bands, are compared to (markers) NNLO predictions with different parameterisations of the PDFs.

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(top) $ W ^+$ to $ W ^-$ cross-section ratio in bins of muon pseudorapidity. (bottom) $ W ^+$ ( $ W ^-$ ) to $ Z $ cross-section ratio in bins of $\mu ^+$ ( $\mu ^-$ ) pseudorapidity. Measurements, represented as bands, are compared to (markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs.

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$ W $ production charge asymmetry in bins of muon pseudorapidity. Measurements, represented as bands, are compared to (markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs.

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Summary of the $ W $ and $ Z $ cross-section ratios at different centre-of-mass energies. Measurements, represented as bands, are compared to (markers) NNLO predictions with different parameterisations of the PDFs.

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Summary of the cross-section double ratios at different centre-of-mass energies. Measurements, represented as bands, are compared to (markers) NNLO predictions with different parameterisations of the PDFs.

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Double ratios of cross-sections at different centre-of-mass energies as a function of muon pseudorapidity. Measurements, represented as bands, are compared to (markers, displaced horizontally for presentation) NNLO predictions with different parameterisations of the PDFs.

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Tables and captions

Summary of the relative uncertainties on the $ W ^+$ , $ W ^-$ and $ Z $ boson cross-sections.

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Summary of the relative uncertainties on the $ R_{ W ^\pm }$ , $ R_{ W ^+ Z }$ , $ R_{ W ^- Z }$ and $ R_{ W Z }$ cross-section ratios.

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Summary of the relative uncertainties on the electroweak boson cross-section ratios at different centre-of-mass energies.

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Cross-section for (top) $ W ^+$ and (bottom) $ W ^-$ boson production in bins of muon pseudorapidity. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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Cross-section for $ Z $ boson production in bins of boson rapidity. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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Cross-section for $ Z $ boson production in bins of boson transverse momentum. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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Cross-section for $ Z $ boson production in bins of boson $\phi^{*}_{\eta}$ . The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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Cross-section for $ Z $ boson production in bins of muon pseudorapidity. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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(Top) $ W ^+$ and (bottom) $ W ^-$ to $ Z $ cross-section ratios in bins of muon pseudorapidity. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement.

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$ W ^+$ to $ W ^-$ cross-section ratio in bins of muon pseudorapidity. The first uncertainties are statistical, the second are systematic and the third are due to the knowledge of the LHC beam energy.

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Lepton charge asymmetry in bins of muon pseudorapidity. The first uncertainties are statistical, the second are systematic and the third are due to the knowledge of the LHC beam energy.

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Ratios of (top) $ W ^+$ and (bottom) $ W ^-$ to $ Z $ cross-section ratios at different centre-of-mass energies in bins of muon pseudorapidity. The first uncertainty is statistical and the second is systematic.

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Cross-section for $ Z $ boson production in bins of muon pseudorapidity at $\sqrt{s}$ = 7 TeV. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement. The last column lists the final-state radiation correction.

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(Top) $ W ^+$ and (bottom) $ W ^-$ to $ Z $ cross-section ratios in bins of muon pseudorapidity at $\sqrt{s}$ = 7 TeV. The first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity measurement.

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Correlation coefficients between the differential $ Z $ cross-sections in bins of (top) $\eta^{\mu^{+}}$ and (bottom) $\eta^{\mu^{-}}$ . The LHC beam energy and luminosity uncertainties, which are fully correlated between cross-section measurements, are excluded.

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Correlation coefficients between the differential $ W $ and $ Z $ cross-sections in bins of (top) $\eta^{\mu^{+}}$ and (bottom) $\eta^{\mu^{-}}$ . The LHC beam energy and luminosity uncertainties, which are fully correlated between cross-section measurements, are excluded.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplementary material for the publication LHCb-PAPER-2015-049. The files are: supplementary.pdf : an overview of the extra figures *.pdf, *.png, *.eps : the figures in various formats

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Created on 20 April 2019.Citation count from INSPIRE on 24 April 2019.