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

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Abstract

A measurement of the inclusive $W \to \mu\nu$ production cross-section using data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s} = 7 {\rm \, TeV}$ is presented. The analysis is based on an integrated luminosity of about $1.0 {\rm \, fb}^{-1}$ recorded with the LHCb detector. Results are reported for muons with a transverse momentum greater than $20 {\rm \, Ge V\!/}c$ and pseudorapidity between 2.0 and 4.5. The $W^{+}$ and $W^{-}$ production cross-sections are measured to be $$\sigma_{W^{+} \to \mu^{+}\nu} = 861.0 \pm 2.0 \pm 11.2 \pm 14.7 {\rm \, pb},$$ $$\sigma_{W^{-} \to \mu^{-}\overline{\nu}} = 675.8 \pm 1.9 \pm 8.8 \pm 11.6 {\rm \, pb},$$ where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity determination. Cross-section ratios and differential distributions as functions of the muon pseudorapidity are also presented. The ratio of $W^{+}$ to $W^{-}$ cross-sections in the same fiducial kinematic region is determined to be $$\frac{\sigma_{W^{+} \to \mu^{+}\nu}}{\sigma_{W^{-} \to \mu^{-}\overline{\nu}}} = 1.274 \pm 0.005 \pm 0.009,$$ where the uncertainties are statistical and systematic, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.

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Fit to (left) the impact parameter distribution and (right) the relative energy deposition in the calorimeters. The data are compared to fitted contributions described in the legends.

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Transverse momentum distribution of the (left panel) positive and (right panel) negative muon candidates in the fiducial pseudorapidity range. The data are compared to fitted contributions described in the legend. The fit residuals normalised to the data uncertainty are shown at the bottom of each distribution.

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Summary of the $ W $ cross-section determinations. Measurements, represented as bands corresponding to the statistical (orange) and total (yellow) uncertainty, are compared to NNLO predictions for various parameterisations of the PDFs (black markers).

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Two-dimensional plot of the measured (total (yellow) and excluding the luminosity (orange) uncertainty) $ W ^+$ and $ W ^-$ cross-sections compared to NNLO predictions for various parameterisations of the PDFs (coloured markers). The uncertainty of the theoretical predictions corresponds to the PDF uncertainty only; the correlation is determined using the different error eigenvector sets. The ellipses correspond to a $68.3\%$ CL coverage.

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Differential $ W ^+$ and $ W ^-$ cross-section in bins of muon pseudorapidity. Measurements, represented as bands corresponding to the statistical (orange (blue) for $ W ^+$ ( $ W ^-$ )) and total (yellow (light blue) for $ W ^+$ ( $ W ^-$ )) uncertainty, are compared to NNLO predictions with different parameterisations of the PDFs (black (blue) markers for $ W ^+$ ( $ W ^-$ ), displaced horizontally for presentation).

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Lepton charge asymmetry in bins of muon pseudorapidity. Measurements, represented as bands corresponding to the statistical (orange) and total (yellow) uncertainty, are compared to NNLO predictions for various parameterisations of the PDFs (black markers, displaced horizontally for presentation).

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Transverse momentum distribution of the (left panel) positive and (right panel) negative muon candidates in eight bins of pseudorapidity. The data are compared to fitted contributions described in the legend. The fit residuals normalised to the data uncertainty are shown at the bottom of each distribution.

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Transverse momentum distribution of the (left panel) positive and (right panel) negative muon candidates in eight bins of pseudorapidity. The data are compared to fitted contributions described in the legend. The fit residuals normalised to the data uncertainty are shown at the bottom of each distribution.

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Animated gif made out of all figures.

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Tables and captions

Summary of the systematic uncertainties on the inclusive cross-sections and their ratio.

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Correction for final state radiation for $ W ^+ \rightarrow \mu ^+ \nu $ and $ W ^- \rightarrow \mu ^- \overline{\nu } $ in bins of muon pseudorapidity. No loss due FSR is observed for $ W ^-$ in the last pseudorapidity bin because of lack of statistics.

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Ratio of $ W ^+$ to $ W ^-$ cross-section in bins of muon pseudorapidity. The uncertainties are statistical and systematic.

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Correlation coefficients (statistical and systematic uncertainties) between $ W ^+$ and $ W ^-$ cross-sections in bins of muon pseudorapidity. The luminosity uncertainty is not included.

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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-2014-033 The files are supplementary.pdf : an overview of the extra figures *.pdf, *.png, *.eps, *.C : the figures in various formats

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.