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Measurement of forward $W\to e\nu$ production in $pp$ collisions at $\sqrt{s}=8$ TeV

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Abstract

A measurement of the cross-section for $W \to e\nu$ production in $pp$ collisions is presented using data corresponding to an integrated luminosity of $2\,$fb$^{-1}$ collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=8\,$TeV. The electrons are required to have more than $20\,$GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive $W$ production cross-sections, where the $W$ decays to $e\nu$, are measured to be \begin{equation*} \sigma_{W^{+} \to e^{+}\nu_{e}}=1124.4\pm 2.1\pm 21.5\pm 11.2\pm 13.0\,\mathrm{pb}, \end{equation*} \begin{equation*} \sigma_{W^{-} \to e^{-}\bar{\nu}_{e}}=\,\,\,809.0\pm 1.9\pm 18.1\pm\,\,\,7.0\pm \phantom{0}9.4\,\mathrm{pb}, \end{equation*} 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. Differential cross-sections as a function of the electron pseudorapidity are measured. The $W^{+}/W^{-}$ cross-section ratio and production charge asymmetry are also reported. Results are compared with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Finally, in a precise test of lepton universality, the ratio of $W$ boson branching fractions is determined to be \begin{equation*} \mathcal{B}(W \to e\nu)/\mathcal{B}(W \to \mu\nu)=1.020\pm 0.002\pm 0.019, \end{equation*} where the first uncertainty is statistical and the second is systematic.

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The inclusive fit to the $p^e_{\rm T}$ distribution of the full dataset. The $\chi^2/\mathrm{ndf}$ of the fit is 1.1 with 33 degrees of freedom.

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The differential $ W ^+$ and $ W ^-$ cross-sections in bins of $\eta^{ e }$. Measurements, represented as bands, are compared to NNLO predictions with different parameterisations of the PDFs (markers are displaced horizontally for presentation). The bottom panel displays the theory predictions divided by the measured cross-sections.

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The $ W ^+$ to $ W ^-$ cross-section ratio in bins of $\eta^{ e }$. Measurements, represented as bands, are compared to NNLO predictions with different parameterisations of the PDFs (markers are displaced horizontally for presentation). The bottom panel displays the theory predictions divided by the measured cross-section ratios.

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The $ W $ boson production charge asymmetry in bins of $\eta^{ e }$. Measurements, represented as bands, are compared to NNLO predictions with different parameterisations of the PDFs (markers are displaced horizontally for presentation). The bottom panel displays the difference between theory predictions and the measured charge asymmetry.

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The differential $ W ^+$ and $ W ^-$ cross-sections in bins of $\eta^{l}$. The measurement using electrons, represented as bands, is compared to the measurement in the muon final state. The bottom panel displays the muon results divided by the measurements in the electron final state.

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The $ W ^+$ to $ W ^-$ cross-section ratio in bins of $\eta^{l}$. The measurement using electrons, represented as bands, is compared to the measurement in the muon final state. The bottom panel displays the muon results divided by the measurements in the electron final state.

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The $ W $ boson production charge asymmetry in bins of $\eta^{l}$. The measurement using electrons, represented as bands, is compared to the measurement in the muon final state. The bottom panel displays the difference between the muon and electron final states.

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The ratio of branching fractions for the electron and muon final states determined for $ W $ , $ W ^+$ , and $ W ^-$ is compared to hadron collider and LEP results. The theory expectation is represented by the red line.

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Fits to $p^e_{\rm T}$ for $ e ^-$ in bins of $\eta^{ e }$. Pulls are shown underneath.

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Fits to $p^e_{\rm T}$ for $ e ^+$ in bins of $\eta^{ e }$. Pulls are shown underneath.

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Animated gif made out of all figures.

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Tables and captions

Summary of the relative uncertainties on the $ W ^+ $ and $ W ^- $ boson cross-sections and on the cross-section ratio. Uncertainties marked with $^{\dagger}$ are assumed to be uncorrelated between bins; all others are taken to be correlated.

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The Born level cross-section for $W^{+}$ boson production in bins of electron 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 rightmost column gives values of the additional factor, $f^{\textup{FSR}}$, by which the results should be multiplied in order to give the cross-sections after FSR.

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The Born level cross-section for $W^{-}$ boson production in bins of electron 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 rightmost column gives values of the additional factor, $f^{\textup{FSR}}$, by which the results should be multiplied in order to give the cross-sections after FSR.

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The $W^{+}$ to $W^{-}$ cross-section ratio in bins of electron 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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The $ W $ boson production charge asymmetry in bins of electron 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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Correlation coefficients of the systematic uncertainties for the differential $W^{+}$ cross-section measurement between bins of $\eta^{e}$.

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Correlation coefficients of the systematic uncertainties for the differential $W^{-}$ cross-section measurement between bins of $\eta^{e}$.

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Correlation coefficients of the systematic uncertainties for the differential $W^{+}$ and $W^{-}$ cross-section measurements between bins of $\eta^{e}$. The horizontal bin indices label bins of $\eta^{e}$ for electrons while vertical indices label bins for positrons.

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Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-024. The files are: supplementary.pdf : An overview of the extra tables chis.tex : Fit qualities signalFractions.tex : Signal fractions in bins of lepton pseudorapidity

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