A measurement of the production crosssection of Z bosons in pp collisions at $\sqrt{s} = 13$ TeV is presented using dimuon and dielectron final states in LHCb data. The crosssection is measured for leptons with pseudorapidities in the range $2.0 < \eta < 4.5$, transverse momenta $p_\text{T} > 20$ GeV and dilepton invariant mass in the range $60<m(\ell\ell)<120$ GeV. The integrated crosssection from averaging the two final states is \begin{equation*}\sigma_{\text{Z}}^{\ell\ell} = 194.3 \pm 0.9 \pm 3.3 \pm 7.6\text{ pb,}\end{equation*} where the first uncertainty is statistical, the second is due to systematic effects, and the third is due to the luminosity determination. In addition, differential crosssections are measured as functions of the Z boson rapidity, transverse momentum and the angular variable $\phi^*_\eta$.
The fiducial crosssection compared between theory and data. The bands correspond to the average of the dimuon and dielectron final states, with the inner band corresponding to the statistical uncertainty and the outer band corresponding to the total uncertainty. The top three points correspond to $\mathcal{O}(\alpha_{s}^2)$ predictions with different PDF sets. The inner error bars on these points are due to the PDF uncertainty, with the outer error bars giving the contribution of all uncertainties. The bottom points correspond to the LHCb measurements in the dielectron and dimuon final states and their average, with the inner error bar showing the statistical uncertainty and the outer error bar the total uncertainty. 
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The differential crosssection as a function of the $ Z$ boson rapidity, compared between theory and data. The bands correspond to the data, with the inner band corresponding to the statistical uncertainty and the outer band corresponding to the total uncertainty. The points correspond to $\mathcal{O}(\alpha_{s}^2)$ predictions with different PDF sets. The inner error bars on these points are due to the PDF uncertainty, with the outer error bars giving the contribution of all uncertainties. The different predictions are displaced horizontally within bins to enable ease of comparison. The upper plot shows the differential crosssection, and the lower plot shows the same information as ratios to the central values of the NNPDF3.0 predictions. 
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The normalised differential crosssection as a function of the $ Z$ boson $\phi^{*}_\eta$, compared between theory and data. The bands correspond to the data, with the inner band corresponding to the statistical uncertainty and the outer band corresponding to the total uncertainty. The points correspond to the theoretical predictions from the different generators and tunes. The different predictions are displaced horizontally within bins to enable ease of comparison. The upper plot shows the normalised differential crosssection, and the lower plot shows the same information as ratios to the central values of the predictions produced using the Monash 2013 tune of Pythia 8. The uncertainties on the theoretical predictions, visible at high $\phi^{*}_\eta$ , are statistical. 
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The normalised differential crosssection as a function of the $ Z$ boson transverse momentum, compared between theory and data. The bands correspond to the data, with the inner band corresponding to the statistical uncertainty and the outer band corresponding to the total uncertainty. The points correspond to the theoretical predictions from the different generators and tunes. The different predictions are displaced horizontally within bins to enable ease of comparison. The upper plot shows the normalised differential crosssection, and the lower plot shows the same information as ratios to the central values of the predictions produced using the Monash 2013 tune of Pythia 8. 
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The ratio of the normalised differential crosssections to the predictions evaluated using the FxFx scheme. The bands correspond to the data, with the inner band corresponding to the statistical uncertainty and the outer band corresponding to the total uncertainty. The different predictions are displaced horizontally within bins to enable ease of comparison. Alternative schemes give different predictions, shown as points.All predictions are generated using {MadGraph5}\_aMC@NLO. The uncertainties on the theoretical predictions are statistical. The upper plot shows the $\phi^{*}_\eta $ distribution, and the lower plot shows the $p_{\rm T}$ distribution. 
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Animated gif made out of all figures. 
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Summary of the relative uncertainties on the $ Z$ boson total crosssection. 
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The FSR correction applied as a function of the boson rapidity. 
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The FSR correction applied as a function of $\phi^{*}_\eta$ . 
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The FSR correction applied as a function of the boson transverse momentum. 
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The measured differential crosssections as a function of $p_{\rm T}$ . The first uncertainty is due to the size of the dataset, the second is due to experimental systematic uncertainties, and the third is due to the luminosity. 
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Supplementary material full pdf 
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Supplementary figures for LHCbPAPER2016021 
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Created on 14 September 2019.