# Measurement of the forward Z boson production cross-section in pp collisions at $\sqrt{s} = 13$ TeV

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## Abstract

A measurement of the production cross-section of Z bosons in pp collisions at $\sqrt{s} = 13$ TeV is presented using dimuon and dielectron final states in LHCb data. The cross-section 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 cross-section 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 cross-sections are measured as functions of the Z boson rapidity, transverse momentum and the angular variable $\phi^*_\eta$.

## Figures and captions

 The fiducial cross-section 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. Fig1.pdf [13 KiB] HiDef png [131 KiB] Thumbnail [88 KiB] *.C file The differential cross-section 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 cross-section, and the lower plot shows the same information as ratios to the central values of the NNPDF3.0 predictions. Fig2top.pdf [20 KiB] HiDef png [256 KiB] Thumbnail [189 KiB] *.C file Fig2bottom.pdf [22 KiB] HiDef png [478 KiB] Thumbnail [303 KiB] *.C file The normalised differential cross-section 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 cross-section, 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. Fig3top.pdf [18 KiB] HiDef png [196 KiB] Thumbnail [161 KiB] *.C file Fig3bottom.pdf [19 KiB] HiDef png [463 KiB] Thumbnail [282 KiB] *.C file The normalised differential cross-section 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 cross-section, 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. Fig4top.pdf [17 KiB] HiDef png [165 KiB] Thumbnail [155 KiB] *.C file Fig4bottom.pdf [18 KiB] HiDef png [173 KiB] Thumbnail [158 KiB] *.C file The ratio of the normalised differential cross-sections 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. Fig5top.pdf [19 KiB] HiDef png [409 KiB] Thumbnail [243 KiB] *.C file Fig5bottom.pdf [18 KiB] HiDef png [172 KiB] Thumbnail [151 KiB] *.C file Animated gif made out of all figures. PAPER-2016-021.gif Thumbnail

## Tables and captions

 Summary of the relative uncertainties on the $Z$ boson total cross-section. Table_1.pdf [41 KiB] HiDef png [70 KiB] Thumbnail [33 KiB] tex code The FSR correction applied as a function of the boson rapidity. Table_2.pdf [44 KiB] HiDef png [284 KiB] Thumbnail [143 KiB] tex code The FSR correction applied as a function of $\phi^{*}_\eta$ . Table_3.pdf [43 KiB] HiDef png [228 KiB] Thumbnail [113 KiB] tex code The FSR correction applied as a function of the boson transverse momentum. Table_4.pdf [44 KiB] HiDef png [206 KiB] Thumbnail [98 KiB] tex code The measured differential cross-sections 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. Table_5.pdf [43 KiB] HiDef png [134 KiB] Thumbnail [75 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supp.pdf [101 KiB] Supplementary figures for LHCb-PAPER-2016-021 Fig6b.pdf [23 KiB] HiDef png [274 KiB] Thumbnail [200 KiB] *C file Fig6t.pdf [14 KiB] HiDef png [171 KiB] Thumbnail [137 KiB] *C file Fig7.pdf [14 KiB] HiDef png [103 KiB] Thumbnail [91 KiB] *C file

Created on 19 October 2019.