The first measurement of the $Z$ boson production cross-section at centre-of-mass energy $\sqrt{s} = 5.02 $TeV in the forward region is reported, using $pp$ collision data collected by the LHCb experiment in year 2017, corresponding to an integrated luminosity of $100 \pm 2 \rm{pb^{-1}}$. The production cross-section is measured for final-state muons in the pseudorapidity range $2.0<\eta<4.5$ with transverse momentum $p_{\rm{T}}> 20 \rm{GeV/}\it{c}$. The integrated cross-section is determined to be \[ \sigma_{Z \rightarrow \mu^{+}\mu^{-}} = 39.6 \pm 0.7 (\rm{stat}) \pm 0.6 (\rm{syst}) \pm 0.8 (\rm{lumi}) \rm{pb} \] for the di-muon invariant mass in the range $60<M_{\mu\mu}<120 \rm{GeV/}\it{c^{2}}$. This result and the differential cross-section results are in good agreement with theoretical predictions at next-to-next-to-leading order in the strong coupling. Based on a previous LHCb measurement of the $Z$ boson production cross-section in $p$Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, the nuclear modification factor $R_{p\rm{Pb}}$ is measured for the first time at this energy. The measured values are $1.2^{+0.5}_{-0.3} (\rm{stat}) \pm 0.1 (\rm{syst})$ in the forward region ($1.53<y^*_{\mu}<4.03$) and $3.6^{+1.6}_{-0.9} (\rm{stat}) \pm 0.2 (\rm{syst})$ in the backward region ($-4.97<y^*_{\mu}<-2.47$), where $y^*_{\mu}$ represents the muon rapidity in the centre-of-mass frame.
Mass distribution of the $ Z \rightarrow \mu ^+\mu ^- $ signal candidates. The data are overlaid with model of the signal and background models. The signal component is scaled such that the sum of the signal and background matches the integral of the data. |
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Final state radiation correction estimated for the (top-left) $ y^{{Z}}$ , (top-right) $ p_{\mathrm{T}}^{Z}$ , and (bottom) $\phi_{\eta}^{*}$ differential cross-section measurements. The error bars represent the total (statistical and systematic) uncertainties. |
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(Left) Measured single differential cross-section as a function of $ y^{{Z}}$ , $ p_{\mathrm{T}}^{Z}$ and $\phi_{\eta}^{*}$ compared with different theoretical predictions. (Right) Ratio of theoretical predictions to measured values, with the horizontal bars showing the uncertainty from the PDFs. The green band, centered at unity, shows the uncertainty of the measurement. |
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Comparison of the integrated cross-section, $\sigma_{ Z \rightarrow \mu ^+\mu ^- }$, between data and theoretical predictions. The bands correspond to the data, with the inner band corresponding to the statistical uncertainty and the outer bands corresponding to the systematic uncertainty and total uncertainty. |
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Measured $\sigma_{ Z \rightarrow \mu ^+\mu ^- }$ for $pp$ collisions, as a function of $\sqrt{s}$. The uncertainty on the data is smaller than the marker size. The data are overlaid with a curve showing the theoretical prediction. |
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Invariant mass distributions of heavy-flavour samples with (left) PV fit quality $\chi^2 > 95$ and (right) $I_{\mu}< 91\%$ applied to both muons. Only statistical uncertainties are shown. |
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Muon tracking, identification and trigger efficiency as a function of pseudorapidity, estimated from the data and simulation at 5.02 $\text{ Te V}$ and 13 $\text{ Te V}$ . |
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Statistical correlation matrices for the differential cross-section measurements as functions of (top-left) $ y^{{Z}}$ , (top-right) $ p_{\mathrm{T}}^{Z}$ and (bottom) $\phi_{\eta}^{*}$ . |
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Correlation matrices for the efficiency uncertainties as functions of (top-left) $ y^{{Z}}$ , (top-right) $ p_{\mathrm{T}}^{Z}$ and (bottom) $\phi_{\eta}^{*}$ . |
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Animated gif made out of all figures. |
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Summary of the background composition in the $ Z \rightarrow \mu ^+\mu ^- $ data sample of candidates satisfying the signal selection. |
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The uncertainties for the integrated $ Z \rightarrow \mu ^+\mu ^- $ cross-section measurement. |
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Final state radiation correction used in the $ y^{{Z}}$ cross-section measurement. The first uncertainty is statistical and the second is systematic. |
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Final state radiation correction used in the $ p_{\mathrm{T}}^{Z}$ cross-section measurement. The first uncertainty is statistical and the second is systematic. |
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Final state radiation correction used in the $\phi_{\eta}^{*}$ cross-section measurement. The first uncertainty is statistical and the second is systematic. |
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The muon tracking, muon identification and trigger efficiency in each pseudorapidity bin. |
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Systematic uncertainties on the single differential cross-sections in bins of $ y^{{Z}}$ , presented in percentage. |
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Systematic uncertainties on the single differential cross-sections in bins of $ p_{\mathrm{T}}^{Z}$ , presented in percentage. |
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Systematic uncertainties on the single differential cross-sections in bins of $\phi_{\eta}^{*}$ , presented in percentage. |
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Measured single differential cross-sections in bins of $ y^{{Z}}$ . The first uncertainty is statistical, the second systematic, and the third is from the uncertainty on the integrated luminosity. |
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Measured single differential cross-sections in bins of $ p_{\mathrm{T}}^{Z}$ . The first uncertainty is statistical, the second systematic, and the third is due to the luminosity. |
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Measured single differential cross-sections in bins of $\phi_{\eta}^{*}$ . The first uncertainty is statistical, the second systematic, and the third is due to the luminosity. |
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Statistical correlation matrix for the one-dimensional $ y^{{Z}}$ measurement. |
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Statistical correlation matrix for the one-dimensional $ p_{\mathrm{T}}^{Z}$ measurement. |
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Statistical correlation matrix for the one-dimensional $\phi_{\eta}^{*}$ measurement. |
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Correlation matrix for the efficiency uncertainty of the one-dimensional $ y^{{Z}}$ measurement. |
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Correlation matrix for the efficiency uncertainty of the one-dimensional $ p_{\mathrm{T}}^{Z}$ measurement. |
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Correlation matrix for the efficiency uncertainty of the one-dimensional $\phi_{\eta}^{*}$ measurement. |
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Created on 27 April 2024.