This article presents the first measurement of the differential $Z$-boson production cross-section in the forward region using proton-lead collisions with the LHCb detector. The dataset was collected at a nucleon-nucleon centre-of-mass energy of $\sqrt{s_\mathrm{NN}}=8.16 \mathrm{TeV}$ in 2016, corresponding to an integrated luminosity of $30.8 \mathrm{nb}^{-1}$. The forward-backward ratio and the nuclear modification factors are measured together with the differential cross-section as functions of the $Z$ boson rapidity in the centre-of-mass frame, the transverse momentum of the $Z$ boson and a geometric variable $\phi^{*}$. The results are in good agreement with the predictions from nuclear parton distribution functions, providing strong constraining power at small Bjorken-$x$.
Dimuon invariant mass of the selected events for (a) forward and (b) backward collisions, respectively. The red histograms show the distributions from simulation normalised to the number of observed candidates. |
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The measured overall $ Z \rightarrow \mu ^+\mu ^- $ production fiducial cross-section compared to the PowhegBox prediction using CTEQ6.1, EPPS16 and nCTEQ15 (n)PDF sets, for forward and backward collisions, respectively. |
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The measured differential fiducial cross-section as a function of $ y^{*}_{ Z }$ for (a) forward and (b) backward collisions. The theoretical predictions are calculated using PowhegBox with CTEQ6.1, EPPS16 and nCTEQ15 (n)PDF sets. |
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The measured differential fiducial cross-section as a function of $ p_{\mathrm{T}}^{ Z }$ for (a) forward and (b) backward collisions, and the corresponding version for (c) forward and (d) backward collisions with fine intervals at low $ p_{\mathrm{T}}^{ Z }$ . The theoretical predictions are calculated using PowhegBox with CTEQ6.1, EPPS16 and nCTEQ15 (n)PDF sets. |
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The measured differential fiducial cross-section as a function of $\phi^{*}$ for (a) forward and (b) backward collisions. The theoretical predictions are calculated using PowhegBox with CTEQ6.1, EPPS16 and nCTEQ15 (n)PDF sets. |
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The measured forward-backward ratio ( $ R_\mathrm{FB}$ ), (a) for the overall measurement, (b) as a function of $ y^{*}_{ Z }$ , (c) as a function of $ p_{\mathrm{T}}^{ Z }$ , and (d) as a function of $\phi^{*}$ , together with the PowhegBox predictions using CTEQ6.1, EPPS16 and nCTEQ15 (n)PDF sets. |
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Measurements of the nuclear modification factor ( $ R_{ p \mathrm{Pb}}$ ) compared to the PowhegBox predictions using the EPPS16 and nCTEQ15 (n)PDF sets. |
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Nuclear modification factors ( $ R_{ p \mathrm{Pb}}$ ) as a function of (top row) $ y^{*}_{ Z }$ , (middle row) $ p_{\mathrm{T}}^{ Z }$ and (bottom row) $\phi^{*}$ , together with the PowhegBox prediction using EPPS16 and nCTEQ15 nPDF sets, where the left column is for forward collisions and the right column is for backward collisions. |
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Animated gif made out of all figures. |
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Observed number of candidates, input values and uncertainties for the overall cross-section and the theory corrections with uncertainties for the $ R_\mathrm{FB}$ and $ R_{ p \mathrm{Pb}}$ measurements. |
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The $ y^{*}_{\mu}$ acceptance correction factors ( $ k_\mathrm{FB}$ ) for $ R_\mathrm{FB}$ measured in intervals of $ y^{*}_{ Z }$ , $ p_{\mathrm{T}}^{ Z }$ , and $\phi^{*}$ . |
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The $ y^{*}_{\mu}$ acceptance correction factors ( $ k_{ p \mathrm{Pb}}$ ) for $ R_{ p \mathrm{Pb}}$ measured in intervals of $ y^{*}_{ Z }$ , $ p_{\mathrm{T}}^{ Z }$ , and $\phi^{*}$ for forward and backward collisions. |
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Correlation matrix of statistical uncertainty in $ y^{*}_{ Z }$ intervals for forward collisions. |
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Correlation matrix of statistical uncertainty in $ y^{*}_{ Z }$ intervals for backward collisions. |
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Correlation matrix of statistical uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for forward collisions. |
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Correlation matrix of statistical uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for backward collisions. |
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Correlation matrix of statistical uncertainty in $\phi^{*}$ intervals for forward collisions. |
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Correlation matrix of statistical uncertainty in $\phi^{*}$ intervals for backward collisions. |
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Correlation matrix of statistical uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for forward collisions in the low $ p_{\mathrm{T}}^{ Z }$ range. |
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Correlation matrix of statistical uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for backward collisions in the low $ p_{\mathrm{T}}^{ Z }$ range. |
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Correlation matrix of efficiency uncertainty in $ y^{*}_{ Z }$ intervals for forward collisions. |
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Correlation matrix of efficiency uncertainty in $ y^{*}_{ Z }$ intervals for backward collisions. |
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Correlation matrix of efficiency uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for forward collisions. |
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Correlation matrix of efficiency uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for backward collisions. |
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Correlation matrix of efficiency uncertainty in $\phi^{*}$ intervals for forward collisions. |
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Correlation matrix of efficiency uncertainty in $\phi^{*}$ intervals for backward collisions. |
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Correlation matrix of efficiency uncertainty in $ p_{\mathrm{T}}^{ Z }$ intervals for forward collisions in the low $ p_{\mathrm{T}}^{ Z }$ range. |
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Differential cross-section of $ Z \rightarrow \mu ^+\mu ^- $ in intervals of $ y^{*}_{ Z }$ for forward and backward collisions, together with the FSR correction. In the differential cross-section results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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Differential cross-section of $ Z \rightarrow \mu ^+\mu ^- $ in intervals of $ p_{\mathrm{T}}^{ Z }$ for forward and backward collisions, together with the FSR correction. In the differential cross-section results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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Differential cross-section of $ Z \rightarrow \mu ^+\mu ^- $ in intervals of $ p_{\mathrm{T}}^{ Z }$ for forward and backward collisions in the low $ p_{\mathrm{T}}^{ Z }$ region, together with the FSR correction. In the differential cross-section results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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Differential cross-section of $ Z \rightarrow \mu ^+\mu ^- $ in intervals of $\phi^{*}$ for forward and backward collisions, together with the FSR correction. In the differential cross-section results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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The forward-backward ratio ( $ R_\mathrm{FB}$ ) in intervals of $| y^{*}_{ Z } |$, together with the FSR correction. For the $ R_\mathrm{FB}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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The forward-backward ratio ( $ R_\mathrm{FB}$ ) in intervals of $ p_{\mathrm{T}}^{ Z }$ , together with the FSR correction. For the $ R_\mathrm{FB}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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The forward-backward ratio ( $ R_\mathrm{FB}$ ) in intervals of $\phi^{*}$ , together with the FSR correction. For the $ R_\mathrm{FB}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
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The nuclear modification factors ( $ R_{ p \mathrm{Pb}}$ ) in intervals of $ y^{*}_{ Z }$ for forward and backward collisions, together with the FSR correction. For the $ R_{ p \mathrm{Pb}}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
Table_27.pdf [67 KiB] HiDef png [80 KiB] Thumbnail [40 KiB] tex code |
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The nuclear modification factors ( $ R_{ p \mathrm{Pb}}$ ) in intervals of $ p_{\mathrm{T}}^{ Z }$ for forward and backward collisions, together with the FSR correction. For the $ R_{ p \mathrm{Pb}}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
Table_28.pdf [61 KiB] HiDef png [138 KiB] Thumbnail [67 KiB] tex code |
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The nuclear modification factors ( $ R_{ p \mathrm{Pb}}$ ) in intervals of $\phi^{*}$ for forward and backward collisions, together with the FSR correction. For the $ R_{ p \mathrm{Pb}}$ results, the first uncertainty is statistical, the second is systematic and the third is from integrated luminosity. |
Table_29.pdf [67 KiB] HiDef png [124 KiB] Thumbnail [69 KiB] tex code |
Supplementary material full pdf |
supple[..].pdf [156 KiB] |
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This ZIP file contains supplementary materials for the publication LHCb-PAPER-2022-009. supplementary.pdf : the supplementary document supplementary.tex : the latex supplementary codes main-supplementary.tex : the main latex codes *.pdf, *.png, *.eps : The figures |
Fig1.pdf [17 KiB] HiDef png [153 KiB] Thumbnail [134 KiB] *C file |
Created on 27 April 2024.