Measurement of the $Z$ boson production cross-section in $pp$ collisions at $\sqrt{s} = 5.02$ TeV

Information

LHCb-PAPER-2023-010
CERN-EP-2023-141
arXiv:2308.12940 [PDF]

(Submitted on 24 Aug 2023)
JHEP 02 (2024) 070
Inspire 2690798

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Abstract

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.

Figures and captions

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.	Fig1.pdf [18 KiB] HiDef png [299 KiB] Thumbnail [174 KiB] *.C file
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.	Fig2a.pdf [14 KiB] HiDef png [65 KiB] Thumbnail [37 KiB] *.C file
	Fig2b.pdf [14 KiB] HiDef png [68 KiB] Thumbnail [39 KiB] *.C file
	Fig2c.pdf [14 KiB] HiDef png [64 KiB] Thumbnail [38 KiB] *.C file
(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.	Fig3a.pdf [18 KiB] HiDef png [338 KiB] Thumbnail [186 KiB] *.C file
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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.	Fig4.pdf [14 KiB] HiDef png [376 KiB] Thumbnail [188 KiB] *.C file
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.	Fig5.pdf [14 KiB] HiDef png [128 KiB] Thumbnail [66 KiB] *.C file
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.	Fig6a.pdf [17 KiB] HiDef png [226 KiB] Thumbnail [134 KiB] *.C file
	Fig6b.pdf [17 KiB] HiDef png [222 KiB] Thumbnail [130 KiB] *.C file
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 $\text{ Te V}$ .	Fig7a.pdf [14 KiB] HiDef png [59 KiB] Thumbnail [34 KiB] *.C file
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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) $\phi_{\eta}^{*}$ .	Fig8a.pdf [14 KiB] HiDef png [160 KiB] Thumbnail [108 KiB] *.C file
	Fig8b.pdf [14 KiB] HiDef png [166 KiB] Thumbnail [114 KiB] *.C file
	Fig8c.pdf [14 KiB] HiDef png [163 KiB] Thumbnail [110 KiB] *.C file
Correlation matrices for the efficiency uncertainties as functions of (top-left) $y^{{Z}}$ , (top-right) $p_{\mathrm{T}}^{Z}$ and (bottom) $\phi_{\eta}^{*}$ .	Fig9a.pdf [14 KiB] HiDef png [168 KiB] Thumbnail [124 KiB] *.C file
	Fig9b.pdf [14 KiB] HiDef png [187 KiB] Thumbnail [131 KiB] *.C file
	Fig9c.pdf [14 KiB] HiDef png [162 KiB] Thumbnail [113 KiB] *.C file
Animated gif made out of all figures.	PAPER-2023-010.gif Thumbnail

Tables and captions

Summary of the background composition in the $Z \rightarrow \mu ^+\mu ^-$ data sample of candidates satisfying the signal selection.	Table_1.pdf [82 KiB] HiDef png [76 KiB] Thumbnail [37 KiB] tex code
The uncertainties for the integrated $Z \rightarrow \mu ^+\mu ^-$ cross-section measurement.	Table_2.pdf [80 KiB] HiDef png [77 KiB] Thumbnail [38 KiB] tex code
Final state radiation correction used in the $y^{{Z}}$ cross-section measurement. The first uncertainty is statistical and the second is systematic.	Table_3.pdf [78 KiB] HiDef png [219 KiB] Thumbnail [113 KiB] tex code
Final state radiation correction used in the $p_{\mathrm{T}}^{Z}$ cross-section measurement. The first uncertainty is statistical and the second is systematic.	Table_4.pdf [79 KiB] HiDef png [187 KiB] Thumbnail [97 KiB] tex code
Final state radiation correction used in the $\phi_{\eta}^{*}$ cross-section measurement. The first uncertainty is statistical and the second is systematic.	Table_5.pdf [86 KiB] HiDef png [177 KiB] Thumbnail [92 KiB] tex code
The muon tracking, muon identification and trigger efficiency in each pseudorapidity bin.	Table_6.pdf [72 KiB] HiDef png [154 KiB] Thumbnail [75 KiB] tex code
Systematic uncertainties on the single differential cross-sections in bins of $y^{{Z}}$ , presented in percentage.	Table_7.pdf [71 KiB] HiDef png [133 KiB] Thumbnail [66 KiB] tex code
Systematic uncertainties on the single differential cross-sections in bins of $p_{\mathrm{T}}^{Z}$ , presented in percentage.	Table_8.pdf [73 KiB] HiDef png [90 KiB] Thumbnail [42 KiB] tex code
Systematic uncertainties on the single differential cross-sections in bins of $\phi_{\eta}^{*}$ , presented in percentage.	Table_9.pdf [79 KiB] HiDef png [107 KiB] Thumbnail [55 KiB] tex code
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.	Table_10.pdf [87 KiB] HiDef png [149 KiB] Thumbnail [82 KiB] tex code
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.	Table_11.pdf [88 KiB] HiDef png [117 KiB] Thumbnail [62 KiB] tex code
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.	Table_12.pdf [88 KiB] HiDef png [110 KiB] Thumbnail [62 KiB] tex code
Statistical correlation matrix for the one-dimensional $y^{{Z}}$ measurement.	Table_13.pdf [54 KiB] HiDef png [71 KiB] Thumbnail [26 KiB] tex code
Statistical correlation matrix for the one-dimensional $p_{\mathrm{T}}^{Z}$ measurement.	Table_14.pdf [54 KiB] HiDef png [60 KiB] Thumbnail [32 KiB] tex code
Statistical correlation matrix for the one-dimensional $\phi_{\eta}^{*}$ measurement.	Table_15.pdf [54 KiB] HiDef png [64 KiB] Thumbnail [31 KiB] tex code
Correlation matrix for the efficiency uncertainty of the one-dimensional $y^{{Z}}$ measurement.	Table_16.pdf [54 KiB] HiDef png [78 KiB] Thumbnail [32 KiB] tex code
Correlation matrix for the efficiency uncertainty of the one-dimensional $p_{\mathrm{T}}^{Z}$ measurement.	Table_17.pdf [54 KiB] HiDef png [67 KiB] Thumbnail [35 KiB] tex code
Correlation matrix for the efficiency uncertainty of the one-dimensional $\phi_{\eta}^{*}$ measurement.	Table_18.pdf [54 KiB] HiDef png [72 KiB] Thumbnail [34 KiB] tex code